Interfacial interactions during soldering with

low melting Bi-In alloys

Aranav Das

M.Sc. Thesis Report

Interfacial interactions during

soldering with low melting Bi-In alloys

Authored by: Aranav Das, 4506464

in partial fulfilment of the requirements for the degree of MSc Mechanical Engineering (MEA)

at Delft University of Technology

to be defended publicly on Tuesday August 29, 2017 at 15:00 hrs.

An electronic version of this thesis is available at: http://repository.tudelft.nl/

Project Duration: December 2016- August 2017

Thesis Committee: Dr. Ir. M.J.M Hermans (supervisor) Delft University of Technology

Prof. Dr. I. Richardson Delft University of Technology

Dr. Ir. M. Janssen Delft University of Technology

Dr. Mo Biglari Mat-Tech B.V.

i

Acknowledgements

Writing a masters thesis is a task which without help cannot be done, not only technically but

also personally.

Firstly, I would like thank Dr. Mo Biglari and Dr. Co van Veen, for giving me an opportunity

to work and pursue my graduation project at Mat-Tech. Both have been really supportive

about my desire to pursue the project with an open mind while also giving their valuable

inputs wherever necessary. My sincere gratitude to Dr. Sasha Kodentsov, mentor and

supervisor, who guided me throughout this entire duration while also enlightening me on

stuff that was not soldering!!

I also express my deepest gratitude to Dr. Ir. Marcel Hermans, my educational supervisor and

mentor, who was really supportive of the progress that I made (sometimes no progress was

made) while also giving me great advice and pushing me to do a better job.

I would like thank Ludo Krassenburg and Ir. Henk Schoonderwalt for allowing me to work

freely at the Mat-Tech facilities but also teaching me to respect the work place which has

been a valuable experience and I feel better equipped now. Also, a thank you to the lab

managers at the University for helping me with any issue that I needed to deal with.

Finally, a thank you to my parents, without whose backing and support I wouldn’t have

reached this far. To my friends, Atish, Rishabh and Zameer, who have helped me tide past the

tough times and have been a constant support throughout. A thanks also to all my friends here

at the University, who have made this experience and journey much easier and fun.

Thank You!

ii

Abstract

Soldering of a large variety of highly stress sensitive electronic devices can be a strenuous

affair. Thermo-mechanical stresses built up upon cooling after the soldering process can

profoundly affect the overall performance of such devices, sometimes leading to failure. To

reduce such detrimental effects, the soldering temperature has to be kept as low as possible

with high amounts of precision.

In this respect, a novel low melting ordered alloy i.e. Bi-In alloy are worthy of consideration.

These alloys can be considered to be the “low temperature counterpart” of a superalloy

system while exhibiting “superalloy” like properties. It will be shown in the current work that

the vapour phase soldering method with its uniform heating capabilities while maintaining

temperature control is particularly suitable for “fluxless” soldering at low temperatures.

Solidification and wetting behaviour of the Bi-In based solders (BiIn34% and BiIn50%) on

commonly used electronic substrates like Cu, Ni, Pd, Au and Ag has been discussed.

Also, the relatively high service temperature for such soldered joints poses serious questions

concerning the influence of the reaction products at the solder/substrate interface on the

performance and reliability of such joints. Therefore, interfacial reactions occurring between

the solder and the metallization constituent upon processing (reflow) and subsequent

annealing have also been rationalized.

iii

Contents Acknowledgements ................................................................................................................... i

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

List of Abbreviations ............................................................................................................... v

Chapter 1 .................................................................................................................................. 1

Introduction .............................................................................................................................. 1

1.1 History of Soldering ......................................................................................................... 1

1.2 Solders and Fluxes ........................................................................................................... 2

1.3 Lead Free Solder in Microelectronics .............................................................................. 3

1.4 Bi-In System ..................................................................................................................... 4

1.5 Aim of the Present Investigation ...................................................................................... 6

Chapter 2 .................................................................................................................................. 7

Theoretical Framework ........................................................................................................... 7

2.1 Binary Multiphase Diffusion ............................................................................................ 7

2.1.1 Diffusion path in a ternary system ........................................................................... 11

2.2 Reactive Diffusion in Solids .......................................................................................... 12

2.3 Error Sources in the Analytical and Experimental Technique ....................................... 15

Chapter 3 ................................................................................................................................ 17

Experimental Setup ............................................................................................................... 17

3.1 Vapour Phase Soldering ................................................................................................. 17

3.2 Experimental Arrangement For Soldering ..................................................................... 18

3.3 Annealing Procedure ...................................................................................................... 20

3.4 Sample Preparation ........................................................................................................ 20

3.5 Analysis .......................................................................................................................... 21

Chapter 4 ................................................................................................................................ 23

BiIn34% on Common Electronic Substrates ...................................................................... 23

4.1 Reflowed BiIn34% on Common Electronic Substrates ................................................. 23

4.2 Aged BiIn34% on Common Electronic Substrates ........................................................ 26

4.2.1 Aged BiIn34% on Au .............................................................................................. 26

4.2.2 Aged BiIn34% on Cu .............................................................................................. 30

4.2.3 Aged BiIn34% on Ag .............................................................................................. 34

4.2.4 Aged BiIn34% on Ni ............................................................................................... 37

4.2.5 Aged BiIn34% on Pd ............................................................................................... 40

iv

Chapter 5 ................................................................................................................................ 43

BiIn50% on Common Electronic Substrates ...................................................................... 43

5.1 Reflowed BiIn50% on Common Electronic Substrates ................................................. 43

5.2 Aged BiIn50% on Common Electronic Substrates ........................................................ 46

5.2.1 Aged BiIn50% on Au .............................................................................................. 46

5.2.2 Aged BiIn50% on Ag .............................................................................................. 49

5.2.3 Aged BiIn50% on Ni ............................................................................................... 50

5.2.4 Aged BiIn50% on Pd ............................................................................................... 51

5.2.5 Aged BiIn50% on Cu .............................................................................................. 52

Chapter 6 ................................................................................................................................ 53

General Discussion ................................................................................................................. 53

Chapter 7 ................................................................................................................................ 55

Conclusions and Future Study .............................................................................................. 55

7.1 Conclusions .................................................................................................................... 55

7.2 Future Study ................................................................................................................... 55

Summary ................................................................................................................................. 57

References ............................................................................................................................... 59

v

List of Abbreviations

RoHS Restriction of Hazardous Substances Directive

PbO Lead Oxide

SAC Tin-Silver-Copper

Co-Ni-Si Cobalt-Nickel-Silicon

OPS Oxide Polishing Suspension

AES Auger Electron Spectroscopy

EPMA Electron Probe Microanalysis

SEM Scanning Electron Microscopy

BES Backscattered Electron Shadow

EDS Electron Dispersive Spectroscopy

Bi Bismuth

In Indium

Au Gold

Ag Silver

Ni Nickel

Cu Copper

Pd Palladium

1

Chapter 1

Introduction

1.1 History of Soldering Soldering is a process in which two or more items (usually metal) are joined together by

melting and adding a filler metal (solder) into the joint, the filler metal having a lower

melting point than the adjoining metal. Soldering differs from welding in that soldering does

not involve melting the work pieces. In principle, soldering also differs from brazing in the

fact that brazing is performed at temperatures above 300°C, even though the process is the

same. In the past, nearly all solders contained lead, but environmental and health concerns

have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

The earliest traces of hard solder (i.e. liquidus above 300°C) that are found date about as

early as 5000 years ago in Mesopotamia as seen in figure 1. Commonly used solders were

Au-Cu, Ag-Cu and Pb-Cu. [1]

Fig. 1 One of the oldest known images of soldering. An Egyptian goldsmith

soldering using a blower on coal fire. Painting found in a tomb, Thebe, 1475 BC. [1]

2

In soft soldering, tin (Sn) was used in order to lower the melting point of the solder. Even

though it was common practice, only a guess can be made about the origins as only limited

artifacts have been discovered with Sn being unknown to Egyptian society until 2000 BC and

soldering being a slave’s task.

1.2 Solders and Fluxes Soldering filler materials are available in many different alloys for differing applications. In

electronics assembly, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost

identical in melting point) has been the alloy of choice. Some examples of soft-solder are tin-

lead for general purposes, tin-zinc for joining aluminium, lead-silver for strength at higher

than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for

aluminium and corrosion resistance, and tin-silver and tin-bismuth for electronics.

A eutectic formulation has advantages such as: the liquidus and solidus temperatures are the

same which means there is no plastic phase, and it has the lowest possible melting point.

Having the lowest possible melting point minimizes thermal load on electronic components

during soldering and having no plastic phase allows for quicker wetting as the solder heats

up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the

temperature drops through the liquidus and solidus temperatures. Any movement during the

plastic phase may result in cracks, resulting in an unreliable joint. [3]

Common solder formulations based on tin and lead are listed below. The fraction represents

percentage of tin first, then lead, totaling 100%:

63/37: melts at 183 °C (eutectic: the only mixture that melts at a point, instead

of over a range)

60/40: melts between 183–190 °C

50/50: melts between 183–215 °C

For environmental reasons (and the introduction of regulations such as the European RoHS

(Restriction of Hazardous Substances Directive)), lead-free solders are becoming to be

widely used. Unfortunately, most lead-free solders are not eutectic formulations, melting at

around 250 °C, making it more difficult to create reliable joints with them [3]. The most

commonly used lead free solder is the tin-silver-copper (SAC). It is a near eutectic having

adequate thermal stability, strength and wettability. Results have shown that it outperforms

the lead containing solder for the same number of thermal cycles, thus making it the solder of

choice. [3]

Other common solders include low-temperature formulations (often containing bismuth),

which are frequently used to join previously-soldered assemblies without un-soldering earlier

connections, and high-temperature formulations (usually containing silver) which are used

for high-temperature operation or for first assembly of items which must not become un-

soldered during subsequent operations. Alloying silver with other metals changes the melting

point, adhesion and wetting characteristics, and tensile strength. Of all the alloys, silver

solders have the greatest strength and the broadest range of applications. Specialty alloys are

3

available with properties such as higher strength, the ability to solder aluminum, better

electrical conductivity, and higher corrosion resistance.

The purpose of flux is to facilitate the soldering process. One of the obstacles to a successful

solder joint is an impurity at the site of the joint, for example, dirt, oil or oxidation. These

impurities can be removed by mechanical cleaning or by chemical means, but the elevated

temperatures required to melt the solder encourages the work piece (and the solder) to re-

oxidize. This effect is accelerated as the soldering temperatures increase and can completely

prevent the solder reflow on the workpiece. One of the earliest forms of flux was charcoal,

which acts as a reducing agent and helps prevent oxidation during the soldering process.

Some fluxes go beyond the simple prevention of oxidation and also provide some form of

chemical cleaning (corrosion). For many years, the most common type of flux used in

electronics (soft soldering) was rosin-based, using the rosin from selected pine trees. [3]

It was ideal in that it was non-corrosive and non-conductive at normal temperatures but

became mildly reactive (corrosive) at the elevated soldering temperatures. Plumbing and

automotive applications, among others, typically use an acid-based (hydrochloric acid) flux

which provides cleaning of the joint. Many fluxes also act as a wetting agent in the soldering

process, reducing the surface tension of the molten solder and causing it to flow and wet the

work-pieces more easily. Figure 2 shows the first Philips printed wire board (PCB) for a radio

in 1954. It used conventional lead containing solder for this PCB.

1.3 Lead Free Solder in Microelectronics Soldering of large variety of highly stress-sensitive electronic devices (e.g. Si-based Photo

Multipliers, etc.) to the printed boards can be a strenuous affair. Thermo-mechanical stresses

built up upon cooling after soldering can profoundly affect the overall performance of such

devices. In order to reduce this detrimental effect, soldering temperature has to be kept as low

as possible.

It is common misconception that lead free soldering requires higher soldering temperatures

than lead/tin solder; the wetting temperature in lead/tin solder is higher than the melting point

and is the controlling factor. Nevertheless, many new technical challenges have arisen with

this endeavour; to reduce the melting point of tin-based solder alloys various new alloys have

had to be researched, with additives of copper, silver, bismuth as typical minor additives to

reduce melting point and control other properties. Lead-free construction has also extended to

components, pins, and connectors.

For a joining process to be adopted, it is essential that the assembling process is developed in

a reliable and cost-effective manner. In this respect, Bi-In based alloys are worthy of serious

consideration mainly for two reasons: 1) these materials are inexpensive and available in

sufficient quantity, and 2) there exist several binary compounds and eutectic compositions in

the Bi-In system with melting point (liquidus temperature) below 120 C. An example of Bi-

In alloys being used for is in photo multipliers used in the scanners in the medical industry.

4

Fig. 2 The first Philips printed wire board in 1954. [2]

With the increase in use of lead free solders, the need to choose the correct soldering

technique has become paramount. This is because these solders operate at a liquidus

temperature below 300°C and any overshooting of this temperature is harmful to the soldered

joints.

While conventional techniques like soft soldering and silver soldering have been used

extensively, for the current work, a relatively new technique called, “vapour phase soldering”

has been used. The reason behind employing this technique has been explained in detail in

chapter 3. Also, to mimic the behaviour and the service life of the soldered joints at service

temperature, annealing has been performed.

Before moving forward, it will be helpful to understand some of the basics behind the Bi-In

solder system.

1.4 Bi-In System As explained in section 1.2, the Bi-In solder system presents itself as a real alternative to the

lead free solders. Even though the system has been around as an alternative, lack of study and

sufficient information has meant that the system has not been able to breakthrough. The Bi-In

solder comes in three different compositions (in weight percent) namely:

Bi67%-In33%

Bi51%-In49%

Bi33%-In67%

5

Fig. 3 Equilibrium Bi-In System Phase Diagram according

to weight percentage of Indium. [22]

According to Ueshima et al. [4], the BiIn compound has a crystal structure of the lead oxide

(PbO) type and is an ordered solder i.e., it is a congruently melting intermetallic. In other

words, the intermetallic is the low temperature counterpart of a superalloy. It is determined

that the composition Bi51%-In49% is a near eutectic alloy having a eutectic melting

temperature of 88.9°C and its constituent phases are all 100% intermetallic compounds

(Bi3In5 and BiIn2) as can be confirmed from figure 3. All these phases as mentioned are

100% intermetallic which means that they exhibit great mechanical properties up until their

melting point. In addition these compounds exhibit greater creep resistance, strength retention

and ductility compared to some of the present SAC solders which are used in the industry,

making it a reliable low temperature solder.

6

1.5 Aim of the Present Investigation As explained in section 1.3, the possibility of research on lead free solders is a broad topic.

Since it is not possible to cover every alternative alloy, the research of this project is limited

to the Bi-In alloy system.

The formation of a new solid phase and the development of a microstructure during chemical

interaction is certainly one of the most fascinating phenomena in solid state science. Since

interfacial reactions effectively control properties and performance of soldered interconnects,

it is imperative for further progress in this field to be able to predict (and control) the reaction

behaviour and morphological evolution of the solder-substrate system.

Although small changes in chemical composition of the interfacial region between solder

alloy and substrate (or metallization constituent) can lead to huge changes in macroscopic

behaviour of the soldered assembly, the underlying mechanisms are but limitedly understood.

The present project aims at providing experimentally validated phenomenological models to

remedy, at least partially, this situation and addresses the following issues:

Evaluation of wetting behaviour of Bi-In based solders on various substrates

commonly used in electronic industry (Cu, Ni, Ag, Au and Pd).

Experimental study of solidification and morphological evolution of the low-melting

Bi-In alloys during reflow on common substrates and metallization constituents.

Investigation of the interfacial reactions occurring between Bi-In alloys and

metallization constituents upon soldering and subsequent annealing of the soldered

interconnects.

Application of equilibrium thermodynamics (phase diagrams) and diffusion kinetics

for elucidation of the reaction phenomena occurring in the solder-substrate systems.

7

Chapter 2

Theoretical Framework

The work for this thesis deals with the interaction between the solder and the substrate

surface. To elucidate this, at this point it is worthwhile to give some general comments about

multiphase diffusion with respect to binary and ternary systems. These comments will

constitute as a natural introduction to this subject and the current work.

A phase diagram is the visual representation of a thermodynamic interaction of phases under

a given set of conditions. Mostly, equilibrium phases diagrams are used which can be

constructed using prior available data. Since prior knowledge of these low melting solder

interaction with a substrate is limitedly found in literature, data has to be determined

experimentally [5]. Furthermore, it is essential to understand the reaction kinetics of the

system for which it is helpful to know diffusion mechanisms involved such as the binary

phase diffusion.

2.1 Binary Multiphase Diffusion If two solid materials are in contact at a sufficiently high temperature, diffusion will occur. If

the materials are completely miscible at that temperature, the concentration profiles are more

or less smooth without any discontinuity. But, if they are only partially miscible or they react

to form new phases, discontinuities occur that are closely reflected by the phase diagram of

the system.

Figure 4 shows an example of a binary system A-B, where compounds γ and δ are formed

additionally to α and β. After sufficient annealing, a concentration profile is obtained for

component B. The exact profile for the various phases cannot be predicted until the diffusion

coefficients with respect to composition are known. The reason for the development of only

straight interfaces with fixed composition gaps follows directly from the phase rule. Three

degrees of freedom are required to fix temperature and pressure and to vary the composition.

Reaction morphologies consisting of two-phase structures (i.e. precipitates or wavy

interfaces) are, therefore, thermodynamically forbidden since then only two degrees of

freedom are allowed, assuming that only volume diffusion takes place. For a binary inter-

diffusion process in which volume diffusion is a rate-limiting step, the evaluation of diffusion

data from concentration profile is the same for single-phase and multiphase systems.

8

Fig. 4 (a) Phase diagram of the binary system A-B.

(b) A diffusion couple A/B annealed together along with the

Concentration profile of B in moles. [5]

In contrast to a diffusion-controlled interaction in a binary system, invariably resulting in a

reaction zone with single-phase product layers separated by parallel interfaces as can be seen

from figure 4, an additional component in a ternary system provides one extra degree of

freedom, which allows for the appearance of two-phase areas in the diffusion zone. The

diffusion zone morphology, which develops during solid-state interaction in a ternary couple,

is defined by type, structure, number, shape and topological arrangement of the newly formed

phases [5]. In general, unlike the case of a binary system, the phases which are formed and

the morphology of the reaction product layers in a ternary diffusion couple cannot

unambiguously be deduced from the corresponding phase diagram. However, the resulting

microstructure of the reaction zone can be visualized with the aid of the so-called diffusion

path. This is a line on the ternary isotherm, representing the locus of the average composition

in planes parallel to the original interface throughout the diffusion zone. Naturally, the

diffusion path in a ternary system must fulfil the law of conservation of mass. In a ternary

system, at a fixed temperature and pressure, the number of degrees of freedom is three minus

the number of phases. This means that for a single phase region, the concentrations can be

varied of the two components while for a two phase system; the concentration of one system

can be varied whereas the other one is fixed. Figure 5 shows an isothermal representation of

9

the system A-B-C with two single phase fields γ and δ and a two phase region with the tie

line being drawn to connect the equilibrium phases.

For this system, if the element C diffuses with the alloy X, the concentration profile might

resemble the hypothetical diffusion paths 1 or 2 or any other as long as the mass balance is

preserved.

Fig. 5 Ternary isotherm with two possible paths of diffusion, 1 and 2

crossing the mass balance once according to law. Mass balance line

is depicted by the solid red line. [5]

Kirkaldy and Brown [6] formulated a number of rules relating the composition of the reaction

zone with the phase diagram. Later these rules were conventionalized by Clark [7].

Referring for details especially [7], a brief summary is presented here of the main ideas (for

the visual appreciation) using a hypothetical reaction couple of an A-B-C ternary system

shown in Figure 6.

10

Fig. 6 A reaction zone structure in a hypothetical couple A/Z of the A-B-C system (on the left)

and the corresponding diffusion path plotted on the isotherm of the ternary diagram (on the

right). The low-case letters relate the structure to the appropriate composition on the

isotherm. (Note that all paths in three-phase fields must be denoted by dashed lines, as a

three-phase layer cannot form in a ternary diffusion couple.) [8]

In the hypothetical system shown in Figure 6, a solid line crossing a single phase field on the

isothermal section (e.g. a-b) denotes an existing layer of that phase in the reaction zone of the

couple A/Z. A dashed line parallel to a tie-line in a two-phase field (g-h) represents a straight

interface between two single phases. A solid line crossing tie-line on the isotherm (b-c)

represents a locally equilibrated two-phase zone (in fact, a wavy interface) in the couple. A

solid line entering a two-phase field and returning to the same phase field (d-e-f) represents a

region of isolated precipitates. A dashed line crossing a three-phase field (e.g. i-j or k-l)

implies an interface in the diffusion structure with equilibrium between three phases, either a

two-phase layer adjacent to a layer consisting of a different phase (e.g. i/j interface) or

adjacent two-phase layers with one common phase (e.g. k/l interface) [8].

Practically, this theory can be used for example in a Co-Ni-Si system. Figure 7 shows BEI

Image of C30Ni70 versus Si diffusion while figure 8 shows the corresponding ternary diagram

along with the calculated diffusion path according to the theory explained above.

11

2.1.1 Diffusion path in a ternary system In an isothermal cross section, numerous diffusion paths can be imagined which would fulfill

the mass balance requirement. But then how is it possible to predict a specific path that is

followed??

A full prediction is still not possible but many of these paths can be excluded based on

irreversible thermodynamic considerations. By principle, the diffusion flux of all species

moves towards the side where its thermodynamic or chemical activity is the lowest. Thus if

the thermodynamics of the system are known, then a plot can be made of the activity of the

element i as a function of the molar ratio of the other elements in the system.

Qualitative assessments are often enough to exclude diffusion paths. For example, the path

C30Ni70 is possible from a mass balance point of view but clearly impossible from a

thermodynamic point. This is because the Ni which diffuses through Co and Co-silicides has

its activity nearly zero in this region whereby it rises again in the Ni2Si region.

Thermodynamically this is not allowed.

Fig. 7 Microstructure of the diffusion zone in Si/Co70 at.% Ni

diffusion couple after reaction at 800°C for 400 h (BEI). [8]

12

Fig. 8 Isothermal cross section through the ternary phase diagram

Co-Ni-Si at 800°C with the diffusion path in black. [8]

Before moving forward, it would be helpful to understand some basics about reactive

diffusion in solids and their influence in a diffusion couple technique which is described

briefly below.

2.2 Reactive Diffusion in Solids At elemental level a classical diffusion couple is nothing more than the juxtaposition of two

“infinite” samples, one of which has a uniform concentration C1 and the other a concentration

C2 of the diffusing species. The first difficulty is the definition of the dimensions for

concentration and distance in the compositional profiles across the reaction zone. In fact, the

frame of reference which is chosen for the description of the inter-diffusion process

determines the definition of the evaluated diffusion coefficients, and indeed quite a variety of

definitions exists. As long as one is working in a consistent system, every evaluation is

equally good and the various diffusion coefficients can be transformed one into another.

According to Oberndoff [9], a binary inter-diffusion process in which volume diffusion is a

rate-limiting step, the evaluation of diffusion data from the concentration profile is the same

for single-phase and multiphase systems. It should be mentioned that the inter-diffusion

coefficient in a solid-state system is a single-valued function of composition only as long as

local thermodynamic equilibrium for the formation of point defects within the reaction zone,

viz. vacancies (and interstitials) is virtually established.

In this case the inter-diffusion coefficient, D~

for a bulk diffusion-controlled process with

constant partial molar volumes of the components, can be derived in a simple and

straightforward manner by a direct application of the Matano-Boltzmann solution to Fick’s

second law. As illustrated by figure 9, the relationship so obtained takes into account the

concentration dependence of the diffusion coefficient and the parabolic time dependence of

the inter-diffusion zone thickness [8].

13

Fig. 9 Concentration curve of a binary diffusion couple between two

alloys with starting compositions BC and

BC , respectively. [8]

The inter-diffusion coefficient is defined by Fick’s first law in the form;

where J~

is the inter-diffusion flux and iC is the concentration of component i in mole/m3.

The inter-diffusion coefficient can be determined at each composition *CB from the

concentration profile of a diffusion couple by Fick’s second law

where x is the position parameter with respect to the Matano plane, which is defined in such a

way that:

B

B

C

C

xdC 0 (2.3)

with

BC and BC are equal to the starting compositions of the end-members of the reaction

couple (in mole/m3). This is the so-called Matano-frame of reference. The position of the

Matano interface, 00 x can be found by making the vertically hatched areas in Figure 9

equal. The integral in Eq. 2.2 equals the hatched area.

x

CD~

J~ i

(2.1)

*BC

CB

*B xdC

*

C

x

tCD

~

2

1

(2.2)

14

It is to be recalled that the inter-diffusion coefficient is only an “overall” measure for the

redistribution of the elements, and give no information on the relative mobility of the species

involved in the interaction. A more fundamental quantity related to the latter issue is the

intrinsic diffusion coefficient, Di of a component i pertinent to the particular alloy

composition. This coefficient is directly related to the atom flux with respect to the position

of inert markers placed in the diffusion zone. In practice, the original interface, also called the

Kirkendall plane, is a convenient plane of reference. The position of that plane can be

revealed by, for example, inert particles introduced at the original contact surface between

end-members prior to annealing [9].

When phases with a very narrow region of homogeneity are formed during reaction, the

concept of the integrated diffusion coefficient, intD~

, can be used to describe the redistribution

of the elements across the interaction zone. This material constant is defined for a line-

compound as the inter-diffusion coefficient, D~

, in this phase integrated over its (unknown)

homogeneity limits N and N (N is the mole fraction);

If phases with a very narrow homogeneity range are only formed, then intD~

of the phase (for

example γ) can be found from the equation 2.4 as:

ba

QaPb

t

x

t

x

ba

baD

γγ

2

Δ

2

Δ~2

int (2.5)

with P and Q equal the hatched areas in Figure 9 with B

γ

B NNa and γ

BB NNb ,

respectively. In the same way the integrated diffusion coefficients for the phases and can

be found as can be seen from the figure above.

Since the integrated diffusion coefficient is a material constant, its value does not depend on

the starting materials of the reaction couple. This means that the values of intD~

in the -

phase (Figure 10) will be the same in an A/B and a / incremental diffusion couple.

N"

N'

dNDD~~

int (2.4)

15

Fig. 10 Schematic concentration profile of a diffusion couple A/B showing the

formation of three line-compounds (, and ) and no terminal solid solutions.

The analysis is also valid for ternary diffusion couples as long as straight bound line

compounds and no terminal solid solutions are formed. The results are consistent with data

found in binary systems as long as the activities at the interfaces are the same [9].

All analysis till here is based on the assumption that the overall interactions are controlled by

the same diffusion process throughout the whole annealing procedure. Then the layer

thickness Δx and the annealing time t are related to the parabolic growth constant kp :

(Δx)2 = 2kpt (2.6)

In practice several deviations from this growth may exist due to, for example impurities or

some oxide films present in the starting materials and due to metallurgical changes with time.

2.3 Error Sources in the Analytical and Experimental Technique

As with any experimental technique, the results obtained from the diffusion couple may also

contain some errors which are related to the sample itself. Referring to the diffusion path

theory, a particular inconvenience is the presence of a liquid phase at the annealing

temperature while the end compositions are solids which can lead to extremely poor results.

Poor wetting at the interfaces and accelerated reactions due to defects leads to results, the

interpretation of which is cumbersome. This problem is particularly likely to occur in

soldering, since liquid is present while soldering.

β γ

16

Another error source is the analytical technique employed to measure the concentrations in

the reaction zone and its limitations. The problems with the techniques used in this

investigation will be described in detail in chapter 3.

Sometimes, phases are thought to be missing or are not detected in a couple when determined

using the microprobe analysis. One of the reasons for the absence of an equilibrium phase

may be the presence of a barrier layer at the interface, such as, for example, oxide films at the

contact surface or the presence of impurities in the starting materials. In the latter case, the

segregation of impurities, which may be present only in the ppm-range in one of the end

members, can cause enrichment in the diffusion zone, rendering nucleation of a certain phase

difficult. An example of this will be provided in chapter 4.

Another area of caution is the growth of the occasional non equilibrium phase stabilized due

to the impurities in the starting materials. Examples are the growth of the oxide scales or

formation of carbide compounds.

Since the growth of interfacial layers is heavily dependent on the annealing times, sometimes

short annealing times lead to slow reactions due to which phases might not grow sufficiently

and quickly. A reason might be difficulty in nucleation, but it is not always the case. An

example is the Ni-Bi system where due to slow reaction rate, the phase NiBi is not observed

even though it is found to be stable from the phase diagram [16]. The behaviour is explained

by the relatively slow diffusion of the NiBi phase i.e. slow growth rates, which means lower

diffusion coefficients when compared to the other phases present (in this case NiBi3).

A method to verify the pure binary nature of the equilibrium phase is to choose the

compositions as close as possible to the phase in question, in such a way that only those

phases are formed. The terminal materials can be single phase or two phase alloys. The phase

then grows parabolically with time with relative thickness while phases due to impurities

cease to exist [5].

17

Chapter 3

Experimental Setup

In this chapter, the standard procedures followed for the preparation and analysis of the

samples used are explained. Specific details for preparation are explained in the relevant sub

sections of this chapter. The following table shows a list of materials used for the

investigation.

Table 3.1 List of materials used for the investigation.

Material Type Thickness/Diameter (mm) Purity (%)

Bi-In34% Rod 3 99.99

Bi-In50% Rod 6.5 99.99

Au Foils/pieces 0.3-1.5 99.99

Ag Disks 2.10/10 99.99

Ni Disks 2.40/10 99.99

Cu Sheet 3.57 99.99

Pd Foils/pieces 0.2-1 99.99

Samples were prepared by reflowing both BiIn34% and BiIn50% on the available substrates

i.e Au, Ag, Cu, Ni and Pd. As mentioned in table 3.1, the available substrates were present in

different sizes and hence the amount of solder reflowed varied with the size.

3.1 Vapour Phase Soldering Since we are dealing here with low melting alloys, it is imperative that the temperature is

controlled to the best possible limit. Also it is imperative that this temperature is maintained

throughout the process of soldering. To ensure this, the methodology used in this

investigation is called vapour phase soldering. In this process, the heat to melt the solder is

generated in the setup by the vapour of the boiling liquid i.e the latent heat of vapourization.

The vapour also acts as the flux needed for soldering effectively making the process

“fluxless” as no external flux is needed. The temperature of the saturated vapour zone is the

same as the boiling point of the vapour phase liquid. The peak soldering temperature is the

boiling temperature of the inert liquid at atmospheric pressure. It heats uniformly, and no

part on the surface (irrespective of its geometry) exceeds the fluid-boiling temperature

thereby maintaining temperature control [2].

18

3.2 Experimental Arrangement For Soldering The first step towards the realization of the soldered joints was to design and construct an

experimental arrangement. Glacial acetic acid has been used as the boiling liquid, the reason

for which is explained below. A hot plate has been used as the heat source along with a large

beaker, which is needed to perform the procedure. A thermometer has been used to constantly

measure the temperature of the acid up until its boiling point. A small ceramic cup acts as a

measure for the amount of acid needed for the procedure. Figure 12 shows the experimental

arrangement.

Before soldering, all the sample substrates were lightly ground on the 4000 SiC paper to

remove any surface impurities. Next, the samples were immersed in Isopropyl Alcohol (IPA)

and ultrasonicated for 10 minutes each time. Then these samples were dried for 2 minutes

using a precision wipe.

Next using forceps, the sample was transferred onto the steel mesh holder. Next a small

chunk of solder with a weight of about 0.07gm was placed on top the sample as shown in

figure 11. It must be mentioned here that forceps have used to prevent any contamination of

the end members.

Fig. 11 Mesh holder with sample substrate and solder chunk on top.

The acid is allowed to boil and reach its boiling temperature of 118°C. The reason for

choosing acetic acid is that its boiling point is some ten degrees above the melting point of

the solder composition BiIn34% and allows for nice heating and fluxing of the solder. As

soon as the acid starts to boil, the mesh is placed inside the beaker while being careful to not

disturb the geometrical arrangement of the sample to be soldered.

19

Fig. 12 a) Schematic b) real time setup for the reflow experiments.

As soon as this happens, the fumes of the acid act as the flux required for soldering while the

heat of the vapour from the boiling acid melts the chunk of solder on top of the substrate.

Figure 13 shows examples of the reflowed solder experiments performed.

a) b)

Fig. 13 a) Top view of reflowed substrates with scale b) Side view of the substrates.

20

3.3 Annealing Procedure

After soldering, some of the samples with the reflow composition of Bi67%-In33% were

aged inside an air furnace at 85°C and samples with the reflow composition of Bi50%-In50%

were aged inside a furnace at 70°C for 720 hours respectively. As mentioned earlier in

section 1.3, annealing has been performed to mimic the behaviour and service life of the

joints at service temperature after one month. The temperature for annealing was chosen

based on two factors; 1) the reflowed solder should not melt while annealing and 2) the

average service temperature range which might be reached when in use. Samples of the same

reflow compositions were also directly cured in resin to check for differences with respect to

the aged samples.

3.4 Sample Preparation The first step involved in the sample preparation of the soldered samples was to embed the

reflowed samples in resin. The samples were attached inside a mould using carbon tape flat

with the solder side on top. Next using the transparent chemical solution Claro-Cit, a solution

with the following quantities is made:

Resin Solution- 14 g

Curing Solution- 2 g

This curing solution is specifically used because it hardens at a lower temperature than the

melting point of the solders. A piece of relatively pure aluminium (1xxx series) is clamped

together with the sample as shown in figure 14.

Fig.14 Curing Moulds with samples and curing resin.

This assures that during SEM analysis, electrical conductivity is present. The chemical

solution is poured into the mould and placed inside the pressure pot for about 8 hours at 2

bars to cure.

Next, these samples were ground (#180 to #4000 SiC paper) and polished upto 1µm

suspension size of polishing liquid. Finally, using the silicon containing oxide polishing

suspension (OPS) solution, the sample was fine polished to remove the remaining scratches.

21

3.5 Analysis Throughout the current investigations, extensive use is made of light optical microscopy in

order to identify the wetting behaviour and the grain structure of the solder.

For identifying chemical composition on either side of the interface, different analytical

techniques are available like Auger Electron Spectroscopy (AES), Electron Probe

Microanalysis (EPMA), Scanning Electron Microscopy (SEM) etc. For the current work, the

last two methods have been applied for making deductions from the obtained results.

In EPMA/SEM, high energy electrons are focused by a fine probe and then directed at the

point of interest in the couple. These electrons on coming in contact with the surface generate

characteristic X-rays. These generated X-rays are then detected and identified using standard

methods thereby giving a nice analysis. The major advantage of EPMA is that it can be used

to measure compositions in very small volumes. Generally, for bulk specimens, the resolution

works well till 1µm. For a nice analysis, the major parameters are the accelerating voltage,

beam current and the counts per second.

For the current investigation, SEM analysis has been performed in low vacuum and

Backscattered Electron Shadow (BES) mode while, Electron Dispersive Spectroscopy (EDS)

has been used to collect the generated X-rays and thus determine the composition of the

formed reaction layer, if any.

As was mentioned in chapter 2 (section 2.3), it is also important to discuss some of the errors

sources encountered in electron microscopy.

First, the determination of the composition has an inherent error due to the resolution of the

detection system, statistical fluctuations in the (low) count rate and mathematical procedures

for extracting the data and the correction involved for it. Second, the X-rays generated in the

bulk samples are due to the scattering effect of the electrons and not the beam size. Now, due

to this scattering effect, it is nearly impossible to generate X-rays for a sample with a

dimension smaller than 1-2µm. Thus interfacial measurements are not possible with an

electron beam technique.

Third, there is an issue with X-ray absorption, which happens when the X-rays generated

travel through the specimen with varying material composition and therefore its absorption

coefficient. Some other issues that might lead to a less than satisfactory analysis is the

fluorescence effect which can very easily effect the measured data since it enhances the X-

rays produced. Corrections for this effect are possible but difficult [9].

It is quite apparent that for a ternary system, the situation can be even more complicated. The

references cited, describe in detail some of the correction procedures for EDS analysis which

if applied can help in a better and more accurate analysis.

22

23

Chapter 4

BiIn34% on Common Electronic Substrates

The following chapter describes the results obtained from the soldering of BiIn34% on

common substrates. Since a comparative result is necessary to investigate the development of

the microstructure and in particular the interface between the solder and substrate, samples of

“as-reflowed solder joints” and “reflowed and aged solder joints” were both studied. This was

done to formulate an idea about the metallization of the solder joints and their reliability at

service temperature. The results are reported in the sub sections below respectively.

4.1 Reflowed BiIn34% on Common Electronic Substrates This section reports the results of the as- reflowed solder on the common electronic substrates

such as Au, Ag, Ni, Pd, Cu respectively in that order. As was mentioned in section 3.4, both

light optical microscopy and SEM Analysis was performed.

Fig. 15 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a gold

substrate b) reaction interface with uneven surface morphology and no detectable diffusion

layer.

a) b)

24

Fig. 16 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a silver

substrate b) reaction interface with uneven surface morphology and no detectable diffusion

layer.

Fig. 17 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a nickel

substrate b) reaction interface and no detectable diffusion layer with presence of Bi grains at

the interface.

a) b)

a) b)

25

Fig. 18 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a

palladium substrate b) reaction interface along with the presence of an oxide scale confirmed

by EDS Analysis on the Bi grains and no detectable diffusion layer.

Fig. 19 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a copper

substrate b) reaction interface at the triple point and no detectable diffusion layer

Analyzing the SEM micrographs of the reflowed solder on the common electronic substrates

from the figure 15 to figure 19, it is can be seen that no detectable layer has formed just after

reflow of the solder. This clearly stems from the reason that for the diffusion process to start,

a certain time bound exposure to a high enough temperature is needed i.e, an incubation time

period only after which the diffusion will occur. It must be clearly noted here that a lack of

visible of detectable layer does not mean that there is no diffusion but due to the incubation

time period, it is extremely slow which means that for us, it is not possible to detect. Hence, it

is necessary to age the reflowed solder substrates to better study its interface interactions.

It is also important to note the wetting behaviour of the solders with these common

substrates. While all of them have a nice adherence to the surface, their reflow varies for

different substrates with samples of Au and Ni showing poor reflow as can be seen from

figure 15a and 17a. It is quite remarkable that this happens as Au is quite extensively used in

the industry to increase wettability of solders. The reason for this is a matter of discussion. As

a) b)

a) b)

26

shown in figure 18b, oxide inclusions are present on the surface of the reaction interface of

the as reflowed solder with Pd. The reason behind this is that this particular sample was

analyzed much later after soldering and hence came in contact with oxygen leading to the

presence of these inclusions.

4.2 Aged BiIn34% on Common Electronic Substrates This section reports the results after annealing was performed at 85°C for 720 h. Both SEM

and Light optical microscopy have been used for analysis.

4.2.1 Aged BiIn34% on Au

Fig. 20 Scanning electron micrographs (BES) after ageing for 720h at 85°C in air of a)

reflowed and aged solder on a gold substrate with unsatisfactory wetting behaviour b)

intermetallic layers at the triple point with a layer of Bi grains segregated on top of (layer1)

c) the reaction interface and the presence of an intermetallic layer d) microstructure within

the bulk solder with Bi grains etched out

c) d)

a) b)

c) d)

27

Anticipating some of the results of the present study, it would be worthwhile to mention the

data on Au-Bi intermetallics available in literature and compare it with the results obtained.

The phase diagram of the Au-Bi system shown in figure 21 is essentially a simple eutectic

involving an intermetallic Au2Bi which is stable in the temperature range of approximately

116-371°C. The structure of this intermetallic is of the Cu2Mg type.

Fig. 21 Au-Bi phase diagram with reported phases. [10]

It has been reported that other phases like Au3Bi and Au2Bi3 may be present within the

system but none of these phases have been proven experimentally. Comparing this data with

the SEM micrographs in figure 20, it is quite evident that the interfacial reaction between Au

and BiIn34% should lead to an intermetallic layer. But since the intermetallic Au2Bi

decomposes below 116°C, thermodynamically it is impossible for this intermetallic to form

since in the current investigation, the annealing temperature is just 85°C. It must also be seen

here that the irregular wetting and reflow of the solder has meant that the reaction layer has

not formed across the whole interface as seen in figure 20a. This has an overall effect on the

diffusion process and hence the reliability of the soldered joint.

Figure 22 shows the EDS Analysis graphs of the intermetallic layers with point analysis done

at different locations from which an average concentration is obtained.

28

29

Fig. 22 EDS Analysis graphs with performed point analysis within the intermetallic layer at

different locations to account for the average concentration in wt%.

30

The EDS Analysis confirms that indeed there is no presence of Bi within the intermetallic

layers 1 and 2. This can also be visually seen in images 20b and 20c where it is clearly visible

that the Bi grains are “left behind” within the solder with only In interacting with the Au

interface. This can be attributed to the thermodynamic reasons mentioned above, which point

out that at our operating temperature, Bi will not interact with Au as the intermetallic Au2Bi

decomposes below 116°C.

4.2.2 Aged BiIn34% on Cu

Fig. 23 Scanning electron micrographs (BES) after ageing for 720h at 85°C in air of a)

reflowed and aged solder on a copper substrate b) presence of Bi grains with possible

diffusion at the triple point c) the reaction interface surface with irregular diffusion pattern

along with presence of Bi d) microstructure within the bulk solder with Bi grains etched out.

Before analyzing the results, it would be worthwhile to mention the data on Cu-Bi

intermetallics and compare it with the results obtained. The phase diagram shown in figure

24, indicates that the equilibrium phases of the Cu-Bi system are: (1) the liquid, miscible in

all proportions; (2) the fcc solid solution, (Cu), with restricted solubility of Bi amounting to

0.003 at.% Bi at ~800°C and (3) the rhombohedral solid solution, (Bi), with presumably

negligible solubility of Cu as described by Chakrabarti et al. [11].

a) b)

c) d)

31

Fig. 24 Cu-Bi phase diagram [11]

Hence, from the above assessment of the binary phase diagram, solid solubilities are

negligible and no intermetallic compound phases are formed. The eutectic point lies very

close to the melting point of pure bismuth and is calculated to be 270.6°C at a copper

concentration of 0.60 at.%.

Comparing this data with the SEM micrographs in figure 23, it is evident that the interfacial

reaction between Cu and BiIn34% would lead to diffusion with no possible interaction

between Cu and Bi. Following the findings of Bahari et al. [12], figure 25 shows the phase

diagram of Cu-In with the presence of stable intermediate phases such as α, β, δ, ε/ε’ and

Cu11In9 at different temperature ranges. The latter intermetallic is formed via a peritectic

reaction at 305.8°C while being involved in the eutectic reaction L In + Cu11In9 at

155.5°C. It is important to keep in mind that the most interesting results from the phase

diagram is that of the lowest temperature phase as our operating temperature is only 85°C.

32

Fig. 25 Cu-In phase diagram [12]

Further, it should also be mentioned here that the intermetallic layer is irregular as seen in

figure 23c and has not formed across the whole reaction interface and is not present at triple

points (see figure 23b). The reason for this is the fact that the Cu11In9 phase starts to form at

temperatures ~305.8°C only while then being stable up until room temperature. But, since our

annealing temperature is so low, the reaction kinetics involved is slow leading to lower

diffusion rates. The lack of a regular layer means that annealing times might need to be

extended to study the interface interactions properly.

33

Fig. 26 EDS Analysis graph with performed point analysis within the intermetallic layer at

different locations to account for the average concentration in wt%.

Figure 26 shows the EDS Analysis graph of the intermetallic layers with point analysis done

at different locations from which an average concentration is obtained. The EDS Analysis

confirms that indeed there is no Bi present in the intermetallic layer. This can also be visually

observed in figure 23c and 23d that the Bi grains are “left behind” within the solder with only

In interacting with the Cu at the interface. This can be attributed to thermodynamic reasons as

no intermetallic compound forms between Cu and Bi at our annealing temperature as is

explained above from the phase diagram of Cu-Bi.

34

4.2.3 Aged BiIn34% on Ag

Fig. 27 Scanning electron micrographs (BES) after ageing for 720h at 85°C in air of a)

reflowed and aged solder on a silver substrate b) complete lack of any detectable diffusion

layer at the reaction interface and the triple point c) the reaction interface surface with

irregular wetting interface and presence of Bi grains d) microstructure within the bulk solder

with Bi grains etched out.

a) b)

c) d)

35

Fig. 28 Ag-Bi phase diagram [13]

The binary Ag-Bi alloy system is a simple eutectic one with eutectic composition of ~95.5

at.% of Bi and eutectic isotherm at 262.5 C. Virtually no silver can be dissolved in solid

bismuth, whereas solid-state solubility of Bi in Ag is about 0.83 at.% at the eutectic

temperature. Figure 28 shows the equilibrium Ag-Bi phase diagram system.

36

Fig. 29 Ag-In phase diagram [14]

Figure 29 shows the equilibrium Ag-In phase diagram system. The Ag-In binary alloy system

is characterized by rather extensive solid-state solubility (more than 20 at. %) of In in Ag and

the existence of five intermediate phases, namely /, , , and stoichiometric compound

AgIn2. The latter intermetallic is formed through the peritectic reaction at 166 C, and is also

involved in the eutectic reaction L AgIn2 + In at 144 C. It is also interesting to notice that

solubility of Ag in solid Indium is negligible.

Analyzing the SEM micrographs, the presence of any reaction/diffusion layer is not detected

as seen in figure 27b and 27c, which is remarkable since it is expected that the product AgIn2

would form. One of the reasons for the absence of an equilibrium phase might be the

presence of a barrier layer at the interface, such as, for example, oxide films at the contact

surface or the presence of impurities in the starting materials. In the latter case, the

segregation of impurities, which may be present only in the ppm-range in one of the end

members, can cause enrichment in the diffusion zone, making nucleation of a certain phase

difficult (see section 2.3). But, it is possible that the phase is present in such a minute quantity

that it cannot be determined easily by the experimental techniques available. Also, the

deficiencies of the analytical techniques employed in the present study mean that the

determination of a chemical composition with EPMA has an inherent experimental error.

37

4.2.4 Aged BiIn34% on Ni

Fig. 30 Scanning electron micrographs (BES) after ageing for 720h at 85°C in air of a)

reflowed and aged solder on a nickel substrate b) complete lack of any detectable diffusion

layer at the reaction interface and the triple point c) the reaction interface with some visual

presence of reaction layer in needle like shape but too minute with presence of Bi gains d)

microstructure within the bulk solder with Bi grains etched out.

a) b)

c) d)

38

Fig. 31 The Ni-Bi phase diagram [15]

Figure 31 shows the Ni-Bi phase diagram. Following the findings of Kao et al. [16], it has

been reported that only the phase NiBi3 was observed, whereas the other intermetallic phase

NiBi, predicted by the Ni-Bi phase diagram to be stable, was not detected at different

temperature ranges. The absence of one or more phases in a binary diffusion couple is not

rare and is frequently explained by the differences in the diffusion coefficients. By principle,

the phases with higher diffusion coefficients will grow faster, and when the difference in

diffusion coefficients is extreme, the phases with higher diffusion coefficients will even grow

at the expense of those with lower diffusion coefficients thus explaining the absence of the

NiBi phase.

39

Fig. 32 The Ni-In phase diagram [17]

According to Waldner et al. [17], the Ni-In system as shown in figure 32 contains five

condensed solution phases and five stoichiometric intermetallic compounds. The Ni-rich

solid solution, δ-Ni2In, δ'-Ni13In9, the δ-NiIn phase with a large homogeneity range, and the

liquid phase belong to the mixture phases. The line compounds Ni2In and Ni3In on the Ni-

rich side, NiIn in the center, and finally Ni2In3 and Ni3In7 on the In-rich side of the binary

system are involved in several invariant equilibrium between the melting points of pure Ni

and In, i. e., 1455°C and 157 °C, respectively.

Upon examination of the SEM micrographs in figure 30, it can be seen that there is a hint of

the presence of reaction/diffusion layer as shown in figure 30c. This growth layer is so thin

and sporadic that it is impossible to make any logical analytical deductions with the

experimental techniques used in this study. The reason that the diffusion layer is hardly

present can be attributed to the operating temperature i.e. the annealing temperature. While

the product layers in a binary Ni-Bi system form only in the range of 650-270°C, those in the

Ni-In system are stable between melting points of its individual components. This means that

at our operating temperature, the diffusion is not fast enough to form detectable reaction

layers and hence larger annealing time is required to form a thick enough layer for further

analysis.

40

4.2.5 Aged BiIn34% on Pd

Fig. 33 Scanning electron micrographs (BES) after ageing for 720h at 85°C in air of a)

reflowed and aged solder on a palladium substrate b) complete lack of any detectable

diffusion layer at the reaction interface and at the triple point with presence of large Bi grain

c) the reaction interface with no visual presence of reaction layer along with presence of Bi

grains d) microstructure within the bulk solder with Bi grains etched out.

c) d)

b) a)

41

Fig. 34 The Bi-Pd phase diagram [18]

Figure 34 shows the binary Bi-Pd system. Okamoto [18] and Scott et al. [19], state that there

are still some uncertainties about the stability of the intermetallics especially on the Pd-rich

side. Okamoto mentions that there are about 6 intermetallic phases in the system having low

and high temperature stabilities. The Bi-rich phases i.e α, β-Bi2Pd has a very narrow

homogeneity range while the phases α, β-BiPd are present as line compounds. These

compounds lie in the temperature range of our interest as they are stable below 250°C as is

confirmed by Oberndorff [9].

42

Fig. 35 The In-Pd phase diagram [20]

The In-Pd system comprises of eight compounds as reported by Okamoto [20]. A more

detailed study by Flandorfer [21], found that the compound In3Pd was not identified within

the system and should be replaced by the compound In7Pd3. Flandorfer also reported the

eutectic reaction at 154 °C as L ➔ (In) + In7Pd3. However, because the Pd concentration of

the eutectic point is negligibly small, the eutectic temperature was determined to be very

close to the melting point of In. Therefore, it is shown at 156 °C in figure 35. It must be

remembered that the system is very complex with various temperature modifications and lack

of experimental data makes it hard to predict phases convincingly.

Examining the SEM micrographs, it can very clearly be seen that there is no visual presence

of reaction/diffusion layer as seen in figure 33b and 33c. The reason for this lack of diffusion

layer can be attributed to the operating temperature and the kinetics and the diffusion

coefficient of the elements involved. As mentioned earlier, the kinetics of the system is not

fast enough as for both the binary systems; the intermetallic compounds are stable and grow

at a temperature range much higher than the operating temperature for this study. This means

that at the annealing temperature, the diffusion coefficient of the elements is so low that the

diffusion is not fast enough and hence larger annealing time is required to form a thick

enough layer for further analysis.

43

Chapter 5

BiIn50% on Common Electronic Substrates

As described in chapter 4, the following chapter describes the results obtained from soldering

of BiIn50% on common substrates. The study for this section was done to realize and

determine solderability and the behavior of the soldered joints at even lower temperatures.

Since a comparative result is necessary to investigate the development of the microstructure

and in particular the interface between the solder and substrate, samples of “as-reflowed

solder joints” and “reflowed and aged solder joints” were both studied. This was done to

formulate an idea about the metallization of the solder joints and their reliability at service

temperature. The results are reported in the sub sections below respectively.

5.1 Reflowed BiIn50% on Common Electronic Substrates The following section reports the results of the reflowed solder on the common electronic

substrates such as Au, Ag, Ni, Pd, Cu respectively in that order. As was mentioned in section

3.4, both light optical microscopy and SEM Analysis was performed.

Fig. 36 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a gold

substrate b) reaction interface with uneven surface morphology and no detectable diffusion

layer.

a) b)

44

Fig. 37 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a silver

substrate b) reaction interface with no visible and detectable diffusion layer.

Fig. 38 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a nickel

substrate b) reaction interface with uneven morphology and no detectable diffusion layer.

a) b)

a) b

)

45

Fig. 39 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a palladium

substrate b) reaction interface with uneven morphology and no detectable diffusion layer.

Fig. 40 Scanning electron micrographs (BES) after reflow of a) reflowed solder on a copper

substrate b) reaction interface with no detectable diffusion layer and an irregular wetting.

Upon examination of the SEM micrographs of the reflowed solder on the common electronic

substrates from figure 36 to figure 40, it is also quite clearly visible from the irregular

structure of the solder that the reflow is quite poor and irregular for this composition of the

solder. According to Ueshima et al. [4], that more the In content in the solder, the more

ductile is the solder and hence the poor reflow characteristics. It has been suggested to add

small quantities of zinc to the solder to enhance wettability and the reflow properties. It is

also clearly visible that no detectable layer has formed just after reflow of the solder. The

reason for such behaviour has been explained in section 4.1. Hence, it is necessary to age the

reflowed solder substrates to better study its interface interactions as was done with the

BiIn34% solder reflow substrates.

a) b)

a) b)

46

5.2 Aged BiIn50% on Common Electronic Substrates The following section reports the results after ageing was performed at 70°C for 720 h. Both

SEM and Light optical microscopy have been used for analysis.

5.2.1 Aged BiIn50% on Au

Fig. 41 Scanning electron micrographs (BES) after ageing for 720h at 70°C in air of a)

reflowed and aged solder on a gold substrate b) possible intermetallic layers at the triple

point with presence of intermetallic phases constituent of the solder c) the reaction interface

surface with some diffusion pattern d) microstructure within the bulk solder with constituent

intermetallic phases

Observing the SEM micrographs in figure 41b and 41c, it is visually not clear if there is a

discernable intermetallic layer that has formed due to diffusion at the reaction surface. It must

be recalled here that following the findings of [4], the wetting and reflow behaviour of the

BiIn50% solder with metallic surfaces is very poor and it is recommended that some quantity

of zinc be added to the solder to make for better wetting. Also as can be seen in figure 41a,

the reflow of the solder is very irregular due to which the diffusion is not proper to form a

thick detectable layer. Another reason could be the slow diffusion process due to a larger

incubation time needed for the solder and the substrate to start the diffusion process as is

explained by the Au-Bi phase diagram in section 4.2.1.

c) d)

b) a)

47

The EDS Analysis graphs are shown in figure 42 below. EDS Analysis was performed at the

reaction interface to check for any possible diffusion but with inconclusive results as Bi is

detected within the interaction layer which according to the Au-Bi phase diagram is not

possible as Bi does not interact with Au below 116°C. A possible explanation for is the

smearing of the solder during metallographic preparations as the solder is extremely soft

leading to these results.

48

Fig. 42 EDS Analysis graphs with performed point analysis at reaction interface at different

locations to account for the average concentration in wt%.

49

5.2.2 Aged BiIn50% on Ag

Fig. 43 Scanning electron micrographs (BES) after ageing for 720h at 70°C in air of a)

reflowed and aged solder on a silver substrate b) no presence of diffusion layer at the triple

point c) the reaction interface surface with irregular surface morphology d) microstructure

within the bulk solder with constituent intermetallic phases

Analyzing the SEM micrographs in figure 43, it is visually clear that there is no discernable

intermetallic layer that has formed due to diffusion at the reaction surface (see figure 43b and

43c). A reason could be the slow diffusion process due to a larger incubation time needed for

the solder and the substrate to start the diffusion process as is explained by the phase

diagrams in section 4.2.3. Another possible reason is that the phase is present in such a

minute quantity that it cannot be determined easily by the experimental techniques available.

Also, the deficiencies of analytical techniques employed in the present study mean that the

determination of a chemical composition with EPMA has an inherent experimental error.

EDS Analysis was carried out but with varied results which were inconclusive in nature.

a) b)

c) d)

50

5.2.3 Aged BiIn50% on Ni

Fig. 44 Scanning electron micrographs (BES) after ageing for 720h at 70°C in air of a)

reflowed and aged solder on a nickel substrate b) no presence of diffusion layer at the triple

point c) the reaction interface surface with irregular surface morphology d) microstructure

within the bulk solder with combination of the two intermetallic phases.

Figure 44 shows the soldering joint between BiIn50% and Ni. It can be visually observed that

there is no intermetallic layer that has formed due to diffusion at the reaction surface as can

be seen in figure 44c. Recalling the phase diagrams from section 4.2.4, the operating

temperature range for the diffusion to start is very high when compared to our service

temperature used in this investigation. Hence, it is not a surprise to find a lack of any

diffusion as the incubation time is much higher and hence longer annealing times are

required. Another possible reason is that the phase is present in such a minute quantity that it

cannot be determined easily by the experimental techniques available. As was mentioned

above in section 5.2.2, EDS Analysis results were inconclusive in nature.

a) b)

c) d)

51

5.2.4 Aged BiIn50% on Pd

Fig. 45 Scanning electron micrographs (BES) after ageing for 720h at 70°C in air of a)

reflowed and aged solder on palladium substrate b) no presence of diffusion layer at the

triple point c) the reaction interface surface with irregular surface morphology d)

microstructure within the bulk solder with combination of the two intermetallic phases.

On analyzing the SEM micrographs in figure 45, it can be observed that the reflow of the

solder is very poor as is follows from [4]. It is also visually clear that there is no intermetallic

layer that has formed due to diffusion at the reaction surface (see figure 45c). Recalling the

phase diagrams from section 4.2.5, and the studies of [9] and [21], it is clear that longer

annealing times are required as the intermetallics formed in the individual binary systems

depend on various temperature modifications and their stability is questionable at the service

temperature used in our investigation. Hence, it is not a surprise to find a lack of any

diffusion layer as the incubation time is much higher for the diffusion to start. It must be

remembered here that lack of a visual layer does not mean lack diffusion but just the fact that

it is present in very minute quantities.

a) b)

c) d)

52

5.2.5 Aged BiIn50% on Cu

Fig. 46 Scanning electron micrographs (BES) after ageing for 720h at 70°C in air of a)

reflowed and aged solder on a copper substrate b) no presence of diffusion layer at the triple

point c) the reaction interface surface with irregular surface morphology d) clear gap at the

mating surface indicating poor wetting at solder/substrate interface.

Figure 46 shows the SEM micrographs of reflowed and aged solder joint between Cu and

BiIn50%. It can be observed that the wetting of the solder is very poor as a clear gap is

visible at the interface as seen in figure 46d. It is also visually clear that there is no

intermetallic layer that has formed due to diffusion at the reaction surface (see figure 46c), an

observation facilitated due to poor wetting. Recalling the phase diagrams from section 4.2.2,

and the studies of [11] and [12], it is clear that longer annealing times are required as the

phase formed in the individual binary systems depend on various temperature modifications

and their stability is questionable at the service temperature used in our investigation. It must

be remembered here that lack of a visual layer does not mean lack diffusion but just the fact

that it is present in very minute quantities.

c)

a) b)

d)

53

Chapter 6

General Discussion

In the present work it is demonstrated that vapour phase soldering using glacial acetic acid is

a viable technique reflowing solder (BiIn34% and BiIn50%) on common electronic substrates

such Au, Ag, Cu, Ni and Pd. The novelty of this technique is that it does not require a flux

while soldering to facilitate wetting as the acid vapour acts as a flux, thereby making this

technique effectively, “fluxless”.

Since we are dealing with soldered joints that are heat sensitive and have melting points at

low temperatures (109.7°C for BiIn34% and 88.9°C for BiIn50%; see figure 3), extreme

precision is required in temperature control, which the aforementioned technique provides

ably. It should be mentioned here that the focus of the current work was primarily on the

investigation of the BiIn34%, whilst the joints made with the BiIn50% alloy have been

analyzed only to test the viability of such a soldering technique.

Examination of the “as-reflowed” soldered joints using light optical microscopy and SEM,

revealed no reaction product at the solder/substrate interface. This can be attributed to the

extremely slow nucleation process and to the fact that a certain type of reaction barrier (e.g.

oxide films) might be present on the mating surfaces. The latter, however, is not very likely

given the reactive nature of the acidic vapour used in the soldering process. Another possible

explanation here is that the reaction products are present in such minute quantities at the

contact interface, that it is simple not possible to detect them using the available analytical

techniques.

However, after exposure of the soldered joints with BiIn34% alloy to the temperature of 85°C

for 720 hours, it was observed that Cu reacts with the solder and forms Cu-In based

intermetallics leaving (virtually) pure Bismuth “behind”. A somewhat similar pattern was

also observed in the case of the Au/BiIn34% alloy joints. Reaction between Au and solder

alloy leads to the formation of binary Au-In intermetallic compounds, and again pure

Bismuth is accumulated behind in the form of a continuous, although somewhat irregular

layer. Such reaction behaviour can, in principle, be rationalized using the binary phase

diagrams of Au-Bi, Cu-Bi and Cu-In, as explained in section 4.2.1 and 4.2.2, respectively.

From a practical standpoint, it is clear that the BiIn34% alloy is not a good option for

soldering to Cu- substrates or Au-metallization. Interfacial reactions occurring during the

exposure of the joints to elevated (service) temperature (85°C) result in the (excessive)

formation of reaction products (binary intermetallics and Bi) with rather poor mechanical

properties.

On the contrary, no intermetallic were found at the interfaces between the Ag-, Ni- or Pd-

substrates and BiIn34% alloy even after exposure to the temperature 85°C for 720 hours.

Perhaps, the main reason behind of this finding is an extremely slow kinetics of the

intermetallic growth in these systems, which, in fact, is very beneficial from a technological

54

point of view. Such reaction behavior allows metallization schemes, like for example, Ni/Pd

or Ni/Ag to be used in soldering with the BiIn34% alloy.

In the case of soldered joints with BiIn50%, with an increase in the Indium content in the

solder one expects increased ductility (and better creep resistance) when compared to the

BiIn34% alloy. While the “as reflowed” joints showed similar results as to the interaction

with the BiIn34% alloy, no product intermetallics were found at the reaction interfaces even

after exposure to temperature 70°C for 720 hours.

It is important to recall here that an increase in the Indium content in the solder has meant

that the melting point has become even lower, viz. 88.9°C, and hence, the service temperature

of the soldered joints is also expected to be somewhat lower, viz. 70°C. Smearing of the

BiIn50% alloy observed during metallographic preparations of the soldered joints cross-

sections also points (although without an absolute proof) in the direction of increased

ductility of the In-rich solder alloy. Unfortunately, the apparent smearing of the BiIn50%

alloy during sample preparation makes the results of microscopic investigation rather

inconclusive.

55

Chapter 7

Conclusions and Future Study

7.1 Conclusions The aim of the current investigation is to effectively study the interaction on soldering with

an ordered low melting Bi-In solder with common electronic substrates such as Au, Ag, Cu,

Ni and Pd. In a more generic view, the feasibility of such a process was examined wherein

temperature control was paramount for effective results and hence the technique of “Vapour

Phase Soldering” was chosen over other conventional soldering techniques. Soldering was

done for two different compositions i.e BiIn34% and BiIn50%.

While the focus was on the interface interaction after reflow, it was important that ample time

be given to the samples to promote better diffusion. To serve this purpose, samples were

annealed at 85°C for BiIn34% and at 70°C for BiIn50% for 720 hours respectively. Samples

of “as reflowed” solder were also studied to give context for the need of annealing.

The soldering and the subsequent annealing of the samples threw up some very interesting

results as has been presented in chapter 6. It must be mentioned here that while one of the

major issues in this study is the sluggish nature of diffusion and the kinetics involved,

soldering for the aforementioned alloys has to be performed at the same temperature range.

Thus, the study of such systems has to be done with patience [9].

It can thus be concluded that while soldering with such a solder is possible and can be a

possible alternative to some current industrial solder, further study is needed to better

understand the process as it is still a highly unexplored field with limited research.

7.2 Future Study The current thesis proves the viability of the vapour phase soldering technique with novel low

melting ordered alloys. Furthermore, while the phenomenological nature of the current work

has helped develop a better understanding to a primarily poorly discussed problem, a lot of

work still needs to be done. As a novel solder and soldering technique, further study into the

method have to be made to reproduce reliable results.

The first step towards that is to improve the wetting and reflow characteristics of the solders

employed. Compositional changes (for eg., adding zinc) as suggested in literature can be

made on a trial and error basis to arrive at the best possible combination. Mechanical testing

of the soldered joints using methods such as Ball Pull test or microhardness analysis are

recommended to be performed to determine the joint strength and its reliability. Such tests

would help get an idea and in creating solutions towards the issue of brittleness due to the

presence of bismuth at the interface (in case of Au/BiIn34% and Cu/BiIn34%). Corrosion

tests can be performed to make an estimate for service lifetime results, an issue poorly

discussed in literature.

At the level of the technique employed while soldering, inert markers can be place in between

the end members to get idea of the diffusion pass after reflow and subsequent annealing. It

56

must be mentioned here that this method would only help develop a model when the driving

force for nucleation and subsequent interaction is very strong. Also the size of the inert

markers to be placed is very important. Even though complex, further study into

thermodynamics involved with improved solder databases and thermodynamic modeling

would enable to make a relation between the phases.

Finally, with increasing miniaturization of the electronic system, it is paramount that the

technique and the systems involved after soldering are optimized to better fit for practical

applications while becoming a game changing breakthrough in lead free solders.

57

Summary

Within the last two to three decades, the progress in technology has taken a significant leap. It

has become easier for people to afford items that reduce human effort and a large part of

those items are electronics. While it is an ever expanding horizon, this boom has also brought

in its own set of challenges like environmental awareness (specially with respect to soldering)

as more and more legislation have led to a blanket ban on the use toxic elements (Pb in most

cases). It has been proposed to look for alternatives that promote lead free soldering while

also serving the demands of the industry that constantly needs to innovate. Much progress has

been made; still ample scope for research is present with different systems that would help in

this endeavor.

In this investigation, an alternative lead free solder system, Bi-In has been studied. The

novelty of this system is that it is an ordered solder i.e., it is stable until its liquidus

temperature thereby melting congruently and thus having superior mechanical properties

which is an advantage. Also it melts at low temperatures (~110°C) thereby reducing the

thermal stresses in the joints of the electronic components. It can be termed to be the low

temperature “counterpart” of a superalloy. The study has been done by reflowing the solder

having different chemical compositions (BiIn34% and BiIn50%) onto common electronic

substrates and analyzed using light optical microscopy, scanning electron microscopy (SEM),

electron probe microanalysis (EPMA) and electron dispersive spectroscopy (EDS).

It is important to mention here that the method of “vapour phase soldering” has been used for

the reflow process. The reason behind this is that a supreme temperature control was needed

while soldering which this method provided over some of the other conventional methods.

Other factors related to this method have been dealt with in Chapter 3. All the systems to be

studied were also aged so as to give about ample time to the systems and better diffusion

could be promoted.

Chapter 4 and Chapter 5 demonstrate and discuss the results of the systems studied. While the

gold systems (for both BiIn34% and BiIn50%) showed the most rapid diffusion with

intermetallics forming at the interface, the nature of the diffusion meant that system itself

became brittle due to segregation of Bi which is not good from a practical standpoint. Some

other systems such as silver and nickel showed no such layer forming even after being

appreciably being annealed at the same service temperature (with respect to the composition

being annealed) and the metallization remaining the same and thus can be considered as an

option for practical applications.

Finally, further studies on this system especially with respect to the thermodynamic

behaviour are recommended in order to better understand the diffusion process thereby also

making it a challenge.

58

59

References

1. Wolters, J, Origins of gold brazing, Frankfurt, 1976.

2. Van Veen, C, mat-tech, Eindhoven.

3. www.wikipedia.org

4. Ueshima, M, Sugomito, J, “Material Design and Package-Level Reliability of a Novel

Low- Temperature Solder based on Intermetallic-Compound Phases with Superior

High-Homologous Temperature Properties, Electronic Components and Technology

Conference, 2008.

5. van Loo, F.J.J., “Multiphase diffusion in binary and ternary solid-state systems”,

Progress in Solid State Chemistry, 1990.

6. Kirkaldy, J, Brown, L, Diffusion Behaviour in Ternary, Multiphase Systems, Can.

Metall. Quat. 2 (1963) pp. 89-117.

7. Clark, J.B, Conventions for Plotting the Diffusion Paths in Multiphase Ternary

Diffusion Couple on the Isothermal Section of a Ternary Phase Diagram, Trans. Met.

Soc. AIME 227 (1963) pp. 1250-1251.

8. Bastin, G.F, van Loo, F.J.J, Scanning 5 (1983) 172

9. Oberndoff, P, PhD Thesis, TU Eindhoven, 2001

10. Massalski, T.B, Binary Alloy Phase Diagrams, ASM, Metals Park, Ohio, 1986.

11. Chakrabarti, D.J, “The Bi-Cu System”, Bulletin of Alloy Phase Diagrams, 1984.

12. Bahari, Z, “The equilibrium phase diagram of the copper–indium system: a new

investigation”, Thermochimica Acta, 2003.

13. Massalski, T.B, Binary Alloy Phase Diagrams, ASM, Metal Park, Ohio, 1986.

14. Binary Phase Diagrams Database, ASM International, 1996.

15. Okamoto, H, “Bi-Ni System”, Journal of Phase Equilibria and Diffusion, Vol. 33,

2012.

16. Kao, C.R, Lee, M.S, “Interfacial Reactions between Ni Substrate and the Component

Bi in Solders”, Journal of Electronic Materials, Vol. 28, No. 1, 1999.

17. Waldner, P, Ipser, H, “Thermodynamic modeling of the Ni-In system”, Z. Metallkd.

93, 2002.

18. Okamoto, H, “Bi-Pd System”, Journal of Phase Equilibria, Vol.15, 1994.

19. Scott, A, Pinkas, J, “Assessment of the thermodynamic properties and phase diagram

of the Bi–Pd system”, Computer Coupling of Phase Diagrams and Thermochemistry,

30 2006, 14–17.

20. Okamoto, H, “In-Pd System”, Journal of Phase Equilibria, Vol. 24, 2003.

21. Flandorfer, H, J. Alloys Compounds, 2002, vol. 336, pp. 176-80.

22. Evans, D.S, Prince, A, “Bi-In System”, Metal Science, 17(1983), 117-122.