Impact of Magnetic Micro-Seeds in Electronic

Package Solder Joint Suitability

Period of Activity

AY 2015/2016 Semester 1 and 2

By: Supervisors:

Yau Ding Hua Tricia Dr. Gopala Krishnan

Ramaswami

A0102362X A/P Tok Eng Soon

In partial fulfilment of PC4199 Honours Project in Physics

Faculty of Science, Physics Department

i

Acknowledgement

I would like to sincerely acknowledge various people who have been with me in this

academic year where I have worked on my thesis. Firstly, a great part of this journey would

not be possible without my supervisors – Dr. Gopala Krishnan Ramaswami and A/P Tok Eng

Soon. Their valuable input and continuous guidance have helped me a great extend to the

completion of this work. I would also like to thank Mr. Arun Kumar Seraaj for his extensive

help thoughout this journey. He has been a great mentor and would always ensure that I do

not make any major mistakes in my work. I would also like to thank Dr. Ong Sheau Wei and

Mr. Chen Chang Pang, whom also belongs in εMaGIC Laboratory, they have helped me in

experimental procedures and discussions. I take this opportunity to also appreciate various

people in NUS Physics Department – Mr Chen Gin Seng, Mr Ho Kok Wen, Mrs Lee Soo

Mien and Mrs. Ng Soo Ngo for their support in equipment and technicalities. Lastly, I wish

to express my sense of gratitude to my family and friends who have been, directly or

indirectly, lending their helping hands and giving me encouragement to get me through the

project.

ii

Abstract

SAC305 is regarded as the top-performing environmentally friendly solder to date. However,

there are challenges to this solder such as its higher melting temperature which may cause

excessive intermetallic growth at solder/substrate interfaces. Adding ferromagnetic materials

into SAC305 has the potential to alleviate the challenges faced as it gives rise to an

alternative to the conventional global reflow process – magnetic induction heating. This

localised heating method prevents the temperature of other parts of the electronic package to

be highly elevated.

The first phase of this work involves the characterization of the as-made material. Fe is

incorporated into SAC305 through the process of ball-milling. The Fe-SAC305 composite

materials were characterized using Scanning Electron Microscopy and Energy-dispersive X-

ray Spectroscopy (SEM-EDX), Vibrating Sample Magnetometer (VSM) and Differential

Scanning Calorimetry (DSC). SEM-EDX was used to analyse the chemical composition of

the as-made solder materials and also to study the microstructure of Fe particles. VSM was

used to characterise its magnetic properties. It has been found that the usefulness of the

magnetic properties of Fe has not been changed after the process of ball-milling. DSC was

used to analyse its thermal properties. The change in melting temperature is less than 1°C

when Fe content increases to 25wt%. This change in melting temperature is insignificant in

the industrial process since it still falls within the solder melting temperature range.

The second phase of this work is a preliminary experimentation on the solder joint. The flake

samples were placed on an industrial Cu Ball Grid Array substrate. From the small sampling,

it seems that wettability is not promoted with the addition of Fe. Due to time constraint,

SEM-EDX was not done to identify the Fe particles at the cross-sections of the solder joint.

This is important as Fe has a higher density than SAC305 and is predicted to sink to the

solder joint interface. This may be detrimental to the strength of the solder joint as well as the

conductivity. However, caution needs to be taken while evaluating the preliminary data. With

the VSM and DSC results, the theoretical magnetic induction heating time is calculated. With

iii

an alternating magnetic field frequency of 280kHz, the induction heating times are able to

reach less than 10s for Fe content of 15wt% and above.

In conclusion, this work shows the potential of ball-milled Fe-SAC305 system solders to be

of great impact to the electronic packaging industry. More studies need to be conducted to

obtain the optimal characteristics of Fe-SAC305 system solder suited to harness Fe as heat-

generating magnetic micro-seeds in magnetic induction heating.

iv

Table of Contents

Acknowledgement ..................................................................................................................... i

Abstract ..................................................................................................................................... ii

Table of Contents .................................................................................................................... iv

List of Figures .......................................................................................................................... vi

List of Tables ............................................................................................................................ x

Chapter 1 Introduction ...................................................................................................... 1

1.1 Microelectronic Packaging ............................................................................................ 1

1.2 Solder and Solder Joint .................................................................................................. 2

1.3 Lead-free Solder Material Journey ................................................................................ 5

1.4 Fe-SAC305 System ........................................................................................................ 8

1.5 Research Objectives and Scope of Thesis ................................................................... 14

Chapter 2 Methodology ................................................................................................... 15

2.1 Overview ...................................................................................................................... 15

2.2 Sample Preparation for Fe-SAC305 Solder ................................................................. 15

2.3 Sample Preparation for solder/substrate Fe-SAC305/Cu joint .................................... 17

2.4 Characterization Techniques ........................................................................................ 21

2.4.1 Scanning Electron Microscopy and Energy-dispersive X-ray Spectroscopy

(SEM-EDX) ...................................................................................................................... 21

2.4.1 Vibrating Sample Magnetometer (VSM)......................................................... 24

2.4.1 Differential Scanning Calorimetry (DSC) ....................................................... 26

Chapter 3 Impact of Fe on SAC305................................................................................ 28

3.1 Solder Composition, and Fe Content and Distribution: SEM-EDX ............................ 28

3.2 Magnetic Properties: VSM .......................................................................................... 33

3.3 Thermal Properties: DSC ............................................................................................. 37

Chapter 4 Fe-SAC Solder Applicability to industrial Cu BGA ................................... 44

4.1 Impact of Fe on Cross-section of Solder/Substrate ...................................................... 44

4.2 Magnetic Induction Heating Applicability .................................................................. 49

Chapter 5 Conclusion and Future Work ....................................................................... 53

v

5.1 Conclusion ................................................................................................................... 53

5.2 Future Work ................................................................................................................. 54

References ............................................................................................................................... 56

Chapter 6 Appendices ...................................................................................................... 62

6.1 Appendix I: Phase Diagram and Gibbs Free Energy of Fe-Sn system ........................ 62

6.2 Appendix II: SEM-EDX .............................................................................................. 63

6.3 Appendix III: Calculated Expected Values of Saturation Magnetization for Fe-

SAC305 System .......................................................................................................... 83

6.4 Appendix IV: Miscellaneous DSC Experimental Results ........................................... 83

6.5 Appendix V: Solder Joint Features .............................................................................. 86

6.6 Appendix VI: IMC Measurement Methodology.......................................................... 89

6.7 Appendix VII: Calculated Values of Induction Heating Time .................................... 90

vi

List of Figures

Figure 1. 1 Multi-level packaging in microelectronics [2]. .................................................................... 1

Figure 1. 2 (a) Schematic of Flip Chip Solder Joint Technology. (b) Optical Micrograph of BGA with

7x6 Cu pads............................................................................................................................................. 2

Figure 1. 3 (a) Optical micrograph of cross-section of SAC305 solder ball on Cu pad. (b) zoom-in of

interfacial region of (a). .......................................................................................................................... 3

Figure 1. 4 Temperature profile for reflow soldering process for SAC305 and Sn96.5/Ag3.5 solders

[12] .......................................................................................................................................................... 4

Figure 1. 5 Reflow window for Sn-Pb and Pb-free solders [5] ............................................................... 6

Figure 1. 6 SEM micrograph of SAC305 [11] (b) and (c) SAC305 solder joint failure in thermal

cycling [14] ............................................................................................................................................. 7

Figure 1. 7 Schematic diagram of reflow process using magnetic induction heating ............................. 8

Figure 1. 8 magnetic induction heating responses of studied alloys with (a) dispersions of different

magnetic elements. (c) Fe dispersions at different concentrations. [25] ............................................... 10

Figure 1. 9 Plot of conductivities and densities of Fe, FeSn, FeSn2, SAC305 and Sn .......................... 12

Figure 1. 10 (a) FeSn2 phase in solder bulk [33] (b) Fe particles near interfacial region; (c) pure Fe

particles surrounded by FeSn2 ............................................................................................................... 13

Figure 2. 1 RETSCH Planetary Ball Mill PM400 ................................................................................ 16

Figure 2. 2 (a) Optical Microscope BX51; (b) Application of flux and solder under optical

microscope; (c) samples placed in porcelain boat................................................................................. 18

Figure 2. 3 Furnace reflow experimental set-up ................................................................................... 19

Figure 2. 4 (a) Preparation of sample for polishing; (b) Struers Tegramin-20 Polishing Machine ...... 20

Figure 2. 5 (a) Electrons and photon signals emanating from volume during electron-beam

impingement. (b) energy spectrum of electrons emitted. (c) effect of surface topography on electro

emission. [44] ........................................................................................................................................ 22

Figure 2. 6 Schematic diagram of x-ray emission................................................................................. 22

Figure 2. 7 JEOL Field Emission Scanning Electron Microscope JSM-6700F .................................... 23

Figure 2. 8 Typical Magnetic hysteresis loop of a ferromagnetic material [46] ................................... 25

Figure 2. 9 Pellet sample ....................................................................................................................... 25

Figure 2. 10 Typical DSC profile for BM SAC305 .............................................................................. 26

vii

Figure 3. 1 SEM micrograph, EDX map spectrum and corresponding EDX elemental maps of Sn, Fe,

C, Cu, Ag and O for 25wt%Fe-SAC. .................................................................................................... 29

Figure 3. 2 At% of elements in all 3 images (I1, I2 and I3) obtained from map spectrum data. Images

can be found in Appendix. .................................................................................................................... 30

Figure 3. 3 Fe/Sn atomic ratio as a function of theoretical Fe wt% ...................................................... 31

Figure 3. 4 All Fe EDX chemical maps from 5wt% to 25wt%. ............................................................ 32

Figure 3. 5 Fe-rich area marked with red marks for (a) 5wt%Fe-SAC and (b) 25wt%Fe-SAC. .......... 32

Figure 3. 6 Hysteresis curves for BM SAC305 and 6 compositions of Fe-SAC system in wide scan

range of -15000Oe to +15000Oe. ......................................................................................................... 34

Figure 3. 7 Graphs of experimental and expected Saturation Magnetization as a function of mass of Fe

present in sample. .................................................................................................................................. 35

Figure 3. 8 Hysteresis curves for BM SAC305 and 6 compositions of Fe-SAC system in narrow scan

range of -100Oe to +100Oe. ................................................................................................................. 36

Figure 3. 9 Hysteresis loss of BM SAC305 and Fe-SAC305 system. .................................................. 36

Figure 3. 10 DSC Curves for ball-milled samples with Fe wt% from 0wt% to 25wt%. Curves are

vertically displaced by -15 mW progressively. ..................................................................................... 38

Figure 3. 11 Data plots of Latent Heat of Fusion against Mass of Sn for all heats of BM SAC305 with

y ≡ Latent Heat of Fusion in mJ and x ≡ Mass of Sn in mg .............................................................. 39

Figure 3. 12 Latent Heat of Fusion against Mass of Sn in sample ........................................................ 40

Figure 3. 13 Minimum and experimental moles of Sn against moles of Fe in sample ......................... 42

Figure 3. 14 DSC curves for 1st, 2nd and 3rd heating for (a) BM SAC (b) 2wt%Fe-SAC (c) 5wt%Fe-

SAC (d) 10wt%Fe-SAC (e) 15wt%Fe-SAC (f) 20wt%Fe-SAC (g) 25wt%Fe-SAC ............................ 43

Figure 4. 1 Optical micrographs of as-received Cu pads with (a) 50x and (b) 100x objective lens

magnifications. ...................................................................................................................................... 46

Figure 4. 2 Cross-section of 7 Cu pads on BGA for (a) B SAC305 and (b) 25wt%Fe-SAC ............... 47

Figure 4. 3 100x objective lens on cross-sections of (a) BM SAC305 and (b) 25wt%Fe-SAC solder

joints ...................................................................................................................................................... 47

Figure 4. 4 Induction heating time as a function of frequency of alternating magnetic field for BM

SAC305 and Fe-SAC system. ............................................................................................................... 51

Figure 6. 1 Fe-Sn phase diagram .......................................................................................................... 62

Figure 6. 2 Graph of Gibbs Free Energy of FeSn and FeSn2 in temperature range of 25°C to 325°C 63

Figure 6. 3 SEM micrographs for 5wt%Fe-SAC flakes. (a), (b) and (c) are 100x100µm sizes while

(d), (e) and (f) are the corresponding 10x10µm images marked by the red boxes. .............................. 64

viii

Figure 6. 4 SEM micrographs for 25wt%Fe-SAC flakes. (a), (b) and (c) are 100x100µm sizes while

(d), (e) and (f) are the corresponding 10x10µm images marked by the red boxes. .............................. 64

SEM image for BM SAC305

Figure 6. 5 250x250µm I1 .................................................................................................................... 65

Figure 6. 6 250x250µm I2 .................................................................................................................... 65

Figure 6. 7 250x250µm I3 .................................................................................................................... 66

Figure 6. 8 100x100µm I1 .................................................................................................................... 66

Figure 6. 9 100x100µm I2 .................................................................................................................... 67

Figure 6. 10 100x100µm I3 .................................................................................................................. 67

SEM image for 5wt%Fe-SAC

Figure 6. 11 250x250µm I1 .................................................................................................................. 68

Figure 6. 12 250x250µm I2 .................................................................................................................. 68

Figure 6. 13 250x250µm I3 .................................................................................................................. 69

Figure 6. 14 100x100µm I1 .................................................................................................................. 69

Figure 6. 15100x100µm I2 ................................................................................................................... 70

Figure 6. 16 100x100µm I3 .................................................................................................................. 70

SEM image for 10wt%Fe-SAC

Figure 6. 17 250x250µm I1 .................................................................................................................. 71

Figure 6. 18 250x250µm I2 .................................................................................................................. 71

Figure 6. 19 250x250µm I3 .................................................................................................................. 72

Figure 6. 20 100x100µm I1 .................................................................................................................. 72

Figure 6. 21 100x100µm I2 .................................................................................................................. 73

Figure 6. 22 100x100µm I3 .................................................................................................................. 73

SEM image for 15wt%Fe-SAC

Figure 6. 23 250x250µm I1 .................................................................................................................. 74

Figure 6. 24 250x250µm I2 .................................................................................................................. 74

Figure 6. 25 250x250µm I3 .................................................................................................................. 75

Figure 6. 26 100x100µm I1 .................................................................................................................. 75

Figure 6. 27 100x100µm I2 .................................................................................................................. 76

Figure 6. 28 100x100µm I3 .................................................................................................................. 76

SEM image for 20wt%Fe-SAC

Figure 6. 29 250x250µm I1 .................................................................................................................. 77

Figure 6. 30 250x250µm I2 .................................................................................................................. 77

Figure 6. 31 250x250µm I3 .................................................................................................................. 78

Figure 6. 32 100x100µm I1 .................................................................................................................. 78

Figure 6. 33 100x100µm I2 .................................................................................................................. 79

Figure 6. 34 100x100µm I3 .................................................................................................................. 79

ix

SEM image for 25wt%Fe-SAC

Figure 6. 35 250x250µm I1 .................................................................................................................. 80

Figure 6. 36 250x250µm I2 .................................................................................................................. 80

Figure 6. 37 250x250µm I3 .................................................................................................................. 81

Figure 6. 38 100x100µm I1 .................................................................................................................. 81

Figure 6. 39 100x100µm I2 .................................................................................................................. 82

Figure 6. 40 100x100µm I3 .................................................................................................................. 82

Figure 6. 41 DSC curves of Stearic Acid 1st, 2nd and 3rd heating. ..................................................... 83

Figure 6. 42 (a), (c) and (e) DSC curves of 1st, 2nd and 3rd heating respectively for 5 different sample

masses. (b), (d) and (f) plots of Latent Heat of Fusion against Mass of Sn for 1st, 2nd and 3rd heating

respectively. Curves are vertically displaced by -15 mW p .................................................................. 85

Figure 6. 43 Cu pads peeling at area without solder joints ................................................................... 86

Figure 6. 44 Black spots in interfacial regions and solder bulk ............................................................ 87

Figure 6. 45 Close up of 7 Cu pads for BM-SAC305 ........................................................................... 87

Figure 6. 46 Close up of 7 Cu pads for 25wt%Fe-SAC ........................................................................ 88

Figure 6. 47 Some solder bulks which appear detached from Cu pads. Thin red arrows show solder

bulks with no IMCs observed. Thick dotted blue arrows show solder bulks with IMCs observed. ..... 88

Figure 6. 48 Rectangular features which may be Ag3Sn platelets ....................................................... 89

Figure 6. 49 IMC thickness measurement methodology (a) average height and (b) area/length

methodologies ....................................................................................................................................... 89

Figure 6. 50 Both methodologies used to measure IMC thickness with x length of 30µm, 70µm and

100µm. .................................................................................................................................................. 90

x

List of Tables

Table 1. 1 Different stages of reflow process and their purposes. .......................................................... 4

Table 1. 2 Family of lead-free solder compounds and their respective melting temperature and relative

price [10] ................................................................................................................................................. 5

Table 1. 3 Addition of reinforcements to SAC305 [6]............................................................................ 7

Table 1. 4 Summary of impact on solder after Fe addition ................................................................... 11

Table 2. 1 Compositions of Fe-SAC305 samples made in mass and atomic weight percentages ........ 17

Table 2. 2 Typical industrial and furnace reflow profiles ..................................................................... 19

Table 2. 3 Polishing parameters ............................................................................................................ 20

Table 2. 4 Values of Characteristic X-Ray Energies [45] ..................................................................... 23

Table 2. 5 Values of weight and atomic percentages of Fe and Sn for different compositions of Fe-

SAC305 ................................................................................................................................................. 24

Table 2. 6 (a) Masses of pellet samples used; (b) VSM applied magnetic field range ......................... 25

Table 2. 7 Experimental and reference values for onset temperature and specific latent heat of fusion

for Indium ............................................................................................................................................. 27

Table 2. 8 Various masses of (a) SAC305 and BM SAC305, (b) different compositions of Fe-

SAC305. ................................................................................................................................................ 27

Table 3. 1 At% of Fe, Sn and Stearic Acid (S.A.) present in different compositions of Fe-SAC system.

.............................................................................................................................................................. 30

Table 3. 2 Values of Retentivity, Coercivity and Saturation Magnetization of Fe, BM Fe, SAC305 and

BM SAC305 for wide scan range of -15000Oe to +15000Oe. ............................................................. 33

Table 3. 3 Values of Retentivity, Coercivity and Saturation Magnetization of BM SAC305 and 6

compositions of Fe-SAC system. .......................................................................................................... 34

Table 3. 4 Average onset and peak melting temperatures of SAC305 and BM SAC305. .................... 38

Table 3. 5 Onset and peak melting temperatures for 1st heating. ......................................................... 39

Table 3. 6 Values of calculations leading to masses and moles of Sn reacted. ..................................... 41

Table 3. 7 Values of calculations leading to minimum masses and moles of Sn if all Fe has reacted. 41

Table 4. 1 Values of heat capacities used ............................................................................................. 49

Table 4. 2 Values required for induction heating time calculation. ...................................................... 50

xi

Table 4. 3 Heating power with alternating magnetic field frequency of 280kHz. ................................ 51

Table 6. 1 SEM-EDX experimental parameters for preliminary experimentation ............................... 63

Table 6. 2 Methodology Parameters used for SEM-EDX Experimentation ......................................... 64

Table 6. 3Experimental and expected values of saturation magnetization from VSM result ............... 83

Table 6. 4 Temperatures for various masses of SAC305 ...................................................................... 84

Table 6. 5 Temperatures for various masses of BM SAC305 ............................................................... 84

Table 6. 6 Temperatures for Fe-SAC305 system .................................................................................. 84

Table 6. 7 BM SAC305 for 1st, 2nd and 3rd heats ............................................................................... 86

Table 6. 8 Fe-SAC305 system for 1st, 2nd and 3rd heats ..................................................................... 86

Table 6. 9 Results from both methodology ........................................................................................... 90

Table 6. 10 Induction heating time between frequency range of 10kHz to 600kHz ............................. 90

1

Chapter 1

Introduction

1.1 Microelectronic Packaging

With the requirements of advanced electronic products (GoPro, iWatch, Fitbit and the like) to

be faster, lighter and smaller, the interconnections between a Si chip and a bonding pad are

driven to be denser than ever. Generally, there are three levels of interconnections in

microelectronic packaging. The first-level refers to connections between pins of the package

substrate and bonding pads on the integrated circuit [1]. The second-level involves discrete

devices on printed circuit boards and the third-level deals with the accessibility by arranging

subsystems together [2].

Figure 1. 1 Multi-level packaging in microelectronics [2].

For high-density Input/Output (I/O) pads, flip chip solder joint technology has been widely

harnessed in the first- and second-level packaging. As in Fig. 1.2 (a), the Si chip is flipped

facing down on a substrate with the electrical connections achieved through an area array of

solder bumps [3]. An optical micrograph of a ball grid array (BGA) is shown in Fig. 1.2(b).

The advantage of this methodology over conventional wire-bonding is that the solder bumps

cover a large part of the chip surface rather than merely the periphery in wire-bonding.

2

Figure 1. 2 (a) Schematic of Flip Chip Solder Joint Technology. (b) Optical Micrograph of

BGA with 7x6 Cu pads

1.2 Solder and solder joint

A solder is the interconnect material for integrated consumer electronic system. In addition to

being an electrical conduit, it acts as a thermal path and mechanically holds the parts together

[4]. A solder joint is essentially the chemical reaction between Sn (in solder) and Cu

(substrate) to form intermetallic compounds (IMCs). Therefore, Cu-Sn is arguably one of the

most important metallurgical binary system that has impacted human civilization [3].

A good solder should henceforth have the following key properties [5,6,7]:

I. Eutectic composition. Its melting point has to be lower than its individual components

and it behaves like a pure solid with a single melting temperature. This ensures that

both interfaces can join at the same time.

II. Low melting temperature. This is to avoid damage to other parts of the electronic

package.

3

III. Good wetting ability. This refers to the capability of an alloy in molten state to spread

over a metal surface. For good wetting, the contact angle, related surface tensions of

substrate/flux and substrate/solder interfaces, needs to be small.

IV. Good mechanical properties, such as hardness and modulus, tensile properties,

thermomechanical fatigue and the likes. It gauges the reliability of consumer products

when subjected to stress in operation environment.

V. Good thermal properties, such as reasonable thermal conductivity and low coefficient

of thermal expansion. Similarly, it gauges how reliable the product operation is.

VI. Good electrical properties. Since solder serves as an electrical interconnect, its

electrical resistivity should be low to allow current flow. Further, due to the trend

towards high I/O density, electro-migration becomes an increasingly worrying issue.

High current density causes atoms to migrate in the general direction as electron flow

which may produce voids. There may also be directional effects on IMC growth

resulting in reliability issues.

VII. Low cost. Microelectronics industry is extremely cost-conscious to continuously

produce higher performance at lower costs.

VIII. Environmentally-friendly. This is due to legislation that eliminates the use of Pb due

to environmental and toxicological concerns.

The physical properties a good solder joint should have are mainly attributed to the

characteristics of the interfacial IMCs as it may adversely affect the reliability and

solderability of solder joints should excessive growth occurs during storage and service [8].

Fig. 1.3 shows an optical micrograph of interfacial IMCs. The IMCs present in the solder

bulk might have been detached from the interfacial IMCs. The characteristics of IMCs are

further discussed in section 1.3.

Figure 1. 3 (a) Optical micrograph of cross-section of SAC305 solder ball on Cu pad. (b)

zoom-in of interfacial region of (a).

4

To form a solder joint, reflow solder is performed. It is the process in which solder paste is

used to attach electrical components to their contact pads i.e. Si chip to Cu pad. The entire

assembly would be subjected to controlled heat so as to melt and permanently connect the

components together [5]. This means that global heating is involved. Fig. 1.4 shows a typical

reflow temperature profile. The profile is developed with the goal of attaining a reliable and

repeatable reflow process which optimises wetting properties and minimises peak reflow

temperature at the same time. It consists of 4 stages – pre-heating, soaking, reflow and

cooling. The purpose of each stage is summarised in Table 1.1.

Table 1. 1 Different stages of reflow process and their purposes.

Figure 1. 4 Temperature profile for reflow soldering process for SAC305 and Sn96.5/Ag3.5

solders [12]

5

1.3 Lead-free solder material journey

The conventional primary solder used, 63Sn-37Pb, is considered to be environmentally

hazardous due to Pb toxicity [7]. According to the Directive on Waste from Electrical and

Electronic Equipment, the use of Pb is to be phased out from January 2004 onwards [7,9].

Since then, the global electronic assembly community has been striving to search for the best

replacement. Table 1.2 shows an example of 5 prominent solders named after their

metallurgical compositions.

Table 1. 2 Family of lead-free solder compounds and their respective melting temperature

and relative price [10]

SAC305 (Sn-3wt%Ag-0.5wt%Cu), in particular, is currently the most favourable

environmentally-friendly solder used. This is due to its low melting temperature of about

217°C, its near-eutectic composition and favourable thermal-mechanical fatigue properties

[11]. This high-performing solder material therefore has many stakeholders such as Indium

Corporation and Infineon which produces the material itself; Global Foundries and ST

Microelectronics which uses the material in their chips; and finally, Samsung and Apple

which uses these chips in the making of consumer products.

However, since it differs from Sn-Pb solders in soldering performance, microstructures and

mechanical behaviours, it creates key challenges which must be addressed in order for it to be

successfully implemented into consumer electronics applications [4]. There are two main

issues at hand – high reflow temperature and reliability issues due to microstructural

phenomena.

6

High Reflow Temperature

As mentioned in section 1.2, packages need to reach a temperature of at least 20°C higher

than the solder melting temperature to form a good solder joint. Fig. 1.5 shows the higher

reflow temperature required as compared to Sn-Pb solders. The peak reflow temperatures are

projected to rise to about 240°C to 260°C while temperature-sensitive electronic components

can tolerate a maximum temperature of about 240°C [13]. Further, due to the inferior wetting

characteristics of SAC305, the reflow soldering window is slightly longer (40-120 seconds)

[5]. These bring about increased damages such as PCB warpages, interfacial delamination

and electrical failure if interconnect lines are broken [4].

Figure 1. 5 Reflow window for Sn-Pb and Pb-free solders [5]

Microstructural Phenomena

The micro-constituents in SAC305 solder are typically large plates of primary Ag3Sn, large

needles of Cu6Sn5 and dendritic β-Sn. Their presence is attributed to the large undercooling

of Sn which starts to nucleate homogeneously after being undercooled for a period of time.

The delay causes the microstructural development of solidified Sn-based alloys which thus

affects the mechanical properties of solder joints adversely [5]. Further, IMCs at

solder/substrate interface also degrade mechanical properties as they can become sites for

crack initiation and propagation if it becomes a significant fraction of the solder joint [4].

7

Figure 1. 6 SEM micrograph of SAC305 [11] (b) and (c) SAC305 solder joint failure in

thermal cycling [14]

With the demands of electronic manufacturing industry based on cost, reliability and shelf

life [10], reinforcements have been added to address the challenges as shown in Table 1.3.

The impact of reinforcements aims for key properties of a good solder mentioned in section

1.2.

Table 1. 3 Addition of reinforcements to SAC305 [6]

Of particular interest would be the addition of ferromagnetic elements (Fe, Ni or Co). Its

magnetic property has the prospective to improve the two main issues of high reflow

temperature and microstructural phenomena which is elaborated in section 1.4.

8

1.4 Fe-SAC305 system

Magnetic Induction Heating

Incorporating ferromagnetic material into solders make them suitable for heating via

magnetic induction – a localised heating methodology (as opposed to global heating in

conventional reflow process). Shown in Fig. 1.7, the assembly would be placed at the centre

of an induction coil. With a high frequency electromagnetic field, ferromagnetic particles

which are homogeneously distributed within the solders are able to dissipate energy via eddy

currents, hysteretic and relaxation processes. This means that they can be harnessed as

localised heat-generating micro-seeds to uniformly heat up and melt the solders.

Additionally, the control of the application of magnetic field can act as a manipulative tool

not only for the control of solder melting during fabrication; but also for other functionalities

such as microelectronic repairs and the like.

Figure 1. 7 Schematic diagram of reflow process using magnetic induction heating

Eddy Currents

Eddy currents are generated in accordance to Lenz’s law - these currents would subsequently

induce an electric field to oppose the change in magnetic flux of the applied field. This results

in Joule heating within the conductors with eddy currents power loss in a material as follows

from Stephensons’ equation:

9

Pec =π2

β

B2f2

ρd2 Wm

-3 with 𝐵 = 𝜇0(𝐻 + 𝑀)

(1.1) [5,21]

where 𝛽 ≡ shape-factor coefficient (equals to 20 for spheres), 𝜇0 ≡ vacuum permeability, H ≡

applied field strength, M ≡ material magnetization, f ≡ frequency of applied field , 𝜌 ≡ bulk

resistivity of the materials and d ≡ cross-sectional dimension (equivalent to diameter for

spheres).

It should be noted that eddy current power loss is negligible for solder powders with diameter

of 50µm or less and do not contribute significantly to heating [21,22].

Hysteresis

Hysteresis is at the heart of the behaviour of magnetic materials [23]. In 1907, Weiss

proposed that a ferromagnetic material is subdivided into regions called magnetic domains.

The magnetization orientation can vary from domain to domain but it is aligned within the

domain [23]. The material gets magnetized when its domains align to an applied field. This

magnetization retains after the applied field is removed unless effort is made to demagnetise

it. Hence, a closed path magnetization curve can be plotted with a varying applied field. This

is known as a hysteresis loop - a detailed discussion is in section 2.4.2.

Relaxation

Magnetic nanoparticles become superparamagnetic below a certain critical size [22].

Relaxation processes then take place due to the thermal relaxation of magnetic moments in

the material [5]. The characteristics of a hysteresis loop do not apply if the measurement time

is larger than the relaxation time of the system which results in relaxation being the dominant

mechanism of power loss [5, 22, 24].

Background

In this project, the ferromagnetic material of choice would be Fe as it is most useful for

magnetic induction heating application. According to Calabro et al. [25], SnAg alloys

containing Fe dispersions are able to inductively heat up at a faster rate than Ni or Co

10

dispersions (Fig. 1.8(a)). It is also noted that higher concentration of Fe produced faster

heating as expected (Fig. 1.8(b)).

Figure 1. 8 magnetic induction heating responses of studied alloys with (a) dispersions of

different magnetic elements. (c) Fe dispersions at different concentrations. [25]

Various studies on Fe reinforcements have been done for Sn-Ag-Cu solders as shown in

Table 1.4.

11

Table 1. 4 Summary of impact on solder after Fe addition

12

With reference to the phase diagram as shown in Appendix, the possible phase formation at

typical reflow temperature would be FeSn2 and FeSn. Therefore, the expected additions to a

solder joint interface would be Fe, FeSn and FeSn2. Fig. 1.9 shows the relative densities and

resistivities of Fe, FeSn, FeSn2, SAC305 and Sn. Since the densities of Fe-Sn phases are

higher than SAC305, when Fe-SAC305 system solder is melted onto a substrate, Fe-Sn

phases are expected to sink to the interface during cooling. The wettability is expected to

decrease since Fe-Sn phases would take up the spaces for IMC growth which is required for a

good solder joint. The conductivity of FeSn and FeSn2 are much lower. Hence, the interfacial

contact conductivity may decrease with their formation. Since the IMCs are denser than their

end members (Fe and Sn), they act as scattering sites for conduction of electrons. A higher

density causes electrons to be scattered more. This in turn causes electrical conductivity to

decrease [34].

Figure 1. 9 Plot of conductivities and densities of Fe, FeSn, FeSn2, SAC305 and Sn

[31, 34-37]

FeSn2 have been observed in solder bulk and interfacial region in previous studies as

mentioned in Table 1.4. Fig. 1.10(a) shows the presence of FeSn2 sparsely distributed in

solder bulk of Fe-SAC105 [33]; Fig. 10(b) and (c) show the presence of Fe-rich regions at

interfacial area in Fe-SAC305. FeSn2 phase forms around pure Fe particles, and the thickness

of FeSn2 increased from hundreds of nanometers to 1µm from reflow time of one minute

onwards [31].

13

Figure 1. 10 (a) FeSn2 phase in solder bulk [33] (b) Fe particles near interfacial region; (c)

pure Fe particles surrounded by FeSn2

The addition of Fe allows localised reflow to occur, which may mitigate some of the current

challenges of SAC305 solders. However, their addition causes concern such as the reduction

in the strength of solder joints and also decrease in conductivity. Both of which are

detrimental to the electronic packaging reliability. Therefore, the impact of Fe in SAC305 as

a solder material and as a solder joint on a substrate is crucial information in the

determination of the capability of Fe-SAC305 system solder.

1.5 Research Objectives and Scope of Thesis

The scope of the thesis would be to investigate the suitability of Fe-SAC305 as a solder

material and as a solder joint on Cu BGA. The modification in properties after Fe-SAC305 is

subjected to thermal impact is studied. There are 3 main objectives in this project.

I. Impact of Fe on SAC305 through magnetic and thermal properties of Fe-SAC305

system.

II. Study of Fe as magnetic micro-seeds by evaluating Fe distribution within SAC305.

III. Assess the impact of Fe-SAC305 system on Cu interface

The first phase involves characterization of Fe-SAC305 system. The magnetic and thermal

properties are studied using Vibrating Sample Magnetometer (VSM) and Differential

Scanning Calorimetry (DSC) respectively (objective 1). The spatial distribution is studied

using Scanning Electron Microscopy and Energy-dispersive X-ray Spectroscopy (SEM-EDX)

(objective 2). The second phase occurs after forming a solder joint with Fe-SAC305 and Cu

14

BGA. The interfacial impact is studied using SEM-EDX and Optical Microscopy (OM)

coherently (objective 3).

15

Chapter 2

Methodology

2.1 Overview

This chapter introduces the sample preparation – how Fe was incorporated into SAC305 by

ball-milling. It describes the methodology used to prepare a solder joint between Fe-SAC

system and a Cu pad by furnace reflow. The characterization techniques used in this project –

SEM-EDX, VSM and DSC are also presented in terms of the working principles, instruments

and parameters involved.

2.2 Sample Preparation for Fe-SAC305 solder

The samples were prepared by a process called ball-milling. Ball-powder-ball collisions

causes powder particles to repeatedly flatten, cold-weld, fracture and re-weld [38]. This is

because a small amount of particles will be trapped between the grinding balls during impact.

Soft metal particles will plastically deform and cold-weld while brittle metal particles (e.g.

Fe) tend to fracture. Work hardening may also occur to the soft metals which results in their

fracture thereafter. Subsequently, the freshly fractured particles may then again cold-weld.

The continual ball-ball, ball-powder and ball-wall collisions will cause a rise in temperature,

which promotes diffusion and thus microstructural refinement. Eventually, the competing

events of cold-welding and fracturing result in a homogenized microstructure. The final result

should be a compositions of individual powders equivalent to the composition of the starting

material. This is known as the steady state equilibrium where there is a balance between the

rate of welding and fracturing. The former tends to increase while the latter decreases particle

size. If the aim is to decrease particle size, the particle surfaces can be modified by adding a

processing control agent (PCA) which impedes fresh surface contact necessary for cold

welding. This is because PCA are surface-active agents which are absorbed on the surfaces to

inhibit agglomeration [38]. Since PCAs such as stearic acid consists of carbon, hydrogen and

16

oxygen, they may become incorporated in the powder particles in the form of inclusions or

dispersoids and also result in the formation of carbides and oxides [38].

Besides obtaining a homogenized microstructure, the reduction of particle size can lead to a

decrease in melting point. This is due to excess energy stored in the grain boundaries as well

as the capillarity which causes a reduction in total enthalpy of melting [39]. Melting point

depression is as abovementioned in Section 1.2 an interest in microelectronic packaging.

Another advantage of milling is the possibility of the retardation of interfacial IMCs

formation [40].

Equipment Parameters and Sample Preparation

In making a homogeneous mixture of Fe and SAC305, Fe powder (Alfa Aesar,

LOT:10174355, 99% purity) was introduced into SAC305 (Qualitek, LOT:305Y13M06T3),

with addition of 3wt% PCA Stearic acid (Alfa Aesar, LOT: 10148625,98% purity) by

mechanical ball-milling. Stearic acid was added to prevent cold-welding of the powder. The

equipment used as shown in Fig. 2.1 was RETSCH Planetary Ball Mill PM400, with stainless

steel milling jars (diameter: 45.0mm; capacity: 50ml; depth: 30.0mm) and ball bearings

(mass: 0.5g/ball; diameter: 5.0mm).

Figure 2. 1 RETSCH Planetary Ball Mill PM400

17

A total of 10g of materials (Fe + SAC305) and 0.3g of Stearic Acid was used in each jar. The

compositions (mass and at%) of the samples made are as shown in Table 2.1. This table gives

the name adopted throughout the thesis and composition of Fe-SAC system used for

subsequent characterization and experimentation. The total milling time was 25 hours, with a

milling-rest interval of 30 minutes and rotation rate of 250 rpm.

Table 2. 1 Compositions of Fe-SAC305 samples made in mass and atomic weight

percentages

2.3 Sample Preparation for solder/substrate Fe-SAC/Cu joint

preparation

Choice of Cu Substrate

The choice of Cu substrate was the industrial Cu BGA as shown in Fig. 2.2(b). This is

because the formation of solder joint with Cu BGA has direct industrial application. The Cu

on BGA differs from usual Cu due to an additional layer of Organic Solderability

Preservative (OSP) on it. OSP contains the activated compound arylphenylimidazole (API)

such as benzimidazole. This additional layer, typically 0.2µm to 0.3µm [41], resists oxygen

penetration thereby minimizing Cu oxidation; prevents degradation of solderability [42] and

also acts as an adhesion promoter [43]. Therefore, as compared to a solder joint with a Cu

foil, the reaction rate is not as quick. This means that the IMC thickness would be smaller.

Sample SAC305 Fe S.A. SAC305 Fe Sn S.A.

BM SAC305 10.0 0.0 0.3 98.53 0.00 94.29 1.47

2wt%Fe-SAC 9.8 0.2 0.3 94.49 4.08 90.41 1.43

5wt%Fe-SAC 9.5 0.5 0.3 88.73 9.88 84.91 1.39

10wt%Fe-SAC 9.0 1.0 0.3 79.90 18.77 76.46 1.32

15wt%Fe-SAC 8.5 1.5 0.3 71.91 26.83 68.81 1.26

20wt%Fe-SAC 8.0 2.0 0.3 64.63 34.17 61.84 1.20

25wt%Fe-SAC 7.5 2.5 0.3 57.98 40.87 55.48 1.15

At%Mass /g

18

Equipment Parameters and Sample Preparation

To study solder joint, the flakes were placed on an industrial Cu BGA with the application of

a small layer of flux in between. The flux used was MULTICORE Loctite 425-01 RWF 37K.

As shown in Fig. 2.2(b), both the application of flux and flakes were done using a fine Cu

wire under Optical Microscope BX51 in reflection mode (Fig. 2.2(a)). The objective lens

series used was Long WD M Plan SemiApochromat Series (LMPLFLN, UIS 2).

Figure 2. 2 (a) Optical Microscope BX51; (b) Application of flux and solder under optical

microscope; (c) samples placed in porcelain boat.

The samples were then subjected to furnace reflow twice (2-times reflow). The methodology

for furnace reflow tries to imitate industrial reflow as much as possible. Table 2.2 shows the

stages of industrial reflow as compared to the furnace reflow used. The furnace used was

Carbolite Wire Wound Single Zone Tube Furnace. The samples were placed in a porcelain

boat (Fig. 2.2(c)) and into the furnace tube. A digital thermometer was attached to the furnace

tube to measure the temperature of the samples.

19

Figure 2. 3 Furnace reflow experimental set-up

Table 2. 2 Typical industrial and furnace reflow profiles

After the reflow, the samples were placed perpendicularly in an epoxy mold as preparation

for polishing as shown in Fig. 2.4(a). The epoxy consist of EpoFix resin and EpoFix hardener

in the ratio of 25:3 by weight. Then, the prepared samples were grinded, polished and

chemically etched to reveal the microstructural features. This was done with Struers

Tegramin-20 (Fig. 2.4(b)) with polishing parameters as shown in Table 2.3. The

microstructural features are observed with Optical Microscope BX51 in reflection mode.

20

Figure 2. 4 (a) Preparation of sample for polishing; (b) Struers Tegramin-20 Polishing

Machine

Table 2. 3 Polishing parameters

21

2.4 Characterization Techniques

2.4.1 Scanning Electron Microscopy and Energy-dispersive X-ray Spectroscopy

(SEM-EDX)

Scanning Electron Microscopy (SEM) is one of the more widely used techniques for surface

morphology. Primary electrons are thermionically emitted from a cathode filament towards

an anode (sample surface), with energies which can range from a few thousands to 50keV.

Fig. 2.5 shows the types of electron and photon signals produced from the electron-matter

interaction. The lowest portion of emitted energy distribution comes from secondary

electrons (SE), as it is generated from near sample surface depth (no larger than several

angstroms). Hence, a sloping surface (Fig. 2.5(c)) produce a larger secondary electron yield

due to the larger portion of interaction volume projected on the emission region [44].

Production of SE can be explained with two mechanisms. Firstly, the primary electrons are

scattered by the conduction or valance band electrons of the sample. Secondly, SE can be

emitted from the inner electronic shell of the sample which is usually followed by the release

of Auger electrons or x-rays. The former is produced when the ionized atoms (after emitting

the SE) de-excites and emit another bound electron. The latter is produced when an x-ray

photon is produced instead of an Auger electron.

At the higher portion of emitted energy distribution are energy levels from backscattered

electron (BSE). BSE are due to elastic interaction of the primary electrons with the sample

atomic nuclei. Hence, they have a deflection angle of larger than 90° (backscattered). Since

the mass of the nucleus of the sample is much larger than that of the primary electron, the

energy loss is negligible (elastic). Therefore, it results in a much larger escape depth as

compared to SE; giving information from deeper areas of the sample.

SEM can be compared to a large X-ray vacuum tube used in conventional X-ray diffraction

systems. As abovementioned, electrons are thermionically accelerated to high energies when

they impinge on the target. This causes x-rays to be emitted due to two distinct mechanisms.

The first mechanism is similar to BSE, but the deceleration of the primary electron now

22

imparts kinetic energy to the target atoms in the form of electromagnetic radiation, giving a

continuous spectrum of electromagnetic waves.

Figure 2. 5 (a) Electrons and photon signals emanating from volume during electron-beam

impingement. (b) energy spectrum of electrons emitted. (c) effect of surface topography on

electro emission. [44]

The second mechanism produces x-rays characteristic of the atoms in the irradiated area. A

primary electron knocks out an inner shell electron of the target atom, allowing electrons

from higher energy states to de-excite and fill its vacancy as depicted in Fig. 2.6. This

simultaneously emits x-rays of specific energies which correspond to the energy between the

higher and lower energy state of the target atom, an alternative form from Auger electron.

Figure 2. 6 Schematic diagram of x-ray emission

Table 2.4 gives the characteristic x-rays for elements of interest. C and O are included due to

Stearic Acid. The imaging would be carried out with an electron accelerating voltage of 15

kV as it is sufficient for the characteristic x-ray energies in Table 2.4 to be produced.

23

Equipment Parameters and Sample Preparation

The equipment used was JEOL Field Emission Scanning Electron Microscope JSM-6700F

with software JEOL PC- SEM 6700. Secondary electron detection was used. For EDX

analysis, x-rays are detected using the equipment Oxford Inca Energy 300 SEM with

software Oxford Instruments AZtecEnergy. The emission current was set to 20 µA and the

working distance was approximately 15mm.

SEM-EDX was used in the study of ball-milled flake sample. BM SAC305 and 5

compositions of Fe-SAC system (5-25wt%) flakes were used. The flakes are placed on the

specimen stage using adhesive carbon tape. Table 2.5 gives the weight and atomic

percentages of Fe and Sn of the Fe-SAC system samples.

Figure 2. 7 JEOL Field Emission Scanning Electron Microscope JSM-6700F

Element Characteristic X-ray Energy / keV

C Kα1 = 0.277

O Kα1 = 0.525

Fe Kα1 = 6.403

Cu Lα1 = 0.930

Ag Lα1 = 2.984

Sn Lα1 = 3.444

Table 2. 4 Values of Characteristic X-Ray Energies [45]

24

Table 2. 5 Values of weight and atomic percentages of Fe and Sn for different compositions

of Fe-SAC305

2.4.2 Vibrating Sample Magnetometer (VSM)

When a material is placed and vibrated in a magnetic field, a sensing coil can detect the

induced electromotive force (emf) of the sample, which is proportional to its magnetization.

A hysteresis loop – the relation between the measured magnetization B and the corresponding

applied field H can be generated. This hysteresis loop can be analysed to obtain magnetic

characterizations such as hysteresis, saturation magnetization, retentivity and coercivity.

With reference to Fig. 2.8, the dotted line shows the initial increase in magnetization M as the

applied field strength H increases. The magnetic domains in the ferromagnetic material align

with the direction of the applied field. At ‘a’, saturation magnetization is reached. Almost all

the magnetic domains are aligned such that an increase in H does not produce significant

increase in M. Following that, as H is decreased to 0, M reaches ‘b’ where there is still M

remaining even though H is 0. This is the point of retentivity – a measure of residual

magnetisation when the external field is removed. Some of the magnetic domains remain

aligned. Next, as the direction of H is reversed, M reaches point ‘c’ with M=0. This is the

point of coercivity – a measure of the amount of applied field in the reversed direction to

return M to 0. As H continues to increase in the opposite direction, M reaches saturation at

point ‘d’. This is similar to point ‘a’. When H is subsequently increased from point ‘d’ to ‘a’,

it will take a path going through points ‘e’ and ‘f’ which are due to similar considerations as

points ‘b’ and ‘c’ respectively. Finally, the area enclosed by the hysteresis loop is called the

hysteresis loss. It is the loss of energy for each cycle.

25

Figure 2. 8 Typical Magnetic hysteresis loop of a ferromagnetic material [46]

The hysteresis loss in a cycle can be calculated with:

Ehys = ∮ H dB = μ0 [ ∮ H dH + ∮ H dM ] = μ0 ∮ H dM (2.1)

The expansion is obtained from the relation B = μ(H + M) which gives dB = μ0 (dH + dM).

Equipment Parameters and Sample Preparation

The equipment used was Lake Shore VSM Model 7407. The samples are prepared by

pelletizing the flake samples into 10mm pellets with a thickness of approximately 0.8mm as

shown in Fig. 2.9. The mass of the pellets are shown in Table 2.6(a). Two scans are done,

with parameters shown in Table 2.6(b) and a time per step of 3s. Wide scan data was used for

the analysis of retentivity, coercivity and saturation magnetization. Narrow scan data was

used for the analysis of hysteresis loss.

Table 2. 6 (a) Masses of pellet samples used; (b) VSM applied magnetic field range

Figure 2. 9 Pellet sample

26

2.4.3 Differential Scanning Calorimetry (DSC)

DSC was conducted for thermal analysis. Calorimetry is the study of thermal energy transfer

during physical and chemical processes. Thereby, a calorimeter is the device used to measure

the thermal energy transferred. DSC is a commonly used thermal analysis technique. It

provides information about thermal changes without a change in sample mass [47]. Fig. 2.10

shows the DSC curve for BM SAC305. During an endothermic (exothermic) event, sample

absorbs (releases) energy. This results in additional (less) energy being supplied to it. More

(less) energy is recorded as a function. The feature highlighted in Fig. 2.10 is that of an

endothermic event. The integrated peak area (shaded in red) is the latent heat required for the

event to occur. Particularly, it corresponds to the latent heat of fusion for a solid-liquid phase

transition. The onset and peak temperature of the event can also be found. The onset

temperature is the intersection between the tangent with the horizontal as marked by the

intersection between the black dotted lines. The peak temperature is the maximum

temperature recorded.

Figure 2. 10 Typical DSC profile for BM SAC305

Equipment Parameters and Sample Preparation

The equipment used was Mettler Toledo DSC1 STARe coupled with the analysis software

Mettler Star SW 9.10. The temperature range was from room temperature to 325°C with a

heating rate of 10°C/minute. A single heat calibration was performed using Indium by

27

comparing the known melting onset temperature and the latent heat of fusion as shown in

Table 2.7.

Experimental

Value

Reference Value

[48,49]

Percentage

Discrepancy / %

Onset Temperature /

°C 156.6 156.6 -

Specific Latent Heat

of Fusion / Jg-1

23.7 28.5 16.8

Table 2. 7 Experimental and reference values for onset temperature and specific latent heat

of fusion for Indium

The samples used were 5 different masses of SAC305 and BM SAC305, 6 different

compositions of Fe-SAC system and Stearic Acid (mass = 10.7 mg). The sample holder used

was Al pan of diameter 0.5cm. The various masses of the samples used are shown in Table

2.8 below. Each sample was heated thrice.

Table 2. 8 Various masses of (a) SAC305 and BM SAC305, (b) different compositions of Fe-

SAC305.

28

Chapter 3

Impact of Fe on SAC305

3.1 Solder Composition, and Fe Content and Distribution: SEM-EDX

This chapter examines the incorporation of Fe into SAC305 by the process of ball-milling.

The Fe-SAC305 system materials synthesized were studied using SEM-EDX, VSM and

DSC.

Chemical Composition

Fig. 3.1 shows a sample, 25wt%Fe-SAC, of a SEM micrograph with EDX map spectrum and

corresponding EDX elemental maps of Sn, Fe, C, Cu, Ag and O. The mapping spectrum data

is summarised in Fig. 3.2 for all images taken. It is observed that besides BM SAC305, the

at% of at least one element is inconsistent in all three images (labelled I1, I2 and I3). All

images can be found in Appendix. It is concluded that three images are insufficient to obtain

a representation of the flakes for the purpose of chemical composition. Nevertheless, there

are some general observations. The at% values of C are large for all images. This may be

attributed to the carbon tape used to hold the flake samples and stearic acid. Table 3.1 shows

the at% of stearic in each sample which are all less than 2at%. Stearic acid is therefore not the

main contributor to the high at% of C. The at% of O is also rather high, ranging between

7.91at% to 16.58at%. Similarly, this suggest an alternative source of O besides stearic acid.

Even though an inert gas condition was used during ball-milling, oxidation of Fe might still

have occurred.

In order to explain composition changes, the experimental ratios of atomic wt% of Fe and Sn

is plotted in Fig. 3.3 as a function of the theoretical Fe wt%. Most of the plots except BM

SAC305 (plotted as theoretical 0wt% of Fe) and 15wt% deviate and lie far from one another.

Since it is concluded that three images are insufficient to obtain a representation, it also mean

that the elemental compositions of the flakes are inhomogeneous at 250x250µm spatial image

29

level. This could possibility be a result of the parameters used during ball-milling. From

Reddy et al. [50], the average crystallite size of Sn-rich phases decreases with increasing

milling time for SAC305 powder. Equilibrium was reached at about 25h of milling time.

However, as our samples contain a mixture of SAC305 and Fe, it is possible that more time is

required to reach the equilibrium state of equal rate of fracturing and cold welding. This is

due to the difference in ductility and density of SAC305 and Fe which may result in less

efficient mixing as compared to pure SAC305. There is a possibility that 25h milling time is

inadequate to reach the equilibrium state.

Figure 3. 1 SEM micrograph, EDX map spectrum and corresponding EDX elemental maps of

Sn, Fe, C, Cu, Ag and O for 25wt%Fe-SAC.

30

Figure 3. 2 At% of elements in all 3 images (I1, I2 and I3) obtained from map spectrum data.

Images can be found in Appendix.

Table 3. 1 At% of Fe, Sn and Stearic Acid (S.A.) present in different compositions of Fe-SAC

system.

Solder Fe Sn S.A.

BM SAC305 0.00 94.29 1.47

2wt%Fe-SAC 4.08 90.41 1.43

5wt%Fe-SAC 9.88 84.91 1.39

10wt%Fe-SAC 18.77 76.46 1.32

15wt%Fe-SAC 26.83 68.81 1.26

20wt%Fe-SAC 34.17 61.84 1.20

25wt%Fe-SAC 40.87 55.48 1.15

at%

31

Figure 3. 3 Fe/Sn atomic ratio as a function of theoretical Fe wt%

Microstructure

100x100µm spatial size images are used to study the microstructure of Fe-SAC305 system.

In particular, the distribution of Fe-rich particles amongst SAC30%. The SEM images

together with the EDX map spectrum and corresponding EDX elemental maps can be found

in Appendix. The 3 images taken for each composition is labelled I1, I2 and I3. The Fe EDX

chemical maps are collated in Fig. 3.4, it is observed that the Fe-rich area becomes larger as

the wt% of Fe increases. Using them to identify Fe-rich area in the corresponding SEM

micrograph, it is found that for the larger Fe-rich area, it is hard to differentiate whether the

area is a whole particle itself or an agglomerate of particulates. To illustrate, the red marks

are identified as Fe-rich regions for 5wt%Fe-SAC and 25wt%Fe-SAC in Fig 3.5. It is

observed that for 5wt%Fe-SAC, the identified areas are smaller and it does not contain

obvious contrast to suggest that it is an agglomeration. However, for 25wt%Fe-SAC, the

identified areas are much larger and the dissimilarities in each area suggest that it is an

agglomeration. Henceforth, Fe-rich particulates histogram analysis would not yield an

accurate result.

32

Figure 3. 4 All Fe EDX chemical maps from 5wt% to 25wt%.

Figure 3. 5 Fe-rich area marked with red marks for (a) 5wt%Fe-SAC and (b) 25wt%Fe-SAC.

SEM-EDX conclusion

There are two approaches to take with regards to the observations from the experimental

results obtained. Firstly, obtain a larger sample size in order to obtain a representative image.

This can be done either by increasing the number of images taken for each composition or by

increasing the spatial size of each image taken. Secondly, increase the milling time during

ball-milling. The size of the particles can be checked with SEM-EDX at regular interval in

order to determine the correlation between particle size as a function of milling time. Since

the second approach gives the possibility of better compositional homogeneity and decreases

particle size, the Fe particulates is expected to decrease. As such, they may be small enough

to allow for quantitative evaluation of Fe particle distribution within the solder. The

usefulness of Fe as magnetic micro-seeds may thus be quantified.

33

3.2 Magnetic properties: VSM

The retentivity, coercivity and saturation magnetization of Fe, BM Fe, SAC305 and BM

SAC305 are summarised in Table for wide scan range. The retentivity and coercivity of BM

Fe are higher than Fe while the saturation magnetization is lower. This may be due to the

presence of impurities and the difference in shape and sizes of the Fe particles after ball-

milling. Coercivity values are particle size-dependent [51,52]. As mentioned, oxidation of Fe

might have occurred. The formation of oxide layers decreases the saturation magnetization of

metallic particles, especially so for very fine particles due to large oxide-layer-to-volume

ratio [53,54].

The retentivity and saturation magnetization for both SAC305 and BM SAC305 are very low.

This is attributed to the paramagnetic nature of Sn in SAC305. Paramagnetic materials have

very weak magnetic properties at room temperature [55].

Table 3. 2 Values of Retentivity, Coercivity and Saturation Magnetization of Fe, BM Fe,

SAC305 and BM SAC305 for wide scan range of -15000Oe to +15000Oe.

Fig. 3.6 shows the hysteresis curve of BM SAC305 and Fe-SAC system for wide scan range.

As expected, saturation magnetization increases with Fe content. This is due to the

ferromagnetic nature of Fe. Table summarises the retentivity, coercivity and saturation

magnetization values. Coercivity values for Fe-SAC system are all lower than that of BM

SAC305 with no observable trend. The retentivity generally increases with Fe addition, with

the exception of 5wt%Fe-SAC value being slightly higher than 10wt%Fe-SAC.

The saturation magnetization is plotted as a function of the mass of Fe present in the samples

in Fig. 3.7 (blue plots). Assuming that the magnetization of Fe-SAC system is contributed by

Fe and SAC305 individually – that there is no Fe and SAC305 interaction dependence, the

‘expected’ values for Fe-SAC system is plotted in red. This calculation is based on the

Sample Retentivity / emug-1 Coercivity / Oe Saturation Magnetization / emug

-1

Fe 23.30 9.3 211.1

BM Fe 34.62 13.9 197.7

SAC305 0.00 30.4 0.0

BM SAC305 0.45 47.0 2.8

34

saturation magnetization values of BM Fe and BM SAC305 which can be found in Appendix.

It is observed that both plots lie close to one another. It suggests that there is no Fe-Sn

interaction or reaction that might have resulted in a change in saturation

magnetization.

Figure 3. 6 Hysteresis curves for BM SAC305 and 6 compositions of Fe-SAC system in wide

scan range of -15000Oe to +15000Oe.

Table 3. 3 Values of Retentivity, Coercivity and Saturation Magnetization of BM SAC305 and

6 compositions of Fe-SAC system.

Sample Retentivity / emug-1 Coercivity / Oe Saturation Magnetization / emug

-1

BM SAC305 0.45 47.0 2.8

2wt%Fe-SAC 0.10 10.8 4.0

5wt%Fe-SAC 0.83 21.4 13.3

10wt%Fe-SAC 0.74 11.0 21.8

15wt%Fe-SAC 1.17 12.6 33.0

20wt%Fe-SAC 1.63 12.2 43.9

25wt%Fe-SAC 2.00 12.7 55.2

35

Figure 3. 7 Graphs of experimental and expected Saturation Magnetization as a function of

mass of Fe present in sample.

Fig. 3.8 shows the hysteresis curves of BM SAC305 and Fe-SAC system in the narrow scan

range - the range in which hysteresis loss behaviour occurs. Table shows the hysteresis loss

based on the hysteresis loops. If the loss of energy is harnessed in magnetic induction

heating, the magnetic induction heating time can be calculated. This is further discussed in

section 4.2.

36

Figure 3. 8 Hysteresis curves for BM SAC305 and 6 compositions of Fe-SAC system in

narrow scan range of -100Oe to +100Oe.

Figure 3. 9 Hysteresis loss of BM SAC305 and Fe-SAC305 system.

37

3.3 Thermal Properties: DSC

General Curve for First Heating

Fig. 3.10 shows the DSC curves for ball-milled samples. Three features in regions (i), (ii) and

(iii) were observed.

The peaks in region (i), occurring between 69-70°C coincide with the melting temperature of

Stearic Acid. The DSC curves of Stearic Acid, which can be found in Appendix, show peaks

at the same temperature.

The slight exothermic areas in region (ii) are present in all ball-milled samples. It is

concluded that it may be due to an interaction between stearic acid and SAC305.

The peaks in region (iii) occur at about 220°C. The literature eutectic melting range for

SAC305 is between 217°C to 219°C [56-58]. The melting temperature of Sn, Ag and Cu are

232°C, 962°C and 1085°C respectively [59]. Hence, only Sn falls within the conventional

solder melting temperature. Further, the peak integrals give a specific latent heat of fusion of

(58.9±0.6) J/g. The literature specific latent heat of fusion of Sn is 59.1 J/g [36] which is

close to the experimental value. Therefore, the peaks at region (iii) are attributed to the phase

transition of pure Sn from solid to liquid.

Additional exothermic areas are observed in the samples after the addition of Fe as marked

by region (iv). Further, the endothermic area increases with higher wt% of Fe. This

exothermic event could be attributed to phase formation due to the interaction between Fe

and SAC305. More specifically, it is most likely to be Fe-Sn interaction since Sn makes up

96.5wt% of SAC305.

From Fe-Sn phase diagram (Appendix), for 2wt% to 25wt% of Fe, the possible phase

formation would be FeSn2. From 20wt% onwards, there is the possibility of FeSn formation

too. From the Gibbs Free energy of FeSn2 and FeSn [60], FeSn2 is more stable up to 234°C

while FeSn is more stable from 234°C onwards. The Gibbs Free energy equations are given

in Appendix.

38

Figure 3. 10 DSC Curves for ball-milled samples with Fe wt% from 0wt% to 25wt%. Curves

are vertically displaced by -15 mW progressively.

Melting Temperature

Based on the data from various masses of SAC305 and BM SAC305 which can be found in

Appendix, the average onset and peak melting temperatures are:

Table 3. 4 Average onset and peak melting temperatures of SAC305 and BM SAC305.

From Table, the onset temperature decreased by less than 1°C as Fe content increases from

0wt% to 25wt%. The peak temperature decreased slightly with ball-milling. A possible

explanation would be due to the purity of the samples as stearic acid was added during the

milling process. Nevertheless, the change in melting temperature is insignificant in

industrial process as it still falls within the solder melting temperature range. The negligible

change in temperatures after Fe addition to solder is also reported by Shnawah et al. (2013)

[11].

39

Table 3. 5 Onset and peak melting temperatures for 1st heating.

Latent Heat of Fusion

Based on the data of various masses of BM SAC305 for all three heats (Appendix), a graph

of latent heat of fusion against the mass of Sn in the samples is plotted in Fig. 3.11. Since all

plots lie close to the best-fit line with a specific latent heat of fusion of (58.9±0.6) J/g, it is

taken that the evaporation of sample material is negligible during the heats.

Figure 3. 11 Data plots of Latent Heat of Fusion against Mass of Sn for all heats of BM

SAC305 with y ≡ Latent Heat of Fusion in mJ and x ≡ Mass of Sn in mg

Fig. 14 shows 2nd

and 3rd

DSC heating which was conducted after the 1st has been cooled

down to room temperature. The peaks in region (i) are less pronounced - two broader peaks

instead of one sharp peak and peak shift to the right. Xu (2015) [5] had the same observation

but the cause is not clearly understood. The decomposition temperature for Stearic Acid is

40

150°C [61]. The emergence of the double-peaks could be due to the interaction between

decomposed Stearic Acid with SAC305. The exothermic areas in region (ii) and (iv) has

disappeared. This suggests the absence of reactions between Stearic Acid and SAC305, and

Fe and SAC305 respectively for 2nd

and 3rd

heats.

The peak areas (latent heat of fusion) in region (iii) have decreased from 1st heating but are

similar for both 2nd

and 3rd

heating. This may be due to the decrease in pure Sn content after

some Sn has reacted with Fe during the 1st heating. Fig. 3.12 shows the latent heat of fusion

of Fe-SAC samples against the mass of Sn present in the sample. The absolute values can be

found in Appendix. The black line is the best-fit line from BM SAC305 samples as in Fig.

3.11. If the Fe-SAC system behaves as pure Sn, the plots will like along the base line of

y=58.9x. 1st heating plots fluctuate around the base line. For 0wt% to 10wt%, 2

nd and 3

rd

heating plots fluctuate around base line although all of them are lower than 1st heating. For

15wt% to 25wt%, 2nd

and 3rd

heating plots deviate more from base line. This suggest that

with more Fe present in the sample, more Sn has reacted with it.

Figure 3. 12 Latent Heat of Fusion against Mass of Sn in sample

41

As above-mentioned, there is the possibility of FeSn and FeSn2 formations. A hypothesis can

be made to determine the amount of Sn that has reacted with the following assumptions:

1. All materials are in elemental form during 1st heating.

2. The peak integral represents phase transition of pure Sn from solid to liquid in solder

3. The specific latent heat of fusion used is 58.9 J/g.

4. The mass of Sn reacted (in mg) can thus be calculated from Latent Heat of Fusion

58.9.

5. From Table, since the number of moles of Sn is more than that of Fe for all Fe-SAC

samples, the minimum number of moles of Sn required can be calculated. This is

equivalent to the scenario if all Fe has reacted to form FeSn.

Table shows the calculated mass of Sn reacted using the above assumptions. Table shows the

calculated values for the minimum number of moles of Sn required.

Table 3. 6 Values of calculations leading to masses and moles of Sn reacted.

Table 3. 7 Values of calculations leading to minimum masses and moles of Sn if all Fe has

reacted.

With the values marked with the red squares and blue diamonds, Fig. 3.13 is plotted. It

summarises the minimum and the experimental moles of Sn. It is observed that the all the

42

experimental values are less than the minimum number of moles. Hence, it is concluded that

there is still Fe present in the samples after 3 heats. Since the 3 DSC heats roughly simulates

3 reflow processes, it suggests that there would still be Fe present in the solders after 3-times

reflow. This may be because when an Fe-Sn phase forms, it forms a protective layer around

Fe. This protects, or at least hinders, subsequent reaction between Fe and Sn. As mentioned

previously, Liu et al. (2009) [31] observed that pure Fe particles were surrounded by FeSn2

phase after it is subjected to a reflow time of 2 minutes.

Fe can hereafter still act as magnetic micro-seeds for magnetic induction heating

application.

Figure 3. 13 Minimum and experimental moles of Sn against moles of Fe in sample

43

Figure 3. 14 DSC curves for 1st, 2nd and 3rd heating for (a) BM SAC (b) 2wt%Fe-SAC (c)

5wt%Fe-SAC (d) 10wt%Fe-SAC (e) 15wt%Fe-SAC (f) 20wt%Fe-SAC (g) 25wt%Fe-SAC

44

Chapter 4

Fe-SAC Solder Applicability to Cu BGA Substrate

4.1 Impact of Fe on cross-section of solder/substrate

A preliminary experimentation was conducted to gauge the impact of Fe on solder joint

formation. The results presented here have used a small sample size due to the limited

timeline. Hence, there may be variations with literature values and caution must be exercised

at this juncture.

General Features

The cross-section of the as-received Cu pad in the BGA is shown in Fig. 4.1 Even with 100x

objective lens magnification, the Cu surface roughness is mild. Fig. 4.2 shows the cross-

sections of the 7 Cu pads for BM SAC305 and 25wt%Fe-SAC solders. The solder/Cu

interfaces are labelled as IF1 to 7 (IF ≡ Interface), and solder bulks of interest are labelled as

SB1 to 6 (SB ≡ Solder Bulk). At the surfaces of Cu pads without solder joint formation, the

Cu appear to have peeled. None of the 7 as-received Cu pads cross-section images indicate

such defect. Hence, this feature could be due to thermal impact from the reflow process.

Black spots are also observed. This is present in both solder samples and it appears at both

interfacial region and solder bulk region. These occurrences may be attributed to trapped flux

residues. The close-up images of the features can be found in Appendix.

Solder Joint Features

Not all Cu pads have solder joint formation, especially for 25wt%Fe-SAC. The Cu surface

with solder joints is rougher than before reflow. This indicates the diffusion and reaction

between Cu with the solder. Therefore, the interfacial IMC layer is attributed to a reaction

between the solder and Cu. This is further suggested by the lack of IMCs for reflowed solder

bulks which are not touching the Cu pads. Two solder bulks seem to have no contact with the

45

Cu pads but they contain IMCs. A possible reason is that these solder bulks are actually in

contact with Cu pad but the solder joints have been polished off. Thereby, these images

display the cross-sections of the area nearing to the end of the solder ball. The images of the

mentioned solder bulks can be found in Appendix.

It is observed that both BM SAC305 and 25wt%Fe-SAC have scallop-like IMCs between

solder and Cu pad. There is also a thin layer of IMC present in between the Cu pad and the

scallop-like features. This is observed from the colour contrast, where the thin layer is darker

in colour. From literature, these IMCs are commonly observed. The thicker layer is most

probably Cu6Sn while the thinner layer is Cu3Sn [31,32]. The thin layer of IMCs appears

thinner in 25wt%Fe-SAC solder joints as it is less obviously observed. However, due to the

limitation of optical microscope, the resolution is insufficient to give a quantitative view of

the difference in thickness between the IMC layers.

Rectangular features were also observed above some of the scallop-like IMCs. From

literature, it is most probably Ag3Sn [11]. Ag3Sn are usually platelets from literature. The

rectangular features are most probably the cross sections of the platelets. This feature is also

found in solder bulk area. As explained above, the solder joint might have been polished off.

There is still the possibility that this rectangular feature is a platelet that is obliquely

positioned. Henceforth, the IMC area which might have been attached to the interfacial

region could have been polished off.

There are irregular features omnipresent in the cross-sections for 25wt%-FeSAC which

appear darker compared to the usual solder colour. There is the possibility that these are Fe-

containing features. According to Liu et al. (2009) [31], when only 1wt% of Fe was added to

SAC305 solder, the volume percentage of Fe in Fe-rich area at the cross-sections of

solder/Cu interface can reach up to 19%. This shows that Fe-rich particles may have

agglomerated during the reflow process.

Discussion

SEM-EDX will be able to determine the phase of the various IMCs by comparing the

elemental atomic ratios. The elemental composition of other features such as the black spots

46

and dark irregular features can also be obtained to predict the possible identities.

Unfortunately, due to time constraint, this is not done and is planned as further studies.

Fe has a higher density (7.86gcm-3

) than SAC305 (7.38gcm-3

) of which gives the possibility

that it may sink to the solder joint interface during reflow. Comparing the contrasts of the

solder joints for BM SAC305 and 25t%Fe-SAC, there is no clear indication where Fe

particles have deposited. Further, from Shnawah et al. [33], large FeSn2 IMCs were also

present in solder bulk which appears darker in colour compared to Sn. This is not observed in

this preliminary data. However, the preparation of Fe-SAC solders were done with induction

furnace above 1000ºC for 40 minutes which gives plenty a time and energy for Fe-Sn

reactions. Similarly. the identification of Fe in solder joint can be done by SEM-EDX by

obtaining the Fe EDX chemical map. It’s impact on wettability can be gauge. If there is little

or no solder joint formation at areas with high Fe content, it suggests that Fe has prevented

solder joint formation. However, as above-mentioned, this is a preliminary data with limited

samples. The use of stencil with solder and flux paste in a professional reflow oven can be

done in the future for the analysis of the wettability of Fe-SAC305 system solder.

Figure 4. 1 Optical micrographs of as-received Cu pads with (a) 50x and (b) 100x objective

lens magnifications.

47

Figure 4. 2 Cross-section of 7 Cu pads on BGA for (a) B SAC305 and (b) 25wt%Fe-SAC

Figure 4. 3 100x objective lens on cross-sections of (a) BM SAC305 and (b) 25wt%Fe-SAC solder joints

48

IMC measurement

It is seen that there are only two solder joints which are larger than 50µm in length for

25wt%Fe-SAC. With the limited data, the IMC thickness for BM SAC305 and 25wt%Fe-

SAC is measured. The thicknesses of two solder joints, with x length of 70µm, are measured

for each solder composition using imageJ software. The measurement methodology can be

found in Appendix. BM SAC305 gives an average thickness of (3.0±0.3)µm while 25wt%Fe-

SAC gives (3.6±0.3)µm. This suggests that Fe addition results in an increase in IMC

thickness. This is consistent with Wang et al. (2009) [32] as thicker interfacial Cu6Sn5 layer

was observed for 0.03wt% Fe addition to Sn-2.5Ag-0.8Cu. The reported Cu6Sn5 thickness

for as-cast solder is approximately 2µm while that with Fe addition is approximately 6µm. On

the other hand, Liu et al. (2009) [31] observed a decrease in interfacial IMC layer as a

function of reflow time. From his graph, with a reflow time of 90s, the thickness for SAC305,

SAC305-0.5Fe and SAC305-1Fe all lie within 2um to 2.5µm. The conditions and

specifications of Cu substrate used was not mentioned. where the relative thickness is

SAC305-1Fe < SAC305 < SAC0.5Fe which does not agree with my result. However, note

that the relative thickness values obtained is from the linear regression of experimental values

with reflow times of 30s, 60s, 150s, 300s, 600s and 1800s. Further, the IMC thickness with

Fe addition below reflow time of 60s is thicker. For the experimental thicknesses at reflow

time of 150s, the relative thickness is SAC305-1Fe < SAC305 < SAC0.5Fe; with the

thickness of SAC305 is marginally larger than SAC-1Fe. Since only one iteration was done,

it may be possible that the difference is within the experimental data error. The thicknesses

with Fe addition with reflow time of 90s is hence questionable from his graph. Nevertheless,

the obtained result is inconclusive and more iterations need to be done to obtain a more

repeatable and reliable conclusion.

49

4.2 Induction Melting Applicability

In order to gauge the magnetic induction heating applicability of Fe-SAC system, the time

taken for the samples to heat up from room to melting temperature can be calculated. The

calculations are made with the following assumptions due to practical reasons:

1. The amount of thermal energy required consists of two processes – (a) heating up of

solder from room to melting temperature and (b) the eutectic melting of the solder.

2. The heat capacity used in the calculation for 1(a) remains constant throughout the

process.

3. Hysteresis loss is the sole contribution to the thermal energy required. Eddy current

and relaxation losses are not considered as they are dependent on the dimensions of

the solder and its particulates.

4. Hysteresis loss values used are the experimental values obtained from VSM

experimentation (Section 3.2) which occurred at room temperature.

5. There is no energy loss to the surroundings.

The heat capacity used for Fe-SAC solders is the proportional sum of the individual heat

capacities of the components i in the solder:

Csolder = ∑ Ci ∙ (wt%)i

i

(4.1)

where Ci is the specific heat capacity and (wt%)i is the weight percent of the component. The

values used is shown in Table 4.1.

Table 4. 1 Values of heat capacities used

The total thermal energy required Etot is:

Etot = Csolder ∙ (Tmp − Troom) + Elat (4.2)

50

where Tmp is the melting temperature of the solder, Troom is the room temperature (=25°C)

and Elat is the enthalpy of latent heat for eutectic melting of the solder.

The values used for Tmp and Elat are the experimental values obtained from DSC

experimentation (section 3.). The calculated values are shown in Table 4.2.

Table 4. 2 Values required for induction heating time calculation.

Ehys is the hysteresis loss per loop obtained from VSM result narrow scan - applied field

range of -100Oe to +100Oe.

The relationship between the total thermal energy required and the induction heating time t is:

where f is the frequency of alternating magnetic field applied.

The right side of equation (4.3) is obtained from W = Phys ∙ t = Ehys ∙ f ∙ t [5].

Fig. 4.4 shows the induction heating time as a function of the frequency of the alternating

magnetic field obtained from equation (4.3). It is observed that the induction heating time

decreased significantly after 5wt% of Fe addition. 15wt%, 20wt% and 25wt% of Fe addition

give similar induction heating times. The heating power and induction heating time values

with a frequency of 280kHz is shown in Table 4.3. For Fe content of 15wt% and above, the

induction heating time is lower than 10s.

Etot = Ehys ∙ f ∙ t (4.3)

51

Figure 4. 4 Induction heating time as a function of frequency of alternating magnetic field for

BM SAC305 and Fe-SAC system.

Table 4. 3 Heating power with alternating magnetic field frequency of 280kHz.

From Xu et al. (2015) [5], with a frequency of 300kHz, it takes about 60s and 15s for solder

composites with 6wt% and 10wt% of FeCo magnetic nano-particles respectively to reach

reflow temperature. In comparison, it takes 12s for 10wt% Fe-SAC with 300kHz. The exact

values of magnetic induction time can be found in Appendix . From Li et al. (2008) [63], total

reflow is able to take place within 20s for Sn3.5-Ag solder balls using eddy current induction

heating at frequency of 300kHz. With 25wt%Fe-SAC solder balls, the same can be attained

with a frequency of 60kHz.

Nonetheless, it should be noted that the calculations have been simplified due to the

assumptions made. In real application, the hysteresis loss is temperature-dependent.

52

Hysteresis loss of Fe decreases with increasing temperature [64]. Further, Fe-Sn IMCs may

form during the reflow process as suggested by DSC result which would give a smaller

amount of Fe to act as heat generating micro-seeds. Hence, the experimental induction

heating time is expected to be lower than that calculated using equation (4.3). Future

experimentation studies can be done to determine the capability of the above model in

estimating induction heating time.

53

Chapter 5

Conclusion and Future Work

5.1 Conclusion

In this thesis, Fe is incorporated into SAC305 by the process of ball-milling, ranging from

weight percentages of 2wt% to 25wt%. The Fe-SAC305 system materials synthesied were

characterised using SEM-EDX, VSM and DSC. A preliminary work on solder joint was also

conducted using furnace reflow. The following summarises the experimental results:

I. Inconsistent elemental composition with SEM-EDX imaging at 250x250µm spatial

size level. Fe particles seem to have agglomerated with increasing wt% of Fe. This is

not preferred as Fe particles should be dispersed homogeneously throughout solder to

act as effective heat-generating magnetic micro-seeds. Fe-SAC305 system with

milling time of 25hours may be insufficient to provide an adequately homogeneous

mixture.

II. Magnetic properties of Fe-SAC305 solders by VSM suggest that Fe magnetic

properties in induction heating is still applicable.

III. DSC suggested that there is negligible change in melting temperature which is

satisfactory for industrial purposes.

IV. After 3 DSC heats, there is still Fe present in the solders. Pure Fe particles could have

been protected by the formation of a layer of Fe-Sn phases. Hence, Fe can still act as

magnetic micro-seeds after 3 times reflow.

V. Solder joint experimentation had only one iteration. The data obtained is hence

limited. Further experimentation has to be conducted for repeatability and reliability.

Nevertheless, the preliminary data showed that solder joint formation seem to have

better success rate for BM SAC305 compared to 25wt%Fe-SAC. The thickness of

interfacial IMC layer increased from (3.0±0.3)µm to (3.6±0.3)µm with 25wt% Fe

addition.

VI. Using DSC and VSM result, the induction heating time is calculated. Fe addition of

more 5wt% onwards shows large decrease in induction heating time. An alternating

frequency of 300kHz can reduce the induction heating time to 12s. This mean that

54

ball-milled Fe-SAC305 system has the potential to be successful in magnetic

induction heating applicability.

5.2 Future Work

With the limited timeline, there are much future work to this project with

regards to the experimental results obtained:

I. Composition of Fe-SAC305 system particles were not homogeneous at the level of

250µmx250µm spatial size from the 3 images we have obtained. Experimentation on

Fe-SAC305 system particle size as a function of ball-milling can be conducted. The

minimum time required for all compositions of Fe-SAC305 system to reach the

equilibrium state in ball-milling can be obtained. This allows us to obtain a more

homogeneous distribution of Fe and SAC305 particles within the solder.

II. The Fe particles agglomerated with increasing Fe content. Too much agglomeration

would be undesirable for magnetic induction heating purposes. Since 3wt% of stearic

acid was added to mitigate cold-welding during ball-milling, it might have been

insufficient to be absorbed on the surfaces of Fe particles – particularly for higher Fe

content. Therefore, it gives the possibility of high amount of Fe agglomeration.

Experimentation on the Fe-SAC305 system particle size as a function of the wt% of

stearic acid can be conducted.

III. The high at% of C is not well-understood. SEM-EDX can be conducted for other Fe-

SAC305 system samples prepared for to determine

IV. The magnetic properties of the Fe-SAC305 system samples have been characterised.

An outlook could be to subject the samples to thermal impact which simulates the

reflow process during solder joint formation. The magnetic properties of the as-

reflowed samples can then be characterized again using VSM. Therefore, the gradual

change in magnetic properties of the solder can be analysed for the purpose of multi-

layer packaging.

V. DSC results gave the feature in region (iv) which is attributed to the interaction Fe

and Sn. It may have been due to Fe-Sn phase formation. DSC experimentation on Fe-

55

SAC305 system can be conducted within the region in order to determine the

temperature and enthalpy of reaction.

VI. Comparing the latent heat of fusion between different heats (DSC experimentation), it

is noted that the latent heat of fusion during the second heating is slightly less than the

third heating. More heats can be performed to discover whether there is a decreasing

trend. If that is the case, it suggests that Sn is still being consumed after the multiple

heats. This in turn suggests that more Fe may be used up and more Fe-Sn phases are

formed.

VII. SEM-EDX of the preliminary experimentation of solder joint work can be done in

order to determine the speculations from the Optical Microscopy work. Further, more

solder joint experimentation should be conducted for repeatability and reliability.

VIII. The induction heating time calculated is based on a simplified model. Fe-SAC305

system can be subjected to alternating magnetic field for induction heating so as to

determine the experimental induction heating time required. The capability of the

model in estimating induction heating time can thus be determined.

56

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62

Chapter 6

Appendices

6.1 Appendix I: Phase Diagram and Gibbs Free Energy of Fe-Sn system

Fe-Sn phase diagram [65]

Figure 6. 1 Fe-Sn phase diagram

Equations and Graphs for Gibbs Free Energy of FeSn and FeSn2

FeSn: -23013 + 19.702T

FeSn2: -27624 + 28.803T [60]

63

Figure 6. 2 Graph of Gibbs Free Energy of FeSn and FeSn2 in temperature range of 25°C to

325°C

6.2 Appendix II: SEM-EDX

Methodology

A preliminary experimentation was done with 5wt% and 25wt%Fe-SAC as follows:

Table 6. 1 SEM-EDX experimental parameters for preliminary experimentation

The SEM micrographs are shown in Fig. 6(a), (b) and (c) are of 100x100µm sizes and (d), (e)

and (f) are the corresponding 10x10µm images marked by the red boxes. 10x10µm spatial

size seems to be insufficient in capturing the microstructure of Fe-rich area as the resolution

of the features were unclear. This means that it is too small for the purpose of attaining

microstructure information. 100x100µm spatial size are able to capture at least a full flake

except for Fig. 6(c). Henceforth, the finalised SEM-EDX methodology for composition and

64

microstructure analyses is summarised in Table 6.2. The intent for 250x250µm spatial size is

that it may provide a more representative image compared to 100x100µm spatial size. The

intent of 100x100µm spatial size is to study the microstructure of Fe-rich area.

Figure 6. 3 SEM micrographs for 5wt%Fe-SAC flakes. (a), (b) and (c) are 100x100µm sizes

while (d), (e) and (f) are the corresponding 10x10µm images marked by the red boxes.

Figure 6. 4 SEM micrographs for 25wt%Fe-SAC flakes. (a), (b) and (c) are 100x100µm sizes

while (d), (e) and (f) are the corresponding 10x10µm images marked by the red boxes.

Table 6. 2 Methodology Parameters used for SEM-EDX Experimentation

SEM micrographs, EDX Map Spectrums and EDX Chemical Maps

Each composition consist of 3 250x250µm and 3 100µmx100µm images each. They are

labelled as I1 to 3 (I ≡ Image).

65

BM SAC305:

Figure 6. 5 250x250µm I1

Figure 6. 6 250x250µm I2

66

Figure 6. 7 250x250µm I3

Figure 6. 8 100x100µm I1

67

Figure 6. 9 100x100µm I2

Figure 6. 10 100x100µm I3

68

5wt%Fe-SAC:

Figure 6. 11 250x250µm I1

Figure 6. 12 250x250µm I2

69

Figure 6. 13 250x250µm I3

Figure 6. 14 100x100µm I1

70

Figure 6. 15100x100µm I2

Figure 6. 16 100x100µm I3

71

10wt%Fe-SAC:

Figure 6. 17 250x250µm I1

Figure 6. 18 250x250µm I2

72

Figure 6. 19 250x250µm I3

Figure 6. 20 100x100µm I1

73

Figure 6. 21 100x100µm I2

Figure 6. 22 100x100µm I3

74

15wt%Fe-SAC:

Figure 6. 23 250x250µm I1

Figure 6. 24 250x250µm I2

75

Figure 6. 25 250x250µm I3

Figure 6. 26 100x100µm I1

76

Figure 6. 27 100x100µm I2

Figure 6. 28 100x100µm I3

77

20wt%Fe-SAC:

Figure 6. 29 250x250µm I1

Figure 6. 30 250x250µm I2

78

Figure 6. 31 250x250µm I3

Figure 6. 32 100x100µm I1

79

Figure 6. 33 100x100µm I2

Figure 6. 34 100x100µm I3

80

25wt%Fe-SAC:

Figure 6. 35 250x250µm I1

Figure 6. 36 250x250µm I2

81

Figure 6. 37 250x250µm I3

Figure 6. 38 100x100µm I1

82

Figure 6. 39 100x100µm I2

Figure 6. 40 100x100µm I3

83

6.3 Appendix III: Calculated Expected Values of Saturation

Magnetization for Fe-SAC305 system

Table 6. 3Experimental and expected values of saturation magnetization from VSM result

6.4 Appendix IV: Miscellaneous DSC Experimental Results

Experimental data for Stearic Acid

Figure 6. 41 DSC curves of Stearic Acid 1st, 2nd and 3rd heating.

84

Onset and Peak melting temperatures of various samples

Table 6. 4 Temperatures for various masses of SAC305

Table 6. 5 Temperatures for various masses of BM SAC305

Table 6. 6 Temperatures for Fe-SAC305 system

DSC curves and Latent Heat of Fusion for various masses of BM SAC305

The mass of Sn is calculated as follows:

Mass of Sn = 0.965 ∗ (Mass of Sample − Mass of Stearic Acid)

85

.

Figure 6. 42 (a), (c) and (e) DSC curves of 1st, 2nd and 3rd heating respectively for 5

different sample masses. (b), (d) and (f) plots of Latent Heat of Fusion against Mass of Sn for

1st, 2nd and 3rd heating respectively. Curves are vertically displaced by -15 mW p

86

Absolute experimental values of latent heat of fusion

Table 6. 7 BM SAC305 for 1st, 2nd and 3rd heats

Table 6. 8 Fe-SAC305 system for 1st, 2nd and 3rd heats

6.5 Appendix V: Solder Joint Features

Zoom in on solder joint features

Figure 6. 43 Cu pads peeling at area without solder joints

87

Figure 6. 44 Black spots in interfacial regions and solder bulk

Figure 6. 45 Close up of 7 Cu pads for BM-SAC305

88

Figure 6. 46 Close up of 7 Cu pads for 25wt%Fe-SAC

Figure 6. 47 Some solder bulks which appear detached from Cu pads. Thin red arrows show

solder bulks with no IMCs observed. Thick dotted blue arrows show solder bulks with IMCs

observed.

89

Figure 6. 48 Rectangular features which may be Ag3Sn platelets

6.6 Appendix VI: IMC Measurement Methodology

Two IMC measurement methodologies were commonly harnessed: (a) Average Height and

(b) Area/Length methodologies with equations as follows:

(a) IMC thickness =1

N∑ HN

1 [66, 67]

(b) IMC thickness =IMC area

length in x direction [68, 69]

Figure 6. 49 IMC thickness measurement methodology (a) average height and (b)

area/length methodologies

From Lin. (2014), both methodologies give similar results. As shown in Fig. 6.50, the two

methodologies were used in IMC measurement and the results are compared in Table 6.9. 3

length in x directions were used.

90

Figure 6. 50 Both methodologies used to measure IMC thickness with x length of 30µm,

70µm and 100µm.

Table 6. 9 Results from both methodology

Methodology (b) is always larger than (a) as some IMCs grow sloping to one side.

Methodology (b) was harnessed in the measurement of the IMC thickness in this thesis with

length of 70µm due to limited x length of interfacial IMCs.

6.7 Appendix VII: Calculated Values of Induction Heating Time

Table 6. 10 Induction heating time between frequency range of 10kHz to 600kHz