i

Investigation of Mixed Solder Assemblies &

Novel Lead-Free Solder Alloys

by

Rishi Kaila

A thesis submitted in conformity with the requirements for the degree of Master

of Applied Science

Graduate Department of Materials Science and Engineering

University of Toronto

Copyright by Rishi Kaila, 2011

ii

Investigation of Mixed Solder Assemblies and Novel Lead-Free

Solder Alloys

By: Rishi Kaila

Master of Applied Science

Graduate Department of Materials Science and Engineering

University of Toronto

2011

ABSTRACT

Due to the introduction of Restriction of Hazardous Substances (RoHS) directive Pb

containing solders have been banned from the electronics industry and a reliable replacement

for the Sn-Pb solder is being sought for by industry around the globe. Medical and Defense

industries are currently exempt from the directive and use Sn-Pb solder in their manufacturing

process. The switch to lead-free has led component manufacturers to use different lead-free

solders, thus causing mixed solder joints of lead-free components with Sn-Pb paste. In this

study, mixed assembly microstructures and mechanical properties were examined.

Furthermore, six novel lead-free solders were prepared using SAC105 solder doped with

elements: Ti, Ni, Mn, La, Ce and Y. The solidification microstructures, fracture behavior and

wetting properties of these solders were evaluated to find a suitable replacement for SAC105

solder.

iii

To my parents, Madhu and Ved Kaila for their endless love and support and teaching me the value of education

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ACKNOWLEDGEMENTS

I would like to thank several people to whom I am indebted to for providing me their foresight

and wisdom to achieve this endeavor. Firstly, I would like to extend my heartfelt thank you to

my supervisor Professor Doug. Perovic for his advice, support and encouragement throughout

this project.

I would like to extend my thanks to all research group members for their moral support and

help. I would like to express my gratitude to the Celestica Metallurgical Lab for allowing me

to use their equipment and expertise. In particular, I would like to thank Marianne Romansky,

Polina Snugovsky, Bob Buick, Jason Bragg, Andrea Rawana and Zohreh Bagheri. I would

like to extend a special thanks to Dr. Leonid Snugovsky who helped me at every stage of my

project. Appreciation is also expressed to the staff of the MSE Department for the technical

and administrative support during the course of the project.

I am also very grateful to the financial support I received from University of Toronto

Fellowship.

Finally, I would like to thank my friends and family, and especially my brother Ashish Kaila

and sister-in-law Elena Zakalinskaya Kaila for their enormous and continued support and

encouragement.

v

TABLE OF CONTENTS

ABSTRACT ii

ACKNOWLEDGEMENTS iv

TABLE OF CONTENTS v

LIST OF TABLES viii

LIST OF FIGURES ix

CHAPTER 1: INTRODUCTION 1

1.1 Introduction and Objectives 1

References 3

CHAPTER 2: LITERATURE SURVEY 4

2.1 RoHS Legislation 5

2.1.1 Penalties for Violating RoHS 7

2.1.2 Other LF Policies 8

2.2 Implications of Implementing LF Solder 8

2.2.1 Higher Melting Temperature 9

2.2.2 Mixed Components on Assemblies 11

2.2.3 Tin Whiskering 12

2.3 Wetting Behavior 14

2.4 Previous studies 15

References 26

CHAPTER 3: EXPERIMENTAL METHODS 30

PART I - MIXED ASSEMBLY

3.1 Solder Ball and Paste combinations 30

3.2 Joint Preparation 29

3.3 Microstructure Observation 30

vi

3.4 Microhardness Testing 30

PART II – NOVEL LEAD FREE SOLDERS

3.5 Solder Preparation 31

3.6 Microstructure observation 31

3.7 Wetting Balance Procedure 32

3.8 Solder Joints with Copper 34

3.9 Tensile Testing 35

3.10 Surface Observation 36

CHAPTER 4: RESULTS & DISCUSSION 37

PART I - MIXED ASSEMBLY

4.1 Solidification Microstructure 37

4.2 Effect of Reflow Temperature and Solder Paste 38

4.2.1 SAC solder ball with Sn-Pb Paste 38

4.2.2 SAC solder ball with SAC Paste 44

4.3 Changes in solder composition 47

4.3.1 SAC Solder ball with SAC paste 47

4.3.2 SAC Solder ball with SnPb paste 51

4.4 Microhardness Properties 54

PART II – NOVEL LEAD FREE SOLDERS

4.5 Solidification Microstructure 57

4.6 Melting Behavior 63

4.7 Wetting Behavior 66

4.7.1 As-received (containing 1wt% additives) 66

4.7.2 SAC105 + 0.1wt% Additives 67

4.7.3 Other wettability parameters 68

4.8 Joint Mechanical Properties 72

4.9 Fracture Mode Examination 73

vii

References 78

CHAPTER 5: CONCLUSIONS & FUTURE WORK 81

5.1 Conclusions 81

5.2 Future Work 82

viii

LIST OF TABLES

Table 3-1 Solder ball and paste combinations 29

Table 4-1 Level of mixing obtained in mixed assembly solidification 45

Table 4-2 Sample calculation of calculating Joint Strength 71

Table 4-3 Avg. calculated Joint Strengths of solders 71

ix

LIST OF FIGURES

Figure 2-1 Temperature Profile on Component Packaging Surface for

Sn-Pb vs. LF Solder 10

Figure 2-2 Scanning Electron Micrograph of Lifted Fillet 11

Figure 2-3 Tin Whiskers emerging from the surface 13

Figure 2-4 Surface Tensions acting on the liquid droplet wetting a solid substrate 14

Figure 2-5 Difference between incomplete and complete mixing in SAC105 16

Figure 2-6 Crack propagation in SAC305 vs SAC405 solders when soldered

using SnPb paste 17

Figure 2-7 As cast microstructures of SAC solder containing micro additives 18

Figure 2-8 Fracture surfaces of SAC solders containing micro additives 19

Figure 2-9 Relationship between contact angles, spread area and RE content 20

Figure 2-10 Optical and SEM micrographs of as processed ingots containing

0.1wt%-0.5wt% La 21

Figure 2-11 Shear stress and strain changes with differences in La content 22

Figure 2-12 Fracture surfaces of lap shear samples for (a) Sn-3.9Ag-0.7Cu,

(b) Sn-3.9Ag-0.7Cu-0.1La, and (c) Sn-3.9Ag-0.7Cu-0.5La. 23

Figure 3-1 Sn-Ag-Cu phase diagram showing the near eutectic compositions

and corresponding melting temperatures 28

Figure 3-2 Schematic of mixed solder joint assembly 30

Figure 3-3 Wetting Balance used to evaluate solder wettability 33

Figure 3-4 Metal assembly holding the copper strips and sheet in place for soldering 34

Figure 3-5 SHIMADZU Model AGS-J Tensile Testing Machine 35

Figure 4-1 Solidification microstructure of SAC405 showing the βSn

dendrites surrounded by eutectic structure 37

Figure 4-2 Incomplete melting of SAC105 solder ball takes place when peak

reflow temperature is 210oC 38

Figure 4-3 SEM micrograph of solder joint made soldering SAC105 with

eutectic SnPb paste reflowed at 210oC 39

Figure 4-4 Coarse Sn-rich bands formed at the interface between mixed

and non-mixed regions of the solder joint 41

Figure 4-5 SAC solder joints soldered with eutectic SnPb paste and reflowed

using different peak temperatures and wt% paste.

(a) 210oC, 20wt% paste (b) 210oC, 29wt% paste

(c) 220oC, 20wt% paste and (d) 220oC, 29wt% paste 42

Figure 4-6 SAC105 solder with SAC305 paste soldered at (a) 235oC and (b) 245

oC 44

Figure 4-7 SEM micrographs of SAC105 soldered with SAC305 and EDX

results of regions (a) bulk solder, (b) away from the interface,

and (c) adjacent to the interface 48

Figure 4-8 EDX results of regions (a) bulk solder, (b) away from the interface

, and (c) adjacent to the interface when SAC105 is soldered using

SAC405 paste 49

x

Figure 4-9 SEM micrographs of SACMO1 soldered with SnPb and EDX

results of regions (a) bulk solder, (b) away from the interface,

and (c) adjacent to the interface 51

Figure 4-10 Pb contamination leads to a shift in ternary eutectic which is

different from the normal SAC ternary 52

Figure 4-11 Optical graph showing the distinct regions of a non-mixed joint 53

Figure 4-12 Differences in Vickers Microhardness of the 3 regions of a

non-mixed joint 54

Figure 4-13 SEM micrograph of Region 2 shows the coarse, elongated

intermetallic compounds formed due to interdiffusion of Sn

and Cu towards the joint interface 54

Figure 4-14 Lanthanum (Top) and Yttrium (Bottom) containing intermetallic

compounds formed after solidification 57

Figure 4-15 Nickel (Top) and Titanium (Bottom) containing intermetallic

compounds formed after solidification 58

Figure 4-16 Manganese (Top) and Cerium (Bottom) containing intermetallic

compounds formed after solidification 59

Figure 4-17 (Top) Fine Ti containing intermetallic compounds are formed

and uniformely dispersed through the bulk of the solder.

(Bottom) Scale shows the size of particles to be < 2μm 61

Figure 4-18 DSC curves for SAC alloys containing (Top) Cerium and

(Bottom) Lanthanum 62

Figure 4-19 DSC curves for SAC alloys containing (Top) Manganese

and (Bottom) Nickel 63

Figure 4-20 DSC curves for SAC alloys containing (Top) Titanium and

(Bottom) Yttrium 64

Figure 4-21 Wetting force as a function of time for solders containing

1wt% additives, eutectic SnPb and SAC105 65

Figure 4-22 Wetting force as a function of time for solders containing

0.1wt% additives, eutectic SnPb and SAC105 66

Figure 4-23 Wetting force after 2 seconds as a function of wt% additive 68

Figure 4-24 Time taken to achieve 2/3 of maximum wetting force as

a function of wt% additive 69

Figure 4-25 (Left to Right) Wettability improves as the solder temperature

is increased from 250oC to 265oC and Cu coupons are preheated 70

Figure 4-26 Average Tensile Strength of solders containing different additives 72

Figure 4-27 Different modes of fractures: (a) ductile and through solder,

(b) ductile and brittle mixed, and (c) brittle failure through

interface separation 73

Figure 4-28 (a) & (b): Microvoids and dimples observed in Ti and Ni

containing samples. (c) & (d): Intermetallic compounds present

at the fracture surface of Ti and Ni containing samples 74

Figure 4-29 (a) Microvoids and flat fracture surface found in La, Ce, Mn

containing samples. (b) & (c): Large macrovoids formed in

RE containing samples. (d) Brittle fractures in RE containing samples 75

1

CHAPTER 1: INTRODUCTION

1.1 Introduction and Objectives

As of July 1, 2006, the European Union’s Restriction of Hazardous Substances (RoHS)

directive went into effect. RoHS bans lead, cadmium, mercury, hexavalent chromium, and

brominated flame retardants from all electrical and electronic equipment that are

manufactured or imported into the European Unions (EU) after July 1, 2006. This new

directive poses a problem for all companies in the electronics industry that must change their

manufacturing processes in order to maintain their market share. In addition, other countries

already have similar directives in place or are planning to implement them in the future.

Although the directive was announced in 2000 [1], the electronics industry is still struggling

to determine the impact of converting all their products and processes from tin-lead (Sn-Pb)

solder to lead-free (LF) solder. Sn-Pb solder has been used successfully for many years and

the industry has a good knowledge of its reliability [2, 3]. With RoHS and similar directives

being developed, the industry must change to lead-free solder with limited long-term

knowledge and experience.

Many LF solders have been developed to replace the traditional SnPb solder. The most

common alloys that are currently in use are of the SAC family of alloys (Sn – Ag – Cu based

alloys) with up to 5wt % Ag and 1wt % Cu. Two major problems have surfaced from the use

of SAC alloys.

1. Suppliers of electronic components are producing RoHS compliant parts using only certain

LF solder alloys. These alloys may be mixed with a solder paste (solder + flux) of a different

composition when forming a solder joint. This results in a solder joint with a modified

microstructure and unknown properties. The problem is worsened because critical industries

(such as aerospace) that are currently exempt from RoHS are forced to solder LF components

with SnPb paste as more and more manufacturers are producing only LF components.

Therefore it is necessary to learn how to properly integrate LF components into various other

LF assembly processes and more importantly into SnPb assemblies.

2

2. The LF solders are more susceptible to failure under impact loading conditions. This can be

attributed to the higher strength and modulus of LF solders which prevents deformation in the

bulk solder region. Consequently, most of the stress is passed on to the solder-substrate

interface and the printed circuit board. The hard and brittle intermetallic compounds present at

this interface fail to withstand such impact forces and it is usually this interface that exhibits

brittle failure. There has been some progress in solving this problem by doping LF alloys with

metal and rare-earth (RE) elements such as Ni, Ti, La and Ce. While low level doping has

resulted in tougher alloys with better wetting properties, more research is required before

these quaternary are accepted in the mainstream assembly processes.

Therefore, the main objectives of this study are as follows:

To study the effects of mixing different types of solders, particularly SnPb and lead-free

solders on the solidification microstructure of the joints.

To identify microstructural features of mixed joints that may be detrimental to the joint

service life and to provide assembly parameters that would discourage such features.

To investigate the effects of doping small amount of additives on the microstructure,

wetting and mechanical properties of lead-free solders.

To relate the deformation behavior and failure mechanisms of these lead-free solders to

the microstructure by investigating the fracture surface of joints.

3

References

[1] L. J.Turbini,G. C. Munie, D. Bernier, J. Gamalski, and D. W. Bergman, “Examining

the Environmental Impact of Lead-Free Soldering Alternatives,” IEEE Transactions

on Electronics Packaging Manufacturing, vol. 24, no. 1, pp. 4-9, 2001.

[2] I. W. Clelland and R. A. Price, “Lead-Free Solder Study,” Electronic Business, pp. 1-

17, 2005. [Online document], Sept. 20, 2005, [cited June 20, 2006]

[3] S. C. Johnson, “RoHS and Non-Compliance: Expect to See Penalties,” Electronic

Business, [Online document], June 13, 2006, [cited June 20, 2006], Available: http://

www.reed-electronics.com/eb-mag/index.asp?layout=articlePrint&articleID=CA6342812

4

CHAPTER 2: LITERATURE SURVEY

The RoHS legislation banning the use of lead in all commercial electronics and electrical

equipment in the European Union (EU) has a major impact on the entire electronics industry.

In the following sections, the details of the RoHS directive are reviewed and their implications

to manufacturing companies are discussed. The electronics industry has devoted several years

of research to optimize the lead-soldering processes in their factories. Now they are forced to

comply with RoHS regulations or risk penalties of fines, imprisonment, and a loss of market

share. The implementation of LF solders introduces higher melting temperatures into their

soldering processes, requiring new equipment to handle the temperature changes, compatible

LF components able to withstand the new processing temperatures, and extensive testing on

the reliability of LF solder.

With the RoHS legislation recently coming into effect on July 1, 2006, there is limited

literature on the studies of LF solder joints undergoing drop tests and other mechanical tests

(thermal, vibration, tensile, bend) in comparison with Sn-Pb. Since multiple arrangements,

sizes, and applications exist for circuit boards, each study is different.

5

2.1 RoHS Legislation

RoHS stands for Restriction of Hazardous Substances and is a directive that was

implemented by the EU on July 1, 2006. The directive was first proposed in 2000 [1], and

limits the use of lead, hexavalent chromium, mercury, polybrominated biphenyls (PBB), and

polybrominated diphenyl ethers (PBDE) in electrical and electronic equipment either

produced inside the EU or imported. According to RoHS, electrical or electronic equipment

(EEE) is defined as:

Equipment which is dependent on electric currents or electromagnetic fields in order to work properly and

equipment for the generation, transfer and measurement of such currents and fields … and designed for use with

a voltage rating not exceeding 1,000 volts for alternating current and 1,500 volts for direct current [2].

The maximum allowable levels of these harmful substances are 0.1 wt % except for cadmium,

which is 0.01 wt % [2]. The driving force for limiting these substances is due to the

hazardous health and environmental effects associated with them [3, 4, 5]. The main concern

with lead is that when EEE becomes obsolete, many discarded boards are dumped into

landfills, where there is a risk for the lead to leach into the surrounding soil and groundwater

supply [3, 5]. The leached lead can spread to animals that are part of humans’ food supply

and local drinking water, while children are at risk from the lead contaminating soil and dust

when spending time outdoors. Studies conducted by the EU [11] indicate that lead can affect

the function of the central nervous system, as well as accumulate in bones and cause

deformities.

There are eight different categories of EEE that are affected by RoHS, which are listed below

[2]:

1. Large household appliances

2. Small household appliances

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3. IT and Telecommunication equipment

4. Consumer equipment

5. Lighting equipment

6. Electrical and electronics tools (with exception of large-scale stationary industrial tools)

7. Toys, leisure and sports equipment

8. Automatic dispensers

RoHS restrictions also include wires, cables, chassis plating, and packaging materials [6].

Some items containing lead are exempted from RoHS [2, 6]:

1. Lead in the glass of cathode ray tubes, electronic components, and fluorescent tubes

2. Lead as an alloying element in steel containing up to a maximum of 0.35 wt %

3. Lead as an alloying element in aluminium with a maximum of 0.4 wt %

4. Lead as an alloying element in a copper alloy containing up to 4 wt %

5. Lead in high melting temperature solders (Sn-Pb solders containing more than 85 wt %

lead)

6. Solders for servers, storage, and storage array systems

7. Solders for network infrastructure equipment, switching, signalling, transmission, and

network management for telecommunication

8. Large-scale stationary industrial tools

9. Lead in electronic ceramic parts (piezoelectronic devices)

7

Spare parts produced for repair, re-use, and upgrading of equipment put on the market before

the July 1, 2006 implementation date are included on the exemption list [7]. EEE used in

military, aerospace, automotive, and medical applications are also exempted since the

reliability of the equipment is critical during operation [3, 8, 9]. This exemption is expected

to last until at least December 2011 to provide companies with more time to improve their

manufacturing processes and documentation for switching to LF solder [3].

2.1.1 Penalties for Violating RoHS

Since RoHS is a directive and not law, it is up to the members of the EU, consisting of 27

countries, to enforce the policy and develop their own penalties [7,10]. The directive

indicates that the EEE manufacturers must produce technical documentation to prove that

their product is in compliance with RoHS, and the documentation must be retained for four

years from the date that the product is put on the market [10]. The documentation must show

that no banned substances were used during manufacturing, assembly, or production of the

product [8]. The penalties for companies failing to show due-diligence in following RoHS

include fines or imprisonment [6,11]. The product could also be banned from entering the

market. In the UK, the maximum fine for violating RoHS is £5000 [10], while Ireland has a

maximum fine of 15 million euros [11]. In Greece, a person found liable for the offense could

spend up to three years in prison [11]. Each new product for RoHS must be inspected by an

organization known as Community European [12].

8

2.1.2 Other LF Policies

RoHS is not the only policy regulating lead in EEE. There are similar policies currently

implemented or are in the process of being implemented that affect EEE worldwide. China

implemented the Regulation for Pollution Control of Electronics Products in March 2007,

which would ban the same substances as RoHS, but is stricter because it does not exclude or

exempt any equipment, such as military and medical [12]. Each product must be inspected

and certified by China Compulsory Certification (CCC) in one of 18 laboratories in China

before it can be sold. On Jan. 1, 2007, California prohibited the sale of covered electronic

devices (TVs, computers) if it violated RoHS, with the possibility of extending the prohibition

to the same product scope as RoHS [9]. Japan has had a law in place to ban lead since 2000,

called the Law for the Promotion of Effective Utilization of Resources [9].

2.2 Implications of Implementing LF Solder

The implementation of RoHS means that companies must find alternate solder alloys from the

conventional eutectic Sn-Pb (63Sn-37Pb) solder composition [3, 4, 13, 14]. LF solder

alternatives suggested by companies have higher concentrations of tin, making them more

difficult and expensive to use compared to Sn-Pb solder [4-6, 12, 13]. Sn-Pb solder has been

used for more than 50 years so companies are familiar with its properties and performance as

opposed to LF [3, 11]. Sn-Pb solder was chosen by the electronics industry due to its

relatively low melting temperature, good wettability, high mechanical integrity, ease of

manufacturing, and low cost [4, 5, 15]. Wettability refers to the “efficiency & speed with

which solder spreads over a metal surface” [15]. The melting temperature for eutectic Sn-Pb

solder is 183 °C, with reflow and wave soldering temperatures at 230 °C and 250 °C,

respectively. The reflow soldering process is used to attach surface-mount components onto a

printed wiring board (PWB), such as BGAs and integrated circuits (ICs), which have very

short pins. The wave soldering process is used to solder pin-through-hole components, such

as resistors and capacitors, which have long leads.

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2.2.1 Higher Melting Temperature

One of the major problems with LF solder is its higher melting and processing temperatures

compared to Sn-Pb. A majority of the LF solders have melting temperatures exceeding 200

°C, with a reflow temperature at least 50 °C greater than the melting temperature [10]. The

LF solder compositions studied in this thesis are also known as SAC solders in the electronics

industry. The SAC acronym stands for Sn-Ag-Cu. SAC105 indicates that the compositions

of Ag and Cu add up to 1.05 wt %, with remaining portion of the alloy consisting of Sn. The

melting temperature for SAC105 is about 227oC [16]. Figure 2-1 shows the typical

temperature profiles for LF and Sn-Pb solder during reflow. The profiles show that

throughout the entire reflow process, LF solder requires higher temperatures for preheating

and the actual soldering process. Sn-Pb can be soldered at a temperature of 210 - 220 °C,

versus 260 °C for LF. Due to the increased temperature profile required for LF solder,

companies must ensure that they have the necessary equipment, PWB materials, and electrical

components that can withstand the higher temperatures.

Components, such as plastic encapsulated components and light emitting diodes, cannot

handle such high temperatures without damage [5, 6]. Components may also be more

sensitive to moisture levels at higher temperatures, requiring special handling and storage to

prevent absorption of moisture, which can result in component fracturing [6].

10

Figure 2-1: Temperature Profile on Component Packaging Surface for Sn-Pb vs. LF Solder 6

If moisture is trapped within the component package, the component can fail due to

“popcorning”, which results from the “rapid vapourization of trapped moisture when the

package is exposed to high thermal stresses” [5], such as LF soldering, causing the package to

explode.

2.2.2 Mixed Components on Assemblies

A major concern for the aerospace and medical industry is the possibility of mixed parts for

LF and Sn-Pb solder applications. Although these industries can continue to use Sn-Pb solder

until at least December 2011, there is a danger that component suppliers will mix LF-

compatible components with Sn-Pb components. Sn-Pb components have their leads coated

with lead to improve the wettability during the soldering process. Using a Sn-Pb temperature

profile to process LF solder can result in fillet lifting of the solder joint, which is the

incomplete adhesion between the pad on the PWB and the component lead [4, 5]. An

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example of fillet lifting is shown in Figure 2-2. Since the aerospace and medical industries

will be using a lower temperature profile to solder their products, LF components accidentally

placed on the board are at risk of falling off the PWB at unknown times in the future, resulting

in catastrophic failure.

Figure 2-2: Scanning Electron Micrograph of Lifted Fillet 4

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2.2.3 Tin Whiskering

One of the major benefits for using Sn-Pb solder was that it impeded the development of tin

whiskers [12]. As defined by Carbone [12], “a tin whisker is a thin filament that can grow on

the surface of the solder plating”. LF solders containing high concentrations of Sn, such as

SAC405, are prone to whisker growth which increases with high-temperature exposure [3].

An example of whisker growth on a solder joint is shown in Figure 2-3. Tin whiskers pose a

problem to the reliability of the circuit board since these whiskers can grow long enough to

break off and cause short circuits on the board [5, 6]. According to Clelland and Price [3],

although the mechanism for this whisker growth is not completely understood, it is believed

that the plated tin contains internal stresses.

With aging, the internal stresses begin to relax by nucleating a single crystal of tin that shoots

out of the surface like a whisker. The whisker continues to grow until all of the internal

stresses are relieved, and can be more than 100 μm in length. This is again important for the

high-risk industries (aerospace, automotive, and medical), where the EEE can experience high

levels of vibration and shock during operation. LF solders are more rigid than Sn-Pb, making

them more susceptible to damage by vibration and shock [6].

13

Figure 2-3: Tin Whiskers emerging from the surface 3

2.3 Wetting Behavior

Wetting is the ability of a liquid to make contact with a solid surface and its degree is

determined by a force balance between the cohesive and adhesive forces. The contact angle

(θ) is defined as the angle between the liquid-vapor and solid-liquid interface. A low contact

angle indicates that the liquid can easily “wet” the surface and spread over a large area on the

solid surface. Figure 2-4 shows the three phases (vapor, liquid and solid) and the relation of

their respective surface tensions.

14

Figure 2-4: Surface Tensions acting on the liquid droplet wetting a solid substrate

Young’s equation defines the relation between the surface tensions of the three phases:

γSG = γSL + γLG Cosθ [17]

Furthermore, the Laplace law dictates that when a solid is immersed in a liquid, the wetting

force is given by [11]:

F = γSG - γSL – buoyancy force

F = γLG Cosθ . P – g . ρ . v

Where,

F = Wetting Force (mN)

= Surface tension of molten solder (0.4 mN/mm)

P = Specimen perimeter (mm)

= Contact angle

g = Acceleration due to gravity (9.81 m/s2)

= Solder density

v = Immersed Sample volume (mm3)

g . . v = calculated buoyancy force

15

2.4 Previous studies

Snugovsky et al [18] conducted a study to review the microstructure and reliability of mixed

assemblies. In this study four types of LF ball grid array components were assembled on test

vehicles. Eutectic Sn-Pb solder paste was used with SAC305 and SAC405 balled components

and the SnPb reflow profile was used with peak temperatures that varied from 205oC to

220oC. Accelerated thermal cycling (ATC) was performed on these test vehicles with a

temperature profile of 0 – 100oC for 6000 cycles. Depending on the peak temperature of the

reflow profile the joints were either partially or fully mixed as shown in Figure 2-5. ATC

experiments concluded that the fatigue life of completely mixed and partially mixed joints

was sufficient and comparable to Sn-Pb and Pb-Free fatigue lives. However, completely

mixed solder joints contained a new ternary (L -> Ag3Sn + Pb + Sn) which melted ~ 39oC

lower than the normal SAC ternary (L ↔ βSn + Ag3Sn + Cu6Sn5).

16

Figure 2-5: (Top) A poorly mixed joint between SAC 105 ball and 20wt% SnPb paste formed at a peak

temperature of 210oC. (Bottom) However, increasing peak temperature and wt% paste to 220

oC and

29wt% respectively results in a complete melting of the solder ball.

17

Although the fatigue life of mixed joints was comparable to the fatigue lives of pure SnPb and

SAC solders it should be noted that the inclusion of this new ternary in mixed joints limits the

service life of such joints at 177oC. Furthermore, partially mixed joints exhibited slightly

lower reliability which was due to the coefficient of thermal expansion mismatch of the two

solders. The reliability of SAC405/SnPb joints was better than that of SAC305/SnPb joints

because of the fine grain structure and more IMC particles in SAC405. In fully mixed joints,

the cracks followed a path that contained more Sn and less intermetallic compounds. The Sn

recrystallized in this region and contained small Pb particles at the boundaries. Crack

propagation followed the path along the grain boundaries between Sn and Pb grains for

SAC305/SnPb joints. This is evident in Figure 2-6 as shown below.

Figure 2-6: (Left) Crack propagation in SAC305 ball / SnPb paste takes place along grain boundaries

between Sn and Pb (2500X). (Right) SAC405 ball / SnPb paste has better thermal reliability (1000X) 18

It was also found that the microstructure in the mixed region was not uniform when the

conventional SnPb reflow profile was used. Microstructural investigation revealed that a Sn

rich coarse phase existed in the region adjacent to the retained solder ball that did not melt.

Thermal cycling results showed that these joints had a lower reliability as this Sn rich region

facilitated crack propagation. It was recommended that such microstructures should be

avoided to ensure better thermal reliability.

The drop performance of SAC solders is generally poor relative to eutectic SnPb. In a study

conducted by Liu et al [19] the effect of various dopants on the drop reliability of SAC105

18

solder was reviewed. BGA coupons of size 40 mm x 40 mm were reflowed with peak

temperatures of 240oC. The assemblies were evaluated for drop reliability as-reflowed and

aging at 150oC for 4 weeks. The height used in the drop test was 0.5 meter and 10 simulated

assemblies were used for each test condition. Post drop test analysis revealed that at 0.13wt%

Mn content the drop performance was several times better than pure SnPb and SAC alloys. It

was found that MnSn2 particles accumulated near the IMC layer which shifted the fracture

mode from interface towards solder fracture. Li-Wet Lin et al [20] concluded that addition of

Mn gives rise to refinement and volume extension of Sn-rich dendrites and allows the

formation of heterogeneous intermetallics, MnSn2.

Kim et al [21] studied the effects of 4th

alloying additive on the microstructure and tensile

properties of bulk SAC305 alloy and joints with Cu. The alloying elements were Ti, Mn, Fe,

Ni and Co at a fixed concentration of 0.1 wt%. Solidification microstructures were studied

using SEM and it was found that Ti, Mn and Ni formed very fine precipitates while Fe and Co

formed coarse, elongated precipitates as shown in Figure 2-7.

Figure 2-7: SEM images of as-cast microstructure of (a) SAC305; (b) SAC-0.1 Ti; (c) SAC-0.1 Mn; (d)

SAC-0.1 Fe; (e) SAC-0.1 Ni; (f) SAC-0.1 Co alloy. 19

(The bars indicate 10μm)

It was therefore expected that the addition of Ti, Mn and Ni would improve the tensile

properties of the bulk alloy. It was found that addition of Co decreased the elongation to

19

fracture significantly while Ni and Mn addition produced a slight increase in the same. The

behavior was attributed to the formation of large and fine precipitates in Co and Ni containing

alloys respectively. The SAC -0.1Ni alloy fracture surface showed fine dimple pattern which

is representative of a ductile fracture. On the other hand fracture surface of SAC – 0.1Co

showed brittle fracture containing cracked particles of precipitates. This is shown in Figure 2-

8.

Figure 2-8: (Top) SEM images of fracture surface of (a) SAC-0.1 Ni and (b) SAC-0.1 Co.

(Bottom): Large CoSn2 precipitate found in the fractograph of SAC-0.1 Co bulk alloy. 19

20

Wang et al. [22] conducted a study to investigate the effects of adding rare earth elements

(RE) on the wetting and tensile properties of Sn-3.5Ag alloys. The alloys were made by

adding up to 1.0 wt% of RE (x = 0, 0.25, 0.5 and 1.0wt %) and the RE used were La and Ce.

It was established using DSC experiments that the influence of adding trace amounts of RE on

the melting temperature was negligible. The wetting properties were investigated by

measuring the final contact angles and spread areas when 1.0gm of solder was soldered on a

Cu substrate. The contact angles were decreased from 45o to about 30

o by adding up to 0.5

wt% RE while the spread areas increased from 125mm2 to 150mm

2 as shown in figure 2-9.

Figure 2-9: Relationship between the (Top) contact angles and RE content; (Bottom) Spread area and RE

content. 22

It was concluded that the wetting properties improved by 0.25-0.5 wt% addition of RE into

the base Sn-3.5Ag alloy. Over addition of RE beyond 0.5 wt% reduced the beneficial effect

through oxidation of surface active RE elements [23].

21

Dudek et al [24] reviewed the microstructure and mechanical behavior of La containing LF

solders. She found that addition of up to 0.5 wt% La resulted in microstructural refinement of

the Sn matrix. The average length of Sn dendrites was 60% smaller than the dendrites found

in eutectic SAC alloy as revealed in figure 2-10. After reflow, it was found that these

intermetallics also accumulated in the vicinity of the solder-substrate reaction layer. The

intermetallic thickness of these La containing alloys was 60% lower than the elongated

Cu6Sn5 scallops found in the eutectic SAC alloy.

Figure 2-10: (TOP) Optical micrographs of as processed ingots of (a) Sn-3.9Ag-0.7Cu; (b) Sn-3.9Ag-

0.7Cu-0.1La and (c) Sn-3.9Ag-0.7Cu-0.5La. (BOTTOM) SEM micrographs of reflow joints of (a) Sn-

3.9Ag-0.7Cu; (b) Sn-3.9Ag-0.7Cu-0.1La and (c) Sn-3.9Ag-0.7Cu-0.5La 24

22

This suppression of the growth of intermetallic layer was explained in two ways. First, it was

believed that the heterogeneous nucleation of Sn on LaSn3 sites suppressed undercooling and

Figure 2-11: (Left) Shear stress vs. shear strain. (Right) Shear strain to failure vs. wt% La.24

prevented the liquid solder to react with the substrate. Secondly, a uniform layer of fine

intermetallics acted as a barrier to the dissolution of Cu substrate into the solder thereby

reducing its reaction with Sn to form Cu6Sn5 intermetallics. Single lap shear joint mechanical

tests were

performed on these specimens at a strain rate of 10-3

s-1

. The results of the tests can be seen in

Figure 2-11.

The La containing specimens were more fracture tolerant and the decrease in stress was more

gradual when compared to the abrupt decrease in stress for Sn-3.9Ag-0.7Cu samples. The

ductility also increased with increased concentration of La, however, this effect was not linear

in nature. This indicated the presence of a critical concentration of La beyond which there

would be no appreciable improvement in ductility. Fracture surface examination of Sn-3.9Ag-

0.7Cu-0.5La revealed a classic void nucleation, growth and coalescence process as shown in

Figure 2-12.

23

Figure 2-12: Fracture surfaces of lap shear samples for (a) Sn-3.9Ag-0.7Cu, (b) Sn-3.9Ag-0.7Cu-0.1La,

and (c) Sn-3.9Ag-0.7Cu-0.5La. 24

The fracture surface of Sn-3.9Ag-0.7Cu-0.5La had elongated dimples that contained LaSn3

intermetallics. In these specimens the fracture occurred through the solder and away from the

intermetallic / solder interface. On the other hand Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-0.1La

specimens resulted in a tearing type fracture along the intermetallic / solder interface. It was

believed that the increase in ductility with increased La concentration was due to the ability of

LaSn3 intermetallic particles to nucleate and grow voids in the solder interior.

Yu et al [25] conducted a study to investigate the improvement on the microstructure stability,

mechanical and wetting properties of SAC alloys with RE additions. The RE elements used

were La and Ce ranging from 0.1wt% to 0.25wt% and were alloyed with Sn-3.5Ag-0.7Cu. Sn-

2.5Ag-0.5Cu and Sn-3.5Ag-0.7Cu were used as control. The microstructure in the Sn-2.5Ag-

24

0.7Cu and Sn-3.5Ag-0.7Cu appeared to have coarse β-Sn grains consisting of Ag3Sn and

Cu6Sn5 intermetallics. With the addition of RE these intermetallics became much finer and

uniform in structure. The addition of RE also decreased the width of the eutectic band from

10μm down to 1μm. These changes in microstructure were also reflected in the mechanical

properties of these alloys when tensile tests were performed as a significant increase in %

elongation was noticed in Sn-3.5Ag-0.7Cu-0.25RE. Addition of RE also improved the

wettability of solder on Cu coupons. Sn-3.5Ag-0.7Cu-0.1RE demonstrated the highest wetting

force and lowest contact angle amongst the alloys compared. Yu et al described this

enhancement to be the effect of lower surface tension caused by active RE elements.

2.5 Summary

The implications of Restriction of Hazardous Substances directive (RoHS) were described in

chapter 2. In addition to that the problems of using lead free alloys in electronics

manufacturing were discussed. The latter part of the chapter looked at previous investigative

studies on the compatibility of SnPb and lead free solders in mixed assemblies. Furthermore,

the effects of doping small amount of additives to various SAC alloys were also listed.

25

References

[1] L. J.Turbini,G. C. Munie, D. Bernier, J. Gamalski, and D. W. Bergman, “Examining

the Environmental Impact of Lead-Free Soldering Alternatives,” IEEE Transactions

on Electronics Packaging Manufacturing, vol. 24, no. 1, pp. 4-9, 2001.

[2] UK Department of Trade and Industry, “UK RoHS Regulations SI2006: No. 1463 –

The Restriction of the Use of Certain Hazardous Substances in Electrical and

Electronic Equipment Regulations 2005,” [Online document], Sept. 25, 2005 [cited

Nov. 4, 2006], Available: http://www.rohs.gov.uk/Docs/uksi_20061463_en1.pdf

[3] I. W. Clelland and R. A. Price, “Lead-Free Solder Study,” Electronic Business, pp. 1-

17, 2005. [Online document], Sept. 20, 2005, [cited June 20, 2006], Available:

http://www.reed-electronics.com/eb-mag/contents/images/

ITWPaktronLeadFreeStudy.pdf#search='LeadFree%20Solder%20Study%20Ian%20

W.%20Clelland%20Rick%20A.%20Price%20ITW%20Paktron'

[4] K. Suganuma, “Advances in lead-free electronics soldering,” Current Opinion in

Solid State and Materials Science, vol. 5, no. 1, pp. 55–64, 2001.

[5] M. Goosey, “An overview of the current status of lead-free assembly and related

issues,” Circuit World, vol. 29, no. 4, pp. 23-27, 2003.

[6] R. A. Quinnel, “RoHS Compliance: It’s Not Easy Being Green,” EDN, pp. 37-41, 2006.

[7] UK Department of Trade and Industry, “Explanatory Memorandum to the Restriction

of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment

Regulations 2005,” [Online document], Sept. 30, 2005, [cited Nov. 4, 2006],

Available: http://www.pb-free.info/ukrohsnotes.pdf

[8] B. Jorgensen, “It’s Tough to be Green,” Electronic Business, [Online document],

26

Mar. 1, 2006, [cited Jun. 20, 2006], Available: http://www.reed-electronics.com/eb-

mag/index.asp?layout=articlePrint&articleID=CA6310931

[9] T. Bradley, “Compliance: Beyond RoHS,” Electronic Business, [Online document],

June 20, 2006, [cited June 20, 2006], Available: http://www.reed-electronics.com/eb-

mag/index.asp?layout=articlePrint&articleID=CA6345431

[10] IPC/JEDEC-9704, “Printed Wiring Board Strain Gauge Test Guideline,” Sept. 2004.

[11] S. C. Johnson, “RoHS and Non-Compliance: Expect to See Penalties,” Electronic

Business, [Online document], June 13, 2006, [cited June 20, 2006], Available: http://

www.reed-electronics.com/eb-mag/index.asp?layout=articlePrint&articleID=

CA6342812

[12] J. Carbone, “RoHS: The Tip of the Environmental Iceberg,” Purchasing Magazine

Online, [Online document], June 15, 2006, [cited June 20, 2006], Available: http://

www.purchasing.com/index.asp?layout=articlePrint&articleID=CA6341085

[13] S. Drumm, “Maintaining Quality Lead-Free,” IEE Manufacturing Engineer, vol. 82,

no. 4, pp. 26-27, 2003.

[14] K. Seelig, “A Study of Lead-Free Alloys,” AIM, Inc., vol. 1, no. 1, 2005.

[15] B. Trumble, “Get the Lead Out: Lead-Free Electronics Come of Age,” IEEE

Spectrum, vol. 35, no. 5, pp. 55-60, 1998.

[16] L. Yang, J.B. Bernstein, and K. Chung, “The Impact of Lead-Free Soldering on

Electronics Packages,” Microelectronics International, vol. 18, no. 3, pp. 20-26,

2001.

[17] S. Dongkai. Lead-Free Solder Interconnect Reliability. 2005

27

[18] P. Snugovsky ,H. McCormick ,S. Bagheri ,Z.Bagheri ,C. Hamilton ,and M. Romanksy,

"Microstructure, Defects, and Reliability of Mixed Pb-Free/Sn-Pb Assemblies," Journal

of ELECTRONIC MATERIALS, Vol. 38, No. 2, 2009

[19] W. Liu and N.C. Lee, "NOVEL SACX SOLDERS WITH SUPERIOR DROP TEST

PERFORMANCE," pp. 134-146, 2004

[20] Li-Wei Lin, Jenn-Ming Song, Yi-Shao Lai, Ying-Ta Chiu, Ning-Cheng Lee and Jun-

Yen Uan, “Alloying modification of Sn–Ag–Cu solders by manganese and titanium,”

Microelectronics Reliability (2008), doi:10.1016/j.microrel.2008.10.001

[21] K.S. Kim, S.H. Huh and K. Suganuma, "Effects of fourth alloying additive on

microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu,"

Microelectronics Reliability 43 (2003) 259–267

[22] L. Wang, D.Q. Yu, J. Zhao*, M.L. Huang, "Improvement of wettability and tensile

property in Sn–Ag–RE lead-free solder alloy," Materials Letters 56 (2002) 1039-1042.

[23] H. Hao, J. Tian, Y.W. Shi, Y.P. Lei and Z.D. Xiai, “Properties of Sn3.8Ag0.7Cu Solder

Alloy with Trace Rare Earth Element Y Additions,” Journal of ELECTRONIC MATERIALS,

Vol. 36, No. 7, 2007

[24] M.A. Dudek, R.S. Sidhu, N. Chawla and M. Renavikar, “Microstructure and

Mechanical Behavior of Novel RareEarth–Containing Pb-Free Solders,” Journal of

ELECTRONIC MATERIALS, Vol. 35, No. 12, 2006

[25] D.Q. Yu, J. Jhao, and L. Wang, “Improvement on the microstructure stability,

mechanical and wetting properties of Sn-Ag-Cu lead-free solder with the addition of rare earth

elements,” J. of Alloys and Compounds, 2004, Vol 376, p 174

28

CHAPTER 3: EXPERIMENTAL METHODS

PART I - MIXED ASSEMBLY

3.1 Solder Ball and Paste combinations

The mixed assemblies that replicated the ball grid array joints were prepared by soldering

solder balls of various compositions on a copper substrate by using a solder paste. Various

combinations of solder ball and paste were used to give joints that were either homogenous or

heterogeneous in nature. The compositions of the alloys containing (a) 1-4 wt% Ag, (b) 0.5

wt% Cu and remaining Sn were selected from the Sn-Ag-Cu phase diagram in the vicinity of

the ternary eutectic as shown in the Figure 3-1.

Figure 3-1: Sn-Ag-Cu phase diagram showing the near eutectic compositions and corresponding melting

temperatures.

29

Table 3-1 illustrates the various combinations of solder ball and paste, weight percentage of

paste and peak soldering temperatures used in the experiments. SAC is another notation for

Sn-Ag-Cu based solder. The numbers followed by SAC denote the wt% of Ag and Cu

respectively. For example, SAC105 is 1wt% Ag, 0.5wt% Cu and balance Sn. SAC MO1 is

SAC 105 with 0.1wt% Mn added to the alloy.

Table 3-1: Solder Ball and Paste combinations

Solder Ball / Paste %Paste,

Tpeak

%Paste,

Tpeak

%Paste,

Tpeak

%Paste,

Tpeak

SAC 105 / SAC 305 20, 235oC 29, 235

oC 20, 245

oC 29, 245

oC

SAC 105 / SAC 405 “ “ “ “

SAC MO1 / SAC 305 “ “ “ “

SAC MO1 / SAC 405 “ “ “ “

SAC 105 / SnPb 20, 210oC 29, 210

oC 20, 220

oC 29, 220

oC

SAC 305 / SnPb “ “ “ “

SAC MO1/ SnPb “ “ “ “

3.2 Joint Preparation

In order to fabricate the joint a depression was made on the copper substrate using a flat end

screw. The substrate was then cleaned using acetone, 20% v/v nitric acid and distilled water

and dried using compressed air. Appropriate amount of solder paste was weighed and

carefully deposited on the depressed substrate region and solder ball was put on top of it. The

assembly, together with a thermocouple, was reflowed in a tube furnace for 90 seconds at the

appropriate peak temperature. A schematic of the assembly can be seen in Figure 3-2.

30

Figure 3-2: Schematic of mixed solder joint assembly.

3.3 Microstructure Observation

The excess copper strips were cut in order to expose the edge of the solder joints. They were

then placed in rubber sample mounts and mounted in epoxy. The samples were then ground

with emery papers from coarse to fine grit numbers 400, 600, 800, 1200 and 2400,

respectively. They were then polished using 6 micron diamond suspension solution and etched

with colloidal silica solution to expose the intermetallic layers.

The samples were coated with carbon for scanning electron microscopy (SEM) to make it

conductive and prevent image distortion due to surface charging. They were then observed

under the scanning electron microscope using 20kV operating voltage and a working distance

of 15mm. EDX was performed in order to identify the intermetallic phases.

3.4 Microhardness Testing

The samples were tested for Vickers microhardness using the ASTM E384 – 09 Standard.

Three different areas were investigated – the top, middle and bottom of the joint when

measured as a function of distance from the copper substrate. A force of 10gm (0.098

Newton) was applied.

Solder ball

Solder paste Thermocouple

Copper Substrate

0.762 mm

31

PART II – NOVEL LEAD FREE SOLDERS

3.5 Solder Preparation

Six different additives were added to the base alloy to make six new lead free solders. The

additives were (a) Titanium, (b) Nickel, (c) Manganese, (d) Lanthanum, (e) Cerium, and (f)

Yttrium. The weight percentage of additives used in the alloys was between 0.1wt% to 1.0

wt%. The additives used in making these alloys had a purity level of 99.999%.

The alloys were fabricated in a vacuum arc furnace where there melted repeatedly at 2500oC

to ensure homogeneity and then cast into 0.25” pellets. High temperature and vacuum

conditions ensured complete melting of additives that melt well above the melting

temperature of base solder. Inductive coupled plasma spectroscopy confirmed the presence of

additives between the ranges of 0.8-1.1 wt%. These alloys were then appropriately diluted by

melting and adding SAC105 base alloy in order to dilute the additive wt% to 0.1wt%.

3.6 Microstructure observation

The copper substrate was cleaned and depressed as described in Section 3.3. 12μm of each of

the six solders were carefully measured and deposited in the depression. The assembly was

then reflowed in a furnace at a peak temperature of 260oC and held above liquidus

temperature for 3 minutes. A higher peak temperature and longer time-above-liquidus ensured

complete melting and collapsing of solder in the absence of solder paste. An extra set of each

sample was prepared that was age hardened at 150oC for 96 hours. The samples were then

mounted and polished as described previously in Section 3.3 and observed under the scanning

electron microscope. EDX and EPMA were utilized to examine the composition of phases and

distribution of elements within the joint respectively.

32

3.7 Wetting Balance Procedure

Solder pellets were prepared by carefully measuring and cutting solder chunks into 200mg

fragments. 1 cm long and 0.72mm gauge diameter copper coupons were carefully pieced in

order to ensure repeatability. The test was carried out using the IPC J-STD 002 standard.

Copper coupons were cleaned with acetone, 20% v/v nitric acid and distilled water for 20

seconds each time and then dried on blotting paper. The wetting balance load cell, flux height

and blotting paper level was calibrated prior to conducting the experiments. The following

parameters were used:

(a) Immersion depth = 0.4mm.

(b) Immersion speed = 2mm/sec.

(c) Dwell time = 10 seconds.

(d) Immersion angle = 90o incident to the solder.

(e) Solder temperature = 235oC (SnPb) and 265

oC (Lead-Free).

The following fluxes were used in the experiment:

(i) Flux 1, used for tin/lead, a standard activated rosin flux, -

25%+/-0.5% by weight of colophony, and 0.15%+/-0.01% by weight

diethyl ammonium hydrochloride in a 74.85%+/-0.5% by weight of isopropyl

alcohol.

(ii) Flux 2, used for lead-free solderability tests, a standard activated

rosin flux, - 25%+/-0.5% by weight of colophony, and 0.39%+/-0.01% by

33

weight diethyl ammonium hydrochloride in a 74.61%+/-0.5% by weight of

isopropyl alcohol.

The copper coupons were carefully loaded on the load cell and the test was run 10 times for

each solder. It was imperative to note the flux level and fill with more whenever necessary as

it was volatile in nature and was also being absorbed by the copper coupons. The wetting

balance was attached to a computer workstation which analyzed the load cell sensor and

recorded the wetting force every 0.01 seconds. It is shown in Figure 3-3. This information was

plotted in Microsoft Excel to produce wetting curves for each solder. SnPb and SAC105 were

used as control in this experiment.

Figure 3-3: Wetting Balance used to evaluate solder wettability

34

3.8 Solder Joints with Copper

In order to investigate the joint strength of the lead-free solders a uniaxial tensile test was

designed. Two strips of electronic grade copper (1cm x 5cm x 2mm) were soldered to lead

free solder sheets (1cm x 2mm x 2mm) using the lead-free flux as described in Section 3.7.

The copper strips were cleaned with acetone, 20% v/v nitric acid and distilled water and then

held together with the solder sheet in between in a metal assembly. The strips were lightly

screwed in order to maintain symmetry. The metal assembly was carefully placed in the

furnace at 270oC for 5 minutes. A higher peak temperature and longer time-above-liquidus

ensured that the thick solder sheet melted and collapsed entirely in order to form an efficient

joint. Figure 3-4 shows the copper strips attached to the solder sheet and held together in the

metal assembly.

Figure 3-4: Metal assembly holding the copper strips and sheet in place for soldering.

Cu Strip # 1

Tightening

Screws

Metal

Assembly

Solder Sheet

Cu Strip # 2

35

3.9 Tensile Testing

SHIMADZU machine, model AGS-J (Figure 3-5), was used for tensile test. Tensile test was

performed on the copper-solder-copper joints in order to investigate the joint strength of each

of the six lead-free solders. Tensile testing on joined specimens was carried out at room

temperature at a strain rate of 3.5 x 10-4

s-1

. SAC105 was used as control in this experiment.

Figure 3-5: SHIMADZU Model AGS-J Tensile Testing Machine.

36

3.10 Surface Observation

After the tensile tests were performed, fracture surfaces were examined using SEM, EDX and

optical microscopy. The same was used to examine the microstucture of mixed assembly

joints.

37

CHAPTER 4: RESULTS & DISCUSSION

Part I: Mixed Solder Assemblies

4.1 Solidification Microstructure

As mentioned in Chapter 3, solder joints were made using different combinations of solder

paste and solder balls on a copper substate and reflowed in a tube furnace. The resulting

solidification microstructure of a near ternary Sn-Ag-Cu system consists of the liquid

solidifying into β-Sn dendrites and Ag3Sn and Cu6Sn5 intermetallic phases. The resulting

microstructures depend upon the reflow parameters that were used such as peak reflow

temperature and weight% of solder paste as described in the previous chapter. For this study

the reflow time was kept constant at 90 seconds. Figure 4-1 shows the resulting solidification

microstructure of a SAC405 joint.

Figure 4-1: Solidification microstructure of SAC405 showing the βSn dendrites surrounded by eutectic

structure.

38

It can be seen clearly that the solidification microstructure contains a network of β-Sn

dendrites as the result of primary solidification [1]. The eutectic colonies that exist between

the dendrites consist of the β-Sn, Ag3Sn and Cu6Sn5 phases. The presence of a large number

of dendrites is attributed to the large undercooling that is associated with Sn-Ag-Cu solders

below the eutectic temperature. [2] has shown that β-Sn does not nucleate heterogenously on

the Ag3Sn or Cu6Sn5 phases that nucleate at smaller undercoolings. Instead, β-Sn was shown

to nucleate homogeneously with an undercooling of 29oC. The study also showed that large

Ag3Sn IMC particles did not facilitate the nucleation of β-Sn and therefore were ineffective

substrates for heterogenous nucleation.

4.2 Effect of Reflow Temperature and Solder Paste

4.2.1 SAC solder ball with Sn-Pb Paste

The next objective was to observe the the effect of different reflow profiles on the resulting

microstructure. Figure 4-2 shows the optical micrograph when SAC105 solder ball was

reflowed using Sn-Pb solder paste at a reflow temperature of 210oC and 20wt% paste.

39

Figure 4-2: Incomplete melting of SAC105 solder ball takes place when peak reflow temperature is 210oC

It is evident from Figure 4-2 that the reflow profile was not sufficient for the complete melting

of SAC105 solder ball and the resulting microstrutre was non uniform. The upper part

contained the retained solder ball and was saturated with Sn while the part adjacent to copper

substrate was rich in Pb containing paste. Snugovsky [3] described the degree of mixing as a

fraction of a mixed region in a cross-sectional area of the solder joint. 100% mixing is

achieved when the solder ball melts completely and the resulting microstructure is uniform.

The electron image of the above microstructure can be seen in Figure 4-3 where the clear

distinction between the SAC and Sn-Pb dominant microstructures is evident once again.

40

Figure 4-3: SEM micrograph of solder joint made soldering SAC105 with eutectic SnPb paste reflowed at

210oC

The incomplete melting of the solder ball can be explained by the insufficient peak melting

temperature that was used in this profile. Sectors such as defence that are currently exempt

from ROHS still use the Sn-Pb paste in their manufacturing operations and use lower peak

reflow temperatures normally associated with Sn-Pb manufacturing. However, more OEM

providers are making lead-free chips to keep up with competition and when this is overlooked,

the resulting part will have heteregenous microstructures as shown above. Snugovsky [4] also

noted that complete mixing of solder ball and paste may still result in non-uniform

microstructure with notable difference in compositions at different regions. When SAC solder

ball is melted using Sn-Pb paste, the paste starts to melt first at 183oC and the SAC ball starts

to dissolve in the molten paste. This enhances the concentration of Sn and Ag in the mixed

region and there is a coarse band of β-Sn that is formed at the interface between the mixed and

non-mixed regions. Microstructures containing the Sn bands have been associated with early

41

failures in many studies [4-6]. A closer look at Figure 4-3 exemplifid the existence of Sn

bands at the mixed and non-mixed interface. This can be seen in Figures 4-4 (a) and (b).

42

(a)

(b)

Figure 4-4: Coarse Sn-rich bands formed at the interface between mixed and non-mixed regions of the

solder joint

43

When the wt% of paste and peak reflow temperatures were increased it was expected that the

solder ball would continually progress to melt more and result in a greater mixed region.

Eventually when there was sufficient paste and the peak reflow temperature was high, it was

expected that the solder ball would melt and totally collapse resulting in a 100% mixed

region. This progression was indeed observed in the current study and is illustrated in Figure

4-5.

(a) (b)

(c) (d)

Figure 4-5: SAC solder joints soldered with eutectic SnPb paste and reflowed using different peak

temperatures and wt% paste. (a) 210oC, 20wt% paste (b) 210oC, 29wt% paste (c) 220oC, 20wt% paste

and (d) 220oC, 29wt% paste

44

The solder joint in Figure 4-5 (a) retained most of the solder ball whereas the joint in 4-5 (d)

melted completely. When more solder paste was available at a higher peak temperature it was

easier for the solder ball to dissolve in the molten paste and form a 100% mixed joint.

4.2.2 SAC solder ball with SAC Paste

For these reflow profiles the peak temperatures varied from 235oC to 245

oC and the wt%

paste varied from 20wt% to 29wt% once again. Common SAC solders melt in the 217 –

227oC range and therefore it was expected that the peak reflow temperature was sufficient for

complete melting of solder ball. Figure 4-6 shows the result of SAC105 solder ball soldered

using SAC305 solder paste (20wt%) at 235oC and 245

oC respectively.

45

(a)

(b)

Figure 4-6: SAC105 solder with SAC305 paste soldered at (a) 235oC and (b) 245

oC

46

Figure 4-6 shows the complete collapse of solder ball and 100% mixing resulting in a

dendritic β-Sn structure surrounded by a lamellar array of β-Sn, Ag3Sn and Cu6Sn5, which is

the expected solidification microstructure of near eutectic SAC solders [7-10].

Table 4-1 summarizes the amount of mixing achieved when different solder balls and pastes

were used under varied reflow parameters to form solder joints in this study. Level of mixing

is defined as the ratio of the area of mixed region to the total joint area.

Table 4-1: Level of mixing attained when solder balls and paste of various compositions were reflowed

together at different peak temperatures and wt% paste.

In the case of SAC solder ball reflowed with SnPb paste, complete mixing was achieved only

when the peak reflow temperature and wt% paste was highest, i.e. 220oC and 29wt% paste

respectively. On the other extreme when only 20wt% paste was used and peak reflow

temperature was 210oC the mixing was poor and a considerable amount of solder ball

remained intact.

47

For SAC solders reflowed using SAC paste there was no mixing problems and 100% mixing

was achieved at all reflow parameters. The peak reflow temperatures used in these profiles

and time above liquidus of 90 seconds were sufficient for the solder balls to completely melt

and form a 100% mixed joint.

4.3 Changes in solder composition

4.3.1 SAC Solder ball with SAC paste

In this study solder balls and paste of different compositions were used to make solder joints.

Therefore it was expected to see changes in solder composition during the reflow process as

published in many studies [3-6]. In order to observe these modifications in composition, EDX

was performed on three regions of the solder joint: bulk, top (away from substrate) and

bottom (adjacent to substrate). In the case of SAC solder balls with SAC paste the changes in

composition were modest due to the proximity in the composition of both sources. Figure 4-7

shows the different regions of the solder joint and their respective compositions that were

examined for EDX spectrum when SAC105 ball was soldered using SAC305 paste.

48

(a)

(b)

49

(c)

Figure 4-7: SEM micrographs of SAC105 soldered with SAC305 and EDX results of regions (a) bulk

solder, (b) away from the interface, and (c) adjacent to the interface

Both SAC105 and SAC305 contain only 0.5wt% Cu but the bulk of the solder contained

1.3wt% Cu as seen in Figure 4-7 (a). This can be explained by the rapid copper dissolution

rates that are associated with SAC solders [11-15]. The rate of dissolution of copper in solder

increases linearly with time when dissolved in a solder bath or in a solder joint [16] and slows

down when its concentration in the molten solder approaches its solubility limit.

Figure 4-7 (b) and (c) show differences in concentration of Sn in the top and bottom regions

of the joint. A considerable amount of Sn is used in forming the Cu6Sn5 (η) intermetallic layer

at the interface of solder and substrate. This requires the Sn atoms adjacent to the interfacial

layer to diffuse towards the substrate and hence the bottom region of EDX has a lower

concentration of Sn. Similarly, the redistribution of Ag in the solder joint can be attributed to

the influx of Ag atoms from the Ag rich solder paste (SAC305) towards the solder ball region

containing only 1wt% silver (SAC105). Similar observations were made when SAC105

50

solder balls were soldered using SAC405 solder paste and the EDX results can be seen in

Figure 4-8.

(a)

(b)

(c)

Figure 4-8: EDX results of regions (a) bulk solder, (b) away from the interface, and (c) adjacent to the

interface when SAC105 is soldered using SAC405 paste

51

4.3.2 SAC Solder ball with SnPb paste

For the case of SAC solder balls soldered with SnPb paste the changes in solder composition

were more extensive as expected. SAC is rich in Sn and when the solder joint is formed the

SnPb paste melts first at 183oC. This dissolves a portion of the solder ball and the joint

becomes richer in Sn and Ag while losing Pb concentration. EDX confirmed the changes in

composition in the bulk, top and bottom regions once again. Figure 4-9 shows the different

regions of the solder joint and their respective compositions that were examined for EDX

spectrum when SACMO1 ball was soldered using SnPb paste.

(a)

52

(b)

(c)

Figure 4-9: SEM micrographs of SACMO1 soldered with SnPb and EDX results of regions (a) bulk solder,

(b) away from the interface, and (c) adjacent to the interface

53

A closer examination of fully mixed SAC and SnPb joints revealed that the introduction of Pb

changed the ternary eutectic of the alloy from a normal SAC ternary (βSn + Cu6Sn5 + Ag3Sn)

to a different ternary (Sn + Pb + Ag3Sn). This ternary melts at 177oC and was confirmed using

differential scanning calirometry [4]. This further limits the service life of the joint from

183oC down to 177

oC where these joints may be subjected to high temperatures. Figure 4-10

shows the presence of binary SnPb eutectic and Ag3Sn-Pb-Sn ternary eutectic.

Figure 4-10: Pb contamination leads to a shift in ternary eutectic which is different from the normal SAC

ternary

54

4.4 Microhardness Properties

SAC solder balls soldered with SnPb paste resulted in heterogenous microstructures. The

resistance of such microstructures to localized deformation was investigated using Vickers

microhardness testing. Three regions of the solder joint were chosen for investigation:

retained solder ball (Region 1), interface between mixed joint and solder ball (Region 2), and

mixed joint (Region 3) as shown in Figure 4-11:

Figure 4-11: Optical graph showing the distinct regions of a non-mixed joint

From the literature [17, 18] it is known that that SAC solders are harder and more stiff than

their SnPb counterpart. Therefore it was expected for Region 1 to have a higher hardness

value than Region 3. The results were as expected and consistent for all three SAC and SnPb

combinations. Figure 4-12 shows the differences in microhardness of the 3 regions in all three

55

solder combinations. An interesting phenomenon was seen in Region 2 just below the Sn band

located at the interface. The hardness of this region was significantly higher than Region 3

and also higher than Region 1. Upon closer examination it was observed that this region

consisted of coarse, plate like Cu6Sn5 intermetallics with average length close to 100μm as

seen in Figure 4-13.

Figure 4-12: Differences in Vickers Microhardness of the 3 regions of a non-mixed joint

Figure 4-13: SEM micrograph of Region 2 shows the coarse, elongated intermetallic compounds formed

due to interdiffusion of Sn and Cu towards the joint interface

56

The presence of these coarse Cu6Sn5 intermetallics within the bulk of the solder and closer to

the solder ball / mixed joint interface can be explained by the interdiffusion of Sn and Cu from

the Sn-rich solder and Cu substrate respectively. This region may be susceptible to early

failures during accelerated thermal cycling due to the differences in thermal expansion

coefficients between solder and intermetallic compounds. They may also act as stress raisers

during drop test experiments. At this point more research is required before commenting on

the reliability of such microstructures.

57

Part II: Novel Lead-Free Solders

4.5 Solidification Microstructure

In this part of the study, six novel lead-free solders were prepared using SAC105 as the base

solder. Three metals (Ti, Ni and Mn) and three rare-earth elements were used as additives in

small quantities (1wt%) and melted with SAC105 in vacuum arc furnace as described in

Chapter 3. Additive concentration was then diluted to 0.1wt% by adding SAC105. The

resulting microstructure was similar to the solidification microstructure of SAC alloys with

the addition of intermetallic particles as expected. Figures 4-14, 15 and 16 show the fine

intermetallic compounds that are dispersed within the solder joint mainly above the Cu6Sn5

intermetallic layer above the copper substrate.

58

Figure 4-14: Lanthanum (Top) and Yttrium (Bottom) containing intermetallic compounds formed after

solidification

59

Figure 4-15: Nickel (Top) and Titanium (Bottom) containing intermetallic compounds formed after

solidification

60

Figure 4-16: Manganese (Top) and Cerium (Bottom) containing intermetallic compounds formed after

solidification

61

The presence of these particles above the IMC layer may strengthen the layer through

precipitation hardening and may impede the movement of dislocations or defects such as

cracks forcing the crack to change its route. These compounds have also shown to reduce the

thickness of IMC layer [19-20] Intermetallic compounds were also formed as finely dispersed

particles in the eutectic network. These compounds melt at higher temperatures than the

ternary eutectic and their presence in the primary Sn dendrites suggests that they may have

acted as nucleation sites and reduced the undercooling of β-Sn as documented in many studies

[21-23]. The formation of these compounds on the IMC layer as seen in parts of Figures 4-14,

4-15 & 4-16 also suggest that they may improve the wettability of SAC105 by reducing the

interfacial tension between the solder and substrate. Addition of Ti formed intermetallic

compounds that were found in the eutectic network around the primary Sn dendrites as well as

inside the dendrites suggesting that may have served as crystallization sites. This can be seen

in Figure 4-17. It is important to note that Ti precipitates were the finest among all the six

additives and this is evident in Figure 4-17 by comparing the precipitate size with the scale.

The presence of fine, randomly dispersed particles have been shown to improve the tensile,

drop and wetting properties in solder alloys [21, 23].

62

Figure 4-17: (Top) Fine Ti containing intermetallic compounds are formed and uniformely dispersed

through the bulk of the solder. (Bottom) Scale shows the size of particles to be < 2μm

63

4.6 Melting Behavior

The melting behavior of solder directly impacts the soldering process. SAC solders generally

melt around 217oC for SAC305 solders and around 227

oC for SAC105 solders. Alloys

melting at higher temperatures are generally avoided due to the detrimental effect of

temperature on chips and printed circuit board causing them to fail or warp. SAC105

containing 0.1wt% additives were analyzed using differential scanning calorimetry (DSC) to

observe any increase in melting temperatures. Figures 4-18 through 4-20 show the DSC

heating curves for all six solders.

64

Figure 4-18: DSC curves for SAC alloys containing (Top) Cerium and (Bottom) Lanthanum

Figure 4-19: DSC curves for SAC alloys containing (Top) Manganese and (Bottom) Nickel

65

Figure 4-20: DSC curves for SAC alloys containing (Top) Titanium and (Bottom) Yttrium

Based on the data shown in the DSC curves it can be seen that addition of 0.1wt% had a

negligible effect on the melting reaction of SAC105 and can safely meet the requirement of

present day Pb-free soldering processes.

66

4.7 Wetting Behavior

The six novel solders with additives were investigated for their wettability on pure electronic

grade copper coupons in a wetting balance as described in Chapter 3. The purpose of this

investigation was to compare the wettability of all six solders and to observe how they

compared with SAC105 and eutectic SnPb solder.

4.7.1 As-received (containing 1wt% additives)

The as received solders contained a significant amount of 4th

additive. The reason for adding

up to 1wt% of these elements was to make the fabrication process easier in the vacuum arc

furnace. The wetting curve for the as-received solders is shown in Figure 4-21.

Figure 4-21: Wetting force as a function of time for solders containing 1wt% additives, eutectic SnPb and

SAC105

67

The poor wettability of solders containing up to 1wt% rare-earth elements can be explained by

the oxidation of these active elements. Rare earth elements are active elements and over-

addition of these elements increase the surface tension due to oxidation. This is in

confirmation with numerous studies that investigated the wettability of rare earth doped

solders [24-26]. SnPb portrays the best wettability as expected due to a thin, coherent

intermetallic layer that it forms on copper substrate. Ti, Mn and Ni containing alloys

displayed wettability that was comparable to SAC105.

4.7.2 SAC105 + 0.1wt% Additives

The wetting curve for solders containing 0.1wt% additives is shown in Figure 4-22.

Figure 4-22: Wetting force as a function of time for solders containing 0.1wt% additives, eutectic SnPb

and SAC105

68

Major improvement in wettability was observed when the additives were decreased to 0.1wt%

in the case of metals as well as rare earth elements (RE). Ti and Mn containing alloys

showed the greatest improvement and compared close to eutectic SnPb solder. This can be

attributed to the segregation of intermetallic compounds on phase boundaries close to the IMC

layer which was observed in Figures 4-14 through 4-16. These compounds decreased the

surface tension and enhanced the solder’s capability to wet the surface. Addition of 0.1wt%

La also improved the wettability and surpassed the wetting force of SAC105.

4.7.3 Other wettability parameters

From the wetting force vs time curves a few more parameters can be established to compare

the wettability of solders namely:

- Wetting force at 2 seconds

- Time taken to 2/3 of maximum force

These have been plotted in Figures 4-23 and 4-24 respectively as a function of wt% additive.

69

Figure 4-23: Wetting force after 2 seconds as a function of wt% additive

From the above plot it is evident that SnPb still has the highest wetting force after 2 seconds

with Ti and Mn containing alloys very close to it at 0.1wt%. At higher wt% the wetting force

of all 3 RE containing alloys drop significantly.

70

Figure 4-24: Time taken to achieve 2/3 of maximum wetting force as a function of wt% additive

In Figure 4-24 it is established that at 0.1wt% Ti and Y containing RE alloys exhibit better

wetting times than SnPb. On the basis of the results obtained from Figures 4-22, 4-23 and 4-

24 it is clear that SAC105 + 0.1wt%Ti exhibits exceptional wettability that is better than

SAC105 and comparable to eutectic SnPb.

In order to achieve good wetting, several trial attempts were made to ensure that the copper

coupons wet the surface without sticking to the coupons as a lump. It was also necessary to

have sufficient temperature above the liquidus before the coupon was inserted in the molten

solder pellet because the coupon extracted the heat from the molten solder and resulted in

lumps attached to the coupon lead. It was established that these problems were discharged

when a temperature of 265oC was used and when the copper coupons were held above the

molten solder pellet for preheating. This ensured that the flux had sufficient temperature to

activate and remove any oxides present on the coupon’s surface. Figure 4-25 shows the results

71

of a few unsuccessful attempts (left) resulting in lumps attached to the coupon leads and the

perfectly wet surface with a uniform film of solder (right).

Figure 4-25: (Left to Right) Wettability improves as the solder temperature is increased from 250oC to

265oC and Cu coupons are preheated

72

4.8 Joint Mechanical Properties

The joint strength of all six novel Pb-free solders were evaluated by means of a Cu-solder-Cu

butt joint tensile test as mentioned in detail in Chapter 3. Solders were melted and poured into

a rectangular mold with uniform thickness and were cut into rectangular sections. They were

then held between copper blocks in a metal frame and the whole assembly was put in an

induction furnace to form a solder joint. Tensile tests were carried out 5 times for each sample

as described in the experimental section. The maximum tensile force was divided by the cross

sectional area to deduce the tensile strength of the joint. Five sets of experiments were carried

out for each solder and average was taken. A sample calculation for average tensile strength of

La containing alloy has been tabulated in Table 4-2 and the final results in Table 4-3.

Table 4-2: Sample calculation of determining the Average Joint Strength of SAC105 + 0.1 wt% La

Sample Area (mm2) Max F. (N)

Tensile

Strength (MPa)

Avg Tensile

Strength (MPa)

La1 11.86 930.80 78.48

78 MPa

La2 11.78 944.29 80.16

La3 11.81 926.45 78.45

La4 11.86 912.12 76.91

La5 11.82 915.79 77.48

Similar calculations resulted in the following average joint strengths (rounded to nearest

decimal):

Table 4-3: Average Calculated Joint Strength of Solders

73

The tensile strength results are plotted together in Figure 4-26.

Figure 4-26: Average Tensile Strength of solders containing different additives

4.9 Fracture Mode Examination

After the tensile tests the fracture surfaces were examined under SEM to investigate the nature

of fracture. It was established that there were three main modes of failure as shown in Figure

4-27:

1. Ductile fracture through the solder

2. Mixed fracture (ductile and brittle)

3. Brittle fracture near the interface

74

(a)

(b)

(c)

Figure 4-27: Different modes of fractures: (a) ductile and through solder, (b) ductile and brittle mixed,

and (c) brittle failure through interface separation

75

The fracture surfaces relating to these failure modes are shown in Figures 4-28 and 4-29.

(a) (b)

(c) (d)

Figure 4-28: (a) & (b): Microvoids and dimples observed in Ti and Ni containing samples. (c) & (d):

Intermetallic compounds present at the fracture surface of Ti and Ni containing samples

76

(a) (b)

(c) (d)

Figure 4-29: (a) Microvoids and flat fracture surface found in La, Ce, Mn containing samples. (b) & (c):

Large macrovoids formed in RE containing samples. (d) Brittle fractures in RE containing samples.

77

Figure 4-28 (a) and (b) show the expected fracture pattern for ductile materials containing

voids and dimples. These are observed in most metallic alloys and are caused by microvoid

nucleation and growth. Eventually when the microvoid density becomes sufficiently large

they fuse together and result in a energetically demanding ductile type of fracture. Figure 4-28

(c) and (d) confirm the presence of intermetallic compounds that are found on the fracture

surface. During tensile or drop test these fine intermetallic compounds may separate from the

matrix causing the nucleation of microvoids. This decohesion of small particles in the matrix

is one of the major source of microvoid nucleation and therefore such microstructures are

preferred. Ti and Ni containing alloys and SAC105 displayed this ductile fracture behavior.

The mixed (ductile and brittle) and brittle fractures are shown in Figure 4-29. Figure 4-29 (a)

shows regions of microvoids, dimples as well as relatively flat surfaces which means that the

extent of plastic deformation was less than as shown in Figure 4-28. La, Ce, Mn containing

alloys portrayed this mixed ductile and brittle nature. Finally, brittle fracture can be seen in

figure 4-29 (b), (c) and (d) where macrovoids and flat fracture surfaces prevailed. Y and some

Ce containing alloys exhibited extensive void formation as well as interfacial seperation

resulting in lower tensile joint strength. RE containing alloys required more flux to prevent

oxidation and this may have been the reason for macro-void formation due excessive flux

vapor that was unable to leave the molten solder.

4.10 Summary

Chapter 4 summarized the results obtained from experiments of this thesis. The chapter was

divied into 2 sections: (1) Results of mixed assembly experiements and (2) Results of doping

various elements to SAC105. The abovementioned results were justified using fundamental

principles of diffusion, mechanical behavior, microstrutural properties and wetting principles.

Addition of elements such as Ti improved the wettability and mechanical properties of

SAC105 whereas rare-earth elements proved to be detrimental to wetting characteristics,

contradicting some of the published studies.

78

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81

CHAPTER 5: CONCLUSIONS & FUTURE WORK

5.1 Conclusions

1. The microstructures obtained by soldering Pb-free solder with Sn-Pb paste are non-uniform

when the Sn-Pb reflow profile is used with peak reflow temperatures of 210oC. A peak reflow

temperature of 220oC and 29wt% paste ensures that the microstructure is uniform.

2. In the case of non-uniform, mixed assembly microstructures the interface between the

mixed region and the unmelted solder contained coarse, elongated intermetallics which were

considerably harder than the rest of the microstructure.

3. Pb introduction in solder changes the ternary composition from the regular SAC ternary [ L

↔ βSn + Cu6Sn5 + Ag3Sn ] to Sn-Pb-Ag ternary [L ↔ Ag3Sn + Pb + Sn ] which melts ~ 39oC

below SAC ternary and therefore service life of joint will be limited to 178oC.

4. Addition of 0.1wt% of additives to SAC105 formed additional intermetallic compounds in

the bulk solder as well as at the solder-substrate intermetallic interface layer.

5. Titanium, Nickel and Manganese improved the butt joint strength of solder joined to copper

coupons and exhibited ductile failure modes. Addition of RE elements (La, Ce and Y) induced

macrovoids within the bulk solder, thus reducing the strength of the joint by causing brittle or

mixed failure modes.

6. Addition of 0.1wt% Titanium further improved the wetting force and time of SAC105

solder. Therefore, based on this study, SAC105+0.1wt% Ti is a suitable replacement

candidate for SAC105 due to increased joint strength and better wettability.

82

5.2 Future Work

1. It would be worthwhile to examine the mechanical behavior of bulk SAC105 solder doped

with additives mentioned in this study in addition to the joint mechanical behavior which as

been analyzed in this study.

2. It will also be worthwhile to examine the effects of micro-additives on tin whisker

formation.

3. The effects of adding small amounts of additives on the drop test properties of SAC105

solder should be investigated to understand the impact behavior of these solders.