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Metcal Connection Validation Technology Characterization and Growth Kinetics of the Formation of Intermetallic

Compounds in the Liquid State during Soldering with Lead-Free Solders

Introduction

The technical paper contained in this document forms part of the research commissioned by Metcal to

develop a technology that provides real time feedback during in hand soldering to indicate the integrity of

the solder joint being formed.

It assumes the industry standard evaluation of a solder joint integrity to be the IMC (Inter-Metallic

Compound) depth and typified by the Cu6Sn5 compound which holds regardless of the tin solder alloy

employed.

By experiment, observation and cross-sectional analysis the paper guides the reader through to a

concluding methodology for determining IMC.

Objective

The objective of the research was to

• Identify the growth rates of the IMC during the liquidous phase

• The activation energy required and

• The resulting growth extent for a given energy input.

Once established to then look at controlling strategies for the IMC based on soldering duration and

temperature.

Results

Hence the paper describes how by empirical methods a mathematical description of the IMC formation

during a joints liquidous phase was developed based on variables of time and temperature that can

reasonably be controlled or measured during a hand soldering process.

This mathematical relationship developed and proven in the research forms the basis of the Metcal

Connection Validation technology.

To realize the technology in a practice the Metcal MX-5210 soldering platform had to undergo several

developments

• Upgrade of the on-board processor to run the Connection Validation Algorithm

• Tip Cartridges capable of bi-directional communication with the system processor

• An improved user interface to allow set up and data gathering

• A hand piece with a visual indicator

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These hardware upgrades to the MX platform resulted in the release of the CV-5210 and CV-510

systems which monitors and processes the solder joint power signatures and hence the thermal energy

demand during soldering.

The resulting data is fed into the on-board Connection Validation algorithm, alongside parameters from

the solder tip cartridge relating to its operating temperature, geometry and expected behavior. This

information together with the now known relationship of IMC growth during the liquidous phase of a solder

joint describe in this research paper, calculates the IMC depth and provides real time feedback to the

operator on successful soldering outcomes.

Research Summary

The research described in this paper developed the experimental techniques necessary for promoting

intermetallic layer growth throughout the melting and solidification cycle of the solder and then measuring

the thickness of these layers. Most of the literature to-date reported on aged soldered joints to investigate

the in-service intermetallic layer growth. To observe intermetallic compound (IMC) growth during the liquid

state of soldering required an experimental setup that quickly raises the temperature of the solder joint

followed by fast cooling (comparable to the actual joint conditions.) Temperatures at increments of 25

degrees above the soldering temperature and times of two, five and eight seconds for growth were

carefully controlled. After the experiments, the copper substrate/solder alloy interface was examined

using light microscopy, scanning electron microscopy (SEM) and SEM-EDS (Energy Dispersive

Spectroscopy). As expected, intermetallic phases such as Cu6Sn5, Cu3Sn, and Ag3Sn were observed,

with Cu6Sn5 being the major phase. Statistical analysis was performed for the IMC thickness data

obtained from the long profile analysis. Empirical equations capable of predicting the IMC layer

thicknesses were determined. Using Arrhenius analysis, the activation energy for intermetallic layer

growth was calculated to be approximately 7kCal/mol. This value is significantly larger than the value of

approximately 2kCal/mol reported in the literature for Cu6Sn5. Even considering the presence of Cu3Sn

(counted together with Cu6Sn5) which has about 15% higher activation energy of formation than Cu6Sn5,

the experimentally measured values were still higher. The discrepancy was attributed to the fact that the

values reported in the literature were mostly based on solid-state transformations while the

measurements and calculations in this work were limited to the intermetallic layer formation from liquid

state followed by growth.

1. Background Information

With the restricted use of lead in industrial applications, binary and ternary alloy systems as those formed

by Sn and elements such as Cu, Ag, Zn, Bi, and In have been developed as possible substitutes to the

Pb-Sn alloys. Sn-Cu and Sn-Ag in varied compositions form the two most common binary alloys. The

most widely used lead-free solder alloys are in the SAC (Sn-Ag-Cu) family. These systems have been

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extensively studied regarding their melting and flow characteristics, wetting and spreading behavior, and

mechanical performance.

In solder joints in general, there is also the interest to study the thermal aging behavior and the

deterioration of joint performance. Intermetallic layers form sometimes relatively early during the life-time

of the joined component at the substrate/solder interface. Due to the different physical properties of the

intermetallic compounds, cracks nucleate and grow in or around these phases, causing failures in the

joints and therefore failures in the components (Humpston and Jacobson). However, intermetallic

compound formation and growth is not limited only to the solid state. In many solder/substrate systems,

the reliability of the components depends on the quality of the joints, those created with the reaction layer

when the solder is still in the liquid state (Harman). Then, the understanding of the kinetics of the

formation of these intermetallic compounds during the joining process in the liquid state becomes very

important.

The research reported in this paper covers the development of an experimental technique necessary to

study the growth of the intermetallic compounds at the solder/substrate interface. This technique entails in

controlling the melting characteristics of the solder, the soldering cycle, and the solidification of the solder.

During solder melting, the characteristics of the substrate surface are as important as the thermal

properties of the solder, because both can enhance or inhibit the wetting behavior. The formation of an

intermetallic layer is linked to the quality of wetting during soldering Yu et al.) Findings such as the

obtained intermetallic layer growth rate, activation energy, and extent of growth can be utilized as part of

the controlling strategies to select soldering time and temperature. In fact, Metcal uses these factors in its

new Connection Validation soldering equipment that is being marketed to control the quality of the

soldered joints.

2. Intermetallic Compound Formation in the Liquid State

Formation of an intermetallic layer at the substrate/solder interaction line starts when the molten solder is

in contact with the solid substrate (Madeni). Therefore, the ability of the solder to wet and spread on the

substrate has a significant influence on the nucleation of the intermetallic compounds and the kinetics of

growth. Solder spreading initially depends on the thermal conditions (temperature and heat flow) and on

the molten solder; later, it involves translational motion of the molten metal flow on the substrate and

within itself by convection, also known as the Marangoni effect. During solder spreading, volume diffusion

of the elements from the substrate into the solder matrix and vice versa occurs in small amounts.

Simultaneously, even smaller amounts of dissolution of the substrate material into the solder occur. This

latter phenomenon is known as the base substrate erosion. The surface of the substrate influences the

formation of intermetallic compounds. If it is not completely smooth, uneven surfaces will allow for more

internal motion of the molten solder which may promote substrate dissolution and affect the local

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composition near the solder/substrate interface and intermetallic compound formation. According to

Boetting et al. the amount of substrate dissolution is related to its solubility in the specific solder, but the

amount of intermetallic compound formation depends more on the solubility of the active element in the

base metal. Both mechanisms depend on the time that the solder resides at temperatures greater than

the liquidus temperature. A schematic representation of this mechanism of intermetallic (Cu3Sn + Cu6Sn5)

formation is shown in Figure 1. As can be seen, the diffusion of both Sn and Cu, particularly at the molten

solder/substrate interface is essential in the reactions that form Cu6Sn5 and Cu3Sn.

3. Experimental Program

In this study, commercially pure copper was used as substrate, and the SAC305 solder as the lead-free

solder material. Copper sheets were sectioned into 25 x 12.5 x 0.2 mm strips and the lead-free solder

alloy wire was cut into cylinders of approximately 0.3 g each. Joining experiments were carried out to

produce solder coupons for analysis.

3.1. Experimental Matrix

The equilibrium binary phase diagram of Cu-Sn is shown in Figure 2. Even though the SAC solder also

contains silver, the amount of Ag is low enough that the solidification behavior of the solder alloy can be

described as a pseudo binary Cu-Sn alloy. The η phase and ε phase are the Cu6Sn5 and Cu3Sn,

respectively. These are the intermetallic phases that will likely form at the colder/substrate interface. As a

result of the lower liquidus temperature, Cu6Sn5 is likely to directly form from the liquid state according to

the following reaction.

𝐿 + ℰ → 𝜂

Figure 1 Schematic representation of the formation of intermetallic (Cu3Sn + Cu6Sn5) layer at the Cu/SAC305 interface. (Boetting et al.)

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Increasing copper concentration and copper atom diffusion in liquid tin at lower temperatures can also

result in the formation of Cu6Sn5.

In order to design the temperatures at which soldering experiments should be performed and to promote

the formation of the Cu6Sn5 intermetallic compound, the formation temperature of this compound is

needed. Jang et al. have performed differential scanning calorimetry (DSC) in small Cu6Sn5 pellets

produced by physical vapor

deposition. The reported DSC

curve is shown in Figure 3.

According to this curve, the

formation of Cu6Sn5 occurs at

approximately Tf=275°C,

therefore, three temperatures

were selected for the

production of the solder joints,

from T1=220°C, T2=275°C to

T3=300°C. T1 is just below

the eutectic temperature of

227°C in the Cu-Sn phase

Figure 2 Equilibrium binary phase diagram of Cu-Sn.

Figure 3 DSC curve for the formation of Cu6Sn5 intermetallic pellets (Jang et al.).

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diagram, T2 is at the Cu6Sn5 formation temperature, and T3 is high enough that should ensure the

formation of the intermetallic compound. Three tests per each Temperature-Time setting were run. The

experimental matrix is shown in Table 1.

As shown in the table, soldering temperatures between 220°C and 300°C and soldering hold times

between 2 and 8 seconds were used in the experiments. Three samples represented by the (x, x, x)

in Table 1 were prepared for each time-temperature condition, making a total of 27 solder samples.

3.2. Experimental Set-Up

Both the solder and substrate were cleaned prior to each

experiment. The solder pellet was cleaned using ethanol to

eliminate any residual oil on its surface. The copper substrate was

ground with 1200 grit grinding paper to provide an oxide-free

surface and cleaned with ethanol just before the soldering

experiment. The copper strips were placed in a holder using a heat

insulator so that heating and cooling of the sample are not affected

by heat transfer to and from the clamp and holder, Figure 4. This

figure also shows the location of a thermocouple at the immediate

adjacency of the molten solder to accurately measuring the actual

temperature of the experiment. The substrate remained static

throughout the experiment. The soldering iron was placed under,

but touching the lower surface of the substrate as shown in Figure

4. As the temperature rose during an experiment, a small amount

of solder would be introduced between the soldering iron tip and

the bottom side of the copper substrate to improve contact and

heat transfer to the copper substrate. The intimate contact between the solder iron tip and the copper

strip provided fast heating and cooling of the solder droplet. A controller, not shown in Figures 4, allowed

for the turning on and off of the soldering iron giving start and end to the melting experiments.

Table 1 Experimental matrix for the production of joints showing the Temperature and Times used.

Figure 4 Experimental set-up showing the copper substrate, soldering iron, and thermocouple positions.

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3.3. Temperature-Time (T-t) Control

The substrate temperature was continuously measured using a thermocouple that is positioned close to

the location of the solder button. Setting the soldering iron for the determined testing temperature allowed

for temperature control, which was activated when the soldering iron was turned on. When the substrate

temperature reached and maintaining a pre-set value, e.g. 220, 275, or 300°C, a previously weighed

piece of 0.30 g of SAC305 solder would be placed on the upper surface of the copper substrate. The

solder reaching molten state would be kept at the pre-set temperature for a determined amount of time (2,

5, or 8 seconds). Immediately after the hold time, the soldering iron would be turned off, removed from

under the substrate and the joint cooled down. All procedures including temperatures and holding times

were video-recorded and saved for further analysis, if necessary. A schematic representation of a

complete T-t cycle is shown in Figure 5. The representation of the actual soldering conditions is shown by

the two points on the curve, i.e. when the solder is applied and when the heat source is removed.

A more detailed description of the actual soldering time is shown in the plot in Figure 6.

When the temperature reached the target value of, for example, 300°C, the solder would be applied. As

melting began, the local temperature decreased slightly because of the absorption of heat by the solid

solder. After complete melting of the solder drop, the temperature again stabilized and joining proceeded

till the set time of eight seconds, in this specific example. During these eight seconds, the molten solder

would interact with the substrate to form the Cu6Sn5 intermetallic compound.

Figure 5 Schematic representation of a complete T-t soldering experiment cycle.

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3.3. Metallographic Preparation

All produced samples were cross-sectioned using a low-speed cutting machine, and cold mounted using

an epoxy resin. Cold mounting is required because of the relatively low curing temperatures (compared

with hot mounting) at which intermetallic compounds may form in the material system. Both procedures,

cutting and mounting, were chosen to avoid thermal aging and further growth of the intermetallic layer

that may not be specific to the liquid state transformations targeted in these experiments. Grinding of the

samples was performed down to 1200 grit on SiC grinding papers and polishing was done using 6μm,

3μm, and 1μm diamond paste. A one-hour vibratory polishing using colloidal silica (0.04μm) was

performed for each of the samples. This final polishing provided a flat surface with slight etching that

revealed the intermetallic compound layer. Optical micrographs were taken of all samples. The interfaces

between the copper substrate and the solder alloy were sequentially photographed under 500X or 1000X

magnifications, depending on the thickness of the intermetallic layer, and over a length of approximately

500μm. Figure 7 shows an example of the solder/substrate interface. The yellow arrows indicate the

varied thickness of the intermetallic layer.

Figure 7 Example of the intermetallic layer thickness measurements.

Figure 6 Temperature-Time cycle experienced by the solder during joining.

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4. Experimental Results

Results of the different tests are presented and discussed in the next sections; these include

characterization of existing samples produced using a commercial soldering iron, and analysis of a few

samples produced with the Metcal soldering iron.

4.1. Joint Interface Characterization in Existing Samples

Existing solder joint samples were analyzed to verify the statistical nature of the data, including the range

of intermetallic layer thicknesses. Once the samples were prepared for metallography, the joint interface

in each sample was identified under an optical microscope and the interlayer thickness measured. Figure

8 shows an example of the joint interface for Sample A, where the intermetallic layer Cu6Sn5 is clearly

identified. The intermetallic layer thickness was measured along the sample at intervals of 0.2mm and a

histogram of the measurements and its statistical parameters assuming a normal distribution are shown in

Figure 9. The mean thickness value (μA) for the distribution is 1.06μm with a standard deviation (sA) of

0.52μm. Since the histogram is skewed to the right slightly, with seven outliers found with respect to the

normal distribution. The morphology of the intermetallic layer is irregular. In some locations the thickness

of the intermetallic layer is uniform, but immediately next to that region, elongated needle-like

morphologies are found, some of which are the outliers.

In the case of Sample B, a representative micrograph is shown in Figure 10, and the respective

intermetallic layer thickness measurements in Figure 11. The histogram resembles a normal distribution

with a mean value (μB) of 1.57um and a standard deviation (sB) of 0.66um, although there is one outlier

in the right-hand side of the distribution. The morphology of the intermetallic layer is irregular in shape but

more consistent in thickness. These results, specially the range of thicknesses of the intermetallic layer,

clearly show the different morphology of Cu6Sn5 formation and growth; some needles grow long while

some others form irregular shapes. Therefore, the mean thickness values should be used with caution,

and should not completely ignore the thinner or thicker layer sections.

Figure 8 Micrograph of Sample A displaying the Cu6Sn5 intermetallic layer.

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Figure 9 Histogram of the intermetallic layer thickness for Sample A and its statistical parameters.

Figure 10 Micrograph of Sample B displaying the Cu6Sn5 intermetallic layer.

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Figure 11 Histogram of the intermetallic layer thickness for Sample B and its statistical parameters.

4.2. Joining Experiments with a Commercial Soldering Gun

The experiments were conducted as described in Section 3. The experiments yielded data that included

the T-t cycles for each test, the solder buttons joined to the copper substrate, the micrographs of the

SAC305/Cu interface displaying the intermetallic layer, and the measurements of the intermetallic

thickness.

The T-t data confirming the soldering cycles are shown in Figure 12. The plots are presented in a scale

that is easy to see the actual soldering temperatures and times. For simple comparison, all the plots used

the same X-Y scales. As can be observed from the T-t evolution, once the copper substrate reached the

set temperature, the solder cylinder was placed on the copper surface and the time measurement

commenced. When the solder is placed on the copper surface it is still in the solid state (solder-solid),

then the time for the solder to completely melt is measured (solder-liquid); and when the heat is removed

the time is also measured (Heat-removed); these points are marked on each T-t graph. Cooling occurs as

soon as the heating source is turned off then the joint is cooled down in ambient conditions to room

temperature. In general, the temperatures were kept to ±3oC, and the time to ±0.5 s. The T-t plots in the

left are for soldering time of 8s at 300oC, 275oC, and 220oC, from top to bottom. The plots in the middle

are for 5s soldering time at the three temperatures, and the plots at the right are for soldering time of 2s.

These plots clearly demonstrate the exact soldering times and the rapid cooling that occurred after the

heat source was removed.

An example of the resulting Solder/Cu joint coupon is shown in Figure 13. The solidified, semi-

hemispherical shaped solder attached to the copper substrate and the surface of the copper next to the

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solder show signs of uniform heat exposure suggesting that the temperature at the interface is consistent

throughout the Solder/Cu interaction area. Therefore, the formation of the intermetallic phase should

occur simultaneously all over the contact area.

Figure 14 displays the joint interface micrographs for one sample in each T-t setting in the experimental

matrix. It is observed that mainly one irregular layer of η-phase (Cu6Sn5) intermetallic compound formed

at the joint interface. However, in some cases, this phase takes on a flat scallop-like shape and in others

the needle-like morphology growing in different orientations into the solder matrix. Both morphologies

make up the thickness of the intermetallic layer. These data demonstrate the fast kinetics of the process

in the liquid state, i.e. intermetallic compounds can form in the order of seconds, especially the

morphologies with specific orientations such as the needles.

Figure 12 Temperature-time cycles for each soldering setting in the experimental matrix

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The micrographs of the reaction layers clearly show the effect of increasing processing temperature and

time. At low temperature (220°C), even if the process time increases from two to eight seconds, the

thickness of the intermetallic layer is miniscule, and the morphology of the layer is somewhat uniform,

without any lenticular-shaped features. At 275°C, the intermetallic layer shows slightly more irregularity

Figure 13 Example of the resulting Solder/Cu joint. The uniform coloration around the solder point reminds of uniform heating.

T=220°C, t=2s T=220°C, t=5s T=220°C, t=8s

T=275°C, t-2s T=275°C, t-5s T=220°C, t=8s

T=300°C, t=2s T=300°C, t=5s T=300°C, t=8s

Figure 13 Optical microscopy micrographs of the SAC305/Cu interface at the different T-t cycles stated in the experimental matrix.

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with some intermetallic particles starting to grow in more random orientations; most of the interface,

however, remained uniform. At the higher temperature (300°C), the morphology of the layer becomes

irregular; needles are thicker and grow in different orientations into the solder matrix. The thickness of the

layers is also much larger. The effect of time on the thickness of the needles at this temperature

appeared to be most prevalent.

Measurements of the intermetallic layer thickness were performed on each micrograph, at 0.2mm space

intervals. The results were plotted in histograms and shown in Figure 14. Statistical analysis was also

conducted for each test condition. Table 2 shows the statistical summary of the measurements. For

220oC, the intermetallic layer varied from 0.38 to 0.69μm; for 275oC, the intermetallic layer coarsened

with thickness ranged between 0.97 to 1.63μm; and for 300oC, the intermetallic layers were the thickest,

from 1.48 to 2.80μm.

Sample No. T (°C) t(s) Mean Value of IMC

Thickness (μm) Std. Dev.

(μm)

27 220 2 0.38 0.125

26 220 2 0.39 0.124

Table 2 Statistical summary of the intermetallic compound thickness for all the tests stated on the experimental matrix. Four additional tests at 275°C conducted for 3, 4, 6, and 7s were also included in the table.

Figure 14 Example histograms of the intermetallic layer measurements for selected experiments in the experimental matrix. Assuming normal distribution, the statistical parameters are also shown.

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25 220 2 0.38 0.116

24 220 5 0.59 0.163

23 220 5 0.65 0.212

22 220 5 0.47 0.153

21 220 8 0.70 0.217

20 220 8 0.62 0.221

19 220 8 0.69 0.204

18 275 2 1.05 0.372

17 275 2 1.04 0.363

16 275 2 0.97 0.313

275 3 1.12

275 4 1.24

15 275 5 1.16 0.32

14 275 5 1.32 0.41

13 275 5 1.16 0.33

275 6 1.54

275 7 1.63

12 275 8 1.36 0.39

11 275 8 1.39 0.34

10 275 8 1.24 0.33

9 300 2 1.54 0.45

8 300 2 1.52 0.42

7 300 2 1.48 0.48

6 300 5 2.03 0.59

5 300 5 1.95 0.68

4 300 5 2.22 0.63

3 300 8 2.51 0.64

2 300 8 2.65 1.13

1 300 8 2.80 1.06

5. Growth Kinetics of the Intermetallic Compound Layer

The kinetics of the IMC layer growth is governed by the time the solder dwells on the copper substrate in

the liquid-state at a specific temperature. For the analysis conducted in this work, the thickness of the

intermetallic compound layer was measured for the different samples produced at different times and

temperatures according to the experimental matrix shown in Table1. Also in this analysis, the IMC layer is

considered to be made up of the η-Cu6Sn5 phase only. In comparison, the ε-Cu3Sn phase is small and

negligible, and therefore not separately used in the analysis. As was observed in Figure 14, the thickness

of the IMC layer is varied, mostly with a unimodal distribution. For that reason, the mean thickness of the

layer is used for comparison.

The mean IMC thickness values are 0.38μm, 0.57μm, and 0.67μm for the samples processed at the

temperature of 220°C and times of 2s, 5s, and 8s. For 275°C, the mean IMC thicknesses are 1.02μ,

1.21μm and 1.33μm for the times of 2s, 5s, and 8s. As expected, for the soldering experiments conducted

at 300°C, larger thicknesses of 1.51μm, 2.07μm, and 2.65μm were observed for 2s, 5s, and 8s growth.

Additional samples were produced at 275°C at 3, 4, 6, and 7s, the IMC thicknesses measured were

1.12um, 1.24um, 1.54um, and 1.63um, respectively.

5.1. Determination of k and Q

Typical solid-state diffusion-controlled growth behavior of a phase can be described using a parabolic

equation of the type as stated in Equation (1),

𝑥 = 𝑘1

2𝑡1

2

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where x is the mean value of the IMC thickness, k is the growth constant, and t is the time of IMC

formation in the liquid-state. If k is known, then the thickness of the intermetallic layer for a particular

temperature (T) can be calculated as a function of dwell time (t) alone. However, the actual mechanism of

an intermetallic growth layer from the liquid state will also involve liquid solder development, liquid metal

flow behavior, substrate erosion, diffusion of Cu and Sn, reaction between Cu and Sn, amongst other

factors. As such, the parabolic behavior may not be observed. Additionally, for very short reaction times

of two, five and eight seconds, the rate equation may tend to follow a linear form,

𝑥 = 𝐴 + 𝑘𝑡

where k is the growth constant.

Plotting the thickness data collected in Table 2 as a function of time, the values for k were determined for

each of the three temperatures, Figure

15. Indeed, the linear relationship

described well the thickening behavior of

the intermetallic layer. It is the belief of

the authors that the coefficients, A and k

from the three graphs, can be used for

predicting Cu6Sn5 growth in the

processing conditions by Metcal.

The slopes (ki) of each of the three

graphs in Figure 15 contain

thermodynamic and kinetic meanings as

k can be written in the form of the

Boltzmann distribution function shown

below.

𝑘𝑇 = 𝑘𝑜exp (−𝑄

𝑅𝑇)

Figure 15 Intermetallic layer thickness as a function of time and temperature.

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Plotting the logarithm of kT as an inverse function of temperature (T), the slope of the new graph shown

in Figure 16 will contain Q , in the form of 𝑄𝑅 , which is the activation energy of formation of the

intermetallic compound. The quantity R represents the universal gas constant.

5.2. Comparison of K with literature

When compared with the data obtained for soldering alloys in the literature (Table 3) of approximately

24kCal/mol (1.0eV/atom) for Cu6Sn5 and 19kCal/mol (0.8eV/atom) for the combined Cu6Sn5 and Cu3Sn

layer, the value derived from this work of 7kCal/mol is significantly smaller. It appears that the liquid state

reactions lowered the activation energy of the process. Faster diffusion and greater abundance of the

reacting species in the liquid state must significantly assist in the kinetics of the intermetallic formation

process. The discrepancy was then attributed to the fact that the values reported in the literature were

mostly based on solid-state transformations while the measurements and calculations in this work were

focused on the intermetallic layer formation from liquid state followed by growth. Continued search of

literature on Cu6Sn5 and Cu3Sn formation brought additional thermodynamic data on the formation of the

two intermetallic compounds. When Cu and Sn are mixed together, Cu3Sn was found to exhibit higher

formation energies, between 7.7 and 8.4 kJ/mol, or approximately 21

2.0kCal/mol, Table 4. Literature also yielded 5.8 to 7.0kJ/mol (approximately 1.6kCal/mol) for the

formation of Cu6Sn5.

Figure 16 Arrhenius Plot for calculation of Q, activation energy, of the intermetallic compound growth.

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Table 3 Activation energies of Cu6Sn5 and Cu3Sn formation reported in the literature.

Joint System

Q (eV/atom) Total Q

(eV/atom)

Reference

Cu3Sn Cu6Sn5

Cu/Sn-3.5Ag

--- 1.20 1.20 Choi et al.22

0.73 1.11 --- Flanders et al.13

0.72 0.93 0.85 This work

--- --- 0.80 Harris et al.5

--- --- 0.67 Lee et al. 23

0.52 --- 0.61 Vianco et al.17

Cu/100Sn 0.74 0.83 --- Vianco et al.20

Table 4 Formation energies of Cu6Sn5 and Cu3Sn reported in the literature. Formation energies of Cu6Sn5 and Cu3Sn reported in the literature.

3𝐶𝑢(𝑠) + 𝑆𝑛(𝑠) → 𝐶𝑢3𝑆𝑛(𝑠)

-8.2 ± 1kJ/mol H. Flandorfer et al., Interface in lead-free solder alloys: Enthalpy of formation of binary Ag-Sn, Cu-Sn and Ni-Sn intermetallic compounds, 2007

-7.82 ± 0.2 kJ/mol J.B. Cohen, J.S.L.I. Leach, M/B. Bever, JOM 6 (1954) 1257-1258

-7.81 ± 0.2 kJ/mol O.J. Kleppa, J. Phys. Chem. 60 (1956) 852-858

-7.53 ± 0.2 kJ/mol R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys, ASM Metals Park, OH, USA, 1971

-8.36 kJ/mol W. Blitz, W. Wagner, H. Pieper, W. Holverscheit, Z. Anorg. Chem 134 (1924) 25-36

-7.7 ± 0.5kJ/mol O. Kubaschewski and J.A. Catterall, Thermochemical Data of Alloys, Pergamon, New York, 1956

-7.6 kJ/mol N. Sauders, A.P. Mlodownlk, The Copper-Tin System, Bulletin of Alloy Phase Diagrams, Vol. 11 No 3, 1990

6𝐶𝑢(𝑠) + 5𝑆𝑛(𝑠) → 𝐶𝑢3𝑆𝑛5(𝑠)

-5.8 ± 0.6kJ/mol O. Kubaschewski and J.A. Catterall, Thermochemical Data of Alloys, Pergamon, New York, 1956

-6.1 ± 1kJ/mol H. Flandorfer et al., Interface in lead-free solder alloys: Enthalpy of formation of binary Ag-Sn, Cu-Sn and Ni-Sn intermetallic compounds, 2007

-7.03 ± 0.05kJ/mol A. Gangulee, G.C. Das, M.B. Bever, Metall. Trans. 4, 1973, p. 2063-2066

6. Electron Microcopy Examination of the Solder/Substrate Interface

Electron microcopy further illustrated the irregular layer morphology at the substrate-solder interface as

shown in Figure 17. Particularly at higher temperatures and longer times, the interlayer began to take on

preferred orientations as well as non-uniform thicknesses. However, detailed electron microscopy

analysis of the interfacial features with regard to crystallographic orientation falls outside the scope of this

work. Sufficient evidence has been provided throughout this report to support the characterization work of

Cu6Sn5 and the measurement of the interlayer thickness, as well as the verification of the statistical

nature of the intermetallic compound growth.

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7. Summary The research described in this paper developed the experimental techniques necessary for promoting

intermetallic layer growth throughout the melting and solidification cycle of the solder and then measuring

the thickness of these layers. To observe intermetallic compound (IMC) growth during the liquid state of

soldering required an experimental setup that quickly raises the temperature of the solder joint followed

by fast cooling. Soldering experiments were conducted at 220, 275 and 300oC for 2, 5 and 8 seconds to

promote formation and growth of the intermetallic compounds when the solder is still in the molten state.

Light microscopy, SEM and SEM-EDS clearly defined the intermetallic compounds as well as their

thicknesses. As expected, intermetallic phases such as Cu6Sn5, Cu3Sn, and Ag3Sn were observed, with

Cu6Sn5 being the major phase. Long profile analysis of the intermetallic layers showed non-uniform and

preferred orientation of the intermetallic phases, particularly under high temperatures and long times.

Empirical kinetic equations with growth coefficients were developed for predicting the IMC layer

thicknesses. Using Arrhenius analysis, the activation energy for intermetallic layer growth was calculated

Sample 27, 220°C,2s

Sample 23, 220°C,5 Sample 21, 220°C,8s

Sample 16, 275°C,2s

Sample 14, 275°C,5 Sample 12, 275°C,8s

Sample 7, 220°C,2s

Sample 5, 220°C,5 Sample 3, 220°C,8s

Figure 17 SEM micrographs for selected samples in the experimental matrix illustrating the intermetallic compound layers and their non-uniform thickness and growth direction. Additional information for each of the samples is given in Table 4.

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to be approximately 7kCal/mol. This value is significantly smaller than the values of 24kCal/mol and

19kCal/mol reported in the literature for Cu6Sn5 and for combined Cu6Sn5 and Cu3Sn layer,

respectively. The discrepancy was attributed to the fact that the values reported in the literature were

mostly based on solid-state transformations while the measurements and calculations in this work were

focused on the intermetallic layer formation from liquid state followed by growth.

8. Conclusions and Recommendations

The carefully planned and executed experiments in this work confirmed that intermetallic compounds

began to form and growth even when the solder was still in the molten state. Cu6Sn5 was created

between the SAC305 solder and Cu substrate. Measurements of the intermetallic layer at regular

intervals increased the statistical reliability of the characterization process. The intermetallic layer was

found to grow from 0.37μm at T=220oC and t=2s to 2.80μm at T=300oC and t=8s as the soldering

processing time and temperature increased. Additionally, the thickness range also increased significantly

from 0.16-0.71μm to 1.26-7.50μm, respectively.

The non-uniform thickness and morphology, and random orientation of the intermetallic phases,

particularly those formed under high temperatures and long times, indicate that mean thickness values

should be used judiciously when attempting to predict growth behavior and mechanical properties.

Empirical kinetic equations with growth coefficients were developed for predicting the IMC layer

thicknesses with time for three temperatures within typical soldering processing range. The data obtained

in this study can reliably be used for controlling soldering processing conditions in the liquid state. Finally,

the activation energy for intermetallic layer formation and growth in the liquid state was calculated to be

approximately 7kCal/mol.

9. References Consulted in this Research

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