In Situ Electromigration and Reliability of Pb-Free Solders at Extremely

In situ Electromigration and Reliability of Pb-Free Solders

At Extremely Small Length Scales

Antony Kirubanandham

A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy

Approved November 2016 by the Graduate Supervisory Committee:

Nikhilesh Chawla, Chair Yang Jiao Minhua Lu Jagannathan Rajagopalan

ARIZONA STATE UNIVERSITY

December 2016 ABSTRACT

Over the past several years, the density of integrated circuits has been increasing at a very fast rate, following Moore’s law. The advent of three dimensional (3D) packaging technologies enable the increase in density of integrated circuits without necessarily shrinking the dimensions of the device. Under such constraints, the solder volume necessary to join the various layers of the package is also extremely small. At smaller length scales, the local cooling rates are higher, so the microstructures are much finer than that obtained in larger joints (BGA, C4). The fraction of intermetallic compounds

(IMCs) present in solder joints in these volumes will be larger. The Cu6Sn5 precipitate size and spacing, and Sn grain structure and crystallography will be different at very small volumes. These factors will most certainly affect the performance of the solder.

Examining the mechanical behavior and reliability of Pb-free solders is difficult, primarily because a methodology to characterize the microstructure and the mechanics of deformation at these extremely small length scales has yet to be developed.

In this study, Sn grain orientation and Cu6Sn5 IMC fraction, size, and morphology are characterized in 3D, in pure Sn based solder joints. The obtained results show differences in morphology of Sn grains and IMC precipitates as a function of location within the solder joint indicating influence of local cooling rate differences. Ex situ and in situ electromigration tests done on 250 m and 500 m pure Sn solder joints elucidate the evolution of microstructure, specifically Sn grain growth, IMC segregation and surface degradation. This research implements 3D quantification of microstructural features over micro and nano-scales, thereby enabling a multi-scale / multi-characterization approach.

DEDICATION

To my parents, Simon Kirubanandham and Josephine Flora Mary, who have inspired and encouraged me to dream big throughout my life.

ACKNOWLEDGMENTS

I would like to first acknowledge my advisor, Dr. Nikhilesh Chawla, for providing me the opportunity to work on this project. His constant guidance, support, and encouragement throughout the past few years has trained me to do high quality research and given me the tools to achieve my Ph.D. I would like express my gratitude toward my committee members, Dr. Jagannathan Rajagopalan, Dr. Yang Jiao, and Dr. Minhua Lu for taking their time in evaluating my doctoral research. I would like to acknowledge the financial support from the Semiconductor Research Corporation (SRC) through tasks

#1292.084/.084, research direction from the SRC review panel, and technical guidance from the Task’s industrial liaisons, including Minhua Lu (IBM), Fay Hua (Intel), and

Brett Wilkerson (Freescale Semiconductor).

I would like to especially thank Research Scientist Dr. Jason Williams, who has been of great help with technical challenges in the lab and in my research. I would also like to also acknowledge my colleagues and seniors in Professor Chawla’s Research Group, especially Sudhanshu Singh and James Mertens for all the technical discussions and collaborations. And finally, I would like to thank my family, my girlfriend Irene, and my friends for believing in me and constantly motivating me towards achieving bigger goals in life.

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TABLE OF CONTENTS

Page

LIST OF TABLES ...... vii

LIST OF FIGURES ...... viii

CHAPTER

1. INTRODUCTION ...... 1

2. LITERATURE REVIEW ...... 6

Solder Joints Types in Microelectronics Packages ...... 7

Microstructural Characterization of Pure Sn Lead-Free Solder Joint ...... 8

Volume Effect On Microstructure of a Pure Sn Lead-Free Solder Joint ...... 12

Electromigration in Lead-Free Solder Joints ...... 13

Fundamentals of Electromigration ...... 15

Thermal Effects of Electromigration ...... 16

Influence of Thermal Cycling ...... 17

3. RESEARCH OBJECTIVES AND APPROACH ...... 18

Investigation of Sn Grain Orientation in 3D ...... 19

Investigation of the Cu6Sn5 IMC Distribution ...... 25

Study of the Volume Effect ...... 25

In Situ Electromigration Testing Jig ...... 26

CHAPTER Page

4. 3D CHARACTERIZATION OF TIN CRYSTALLOGRAPHY AND CU6SN5

INTERMETALLICS IN SOLDER JOINTS BY MULTISCALE

TOMOGRAPHY...... 32

Introduction ...... 32

Materials and Experimental Procedure...... 35

Results and Discussion ...... 39

3D Sn grain orientation ...... 39

Investigation of Cu6Sn5 IMCs …………………………………………….....48

Conclusions ...... 55

5. EX SITU CHARACTERIZATION OF ELECTROMIGRATION DAMAGE IN

PURE SN BASED SOLDER JOINTS ...... 56

Introduction ...... 56

Materials and Experimental Procedure...... 57

Results and Discussion ...... 65

Summary ...... 78

6. AN IN SITU STUDY ON THE INFLUENCE OF TEMPERATURE ON

FAILURE MECHANISMS IN PURE SN BASED SOLDER JOINTS ...... 79

Introduction ...... 79

Materials and Experimental Procedure...... 80

Results and Discussion ...... 83

Interrupted Aging Experiment ...... 83

Uninterrupted Aging Experiment ...... 86

CHAPTER Page

In situ Thermal Cycling Behavior ...... 89

Summary...... 96

7. CONCLUSIONS ...... 97

Summary of Research Findings ...... 97

Future Work ...... 98

REFERENCES ...... 100

LIST OF TABLES

Table Page

1. 2D Quantification Results of the Cu6Sn5 Nanoparticles ...... 49

2. 3D Quantification Results of the Cu6Sn5 IMC Needles ...... 51

3. Parameters Used for Thermal Cycling Test ...... 89

4. Thermal Expansion Coefficients for the Different Phases in Pure Sn Solder Joints...92

vii

LIST OF FIGURES

Figure Page

1. A) SEM Image of Cross-Sections of the 250M Solder Joint Used in This Study. B)

High Magnification Image Showing Distribution of Fine Cu6Sn5 Precipitates

Throughout the Solder Volume……………………………………………………...20

2. Silicon V-Groove Stage Used To Prepare Solder Joints A) Schematic Of Setup And

B) Optical Image Of Silicon Stage Taped On To A Copper Block C) Flowchart

Describing The Preparation Method For The Multi-Groove Silicon Wafer Stage…..21

3. Reflow Profile Followed For Fabricating Solder Joints For This Study………………22

4. A) Optical Images of Fiducial Marks, Which Helps Monitor the Polishing Rate. B)

Plot Showing Standardized Polishing Rate as Thickness Loss Vs Polishing Cycle. C)

Vickers Indent Geometry Using Which the Change In Depth is Calculated………..24

5. CAD Model Of The Sample Fixture. Key Design Feature And Its Description Are

Mentioned…………………………………………………………………………...... 28

6. CAD Model of the Sample Fixture with The Watlow Ceramic Heater Attached...... 30

7. Jeol JSM 6100 Scanning Electron Microscope With EBSD, In Situ Nanoindenter And

In Situ Electromigration Testing Capabilities ...... 31

8. Optical Micrograph Showing Typical Microstructural Features in A 250 M Pure Sn

Solder Joint. The Majority Phase is Β-Sn Which Contains a Dispersion Of Cu6Sn5

IMC Particles.……………………………………………………….……………….36

9. A) Secondary Electron Image B) Backscattered Electron Image C) OIM Map Of 250

M Pure Sn Solder Joint...... 40

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Figure Page

10. A) 2D OIM Stack Of The 250 M Solder Joint And B) 3D Reconstructed Volume Of

Sn Grain Orientation C) Segmented Volume From The 3D Dataset Showing Grains

Colored With Respect To The OIM Map And Rendered In 3D Though The

Thickness……………………………………………………………………………...43

11. A) Aspect Ratio Plot Showing The Extent Of Formation Of Elongated Grains B)

Misorientation Plot Giving The Distribution Of Grain Boundary Rotation Angles

Within The Solder Volume And C) OIM Map Of A Single Cross-Section With High

Angle Boundaries (> 15o) Highlighted In Black ...... 46

12. Segmentation Of Cu6Sn5 Nanoparticles. A) SE Image Of A Magnified Part Of The

Solder Cross Section. B) Cu6Sn5 Phases Are Selectively Thresholded By Their

Grayscale Value. C) Segmented Cu6Sn5 Imcs As Black Features In A White Matrix. . 48

13. 3D Reconstruction Of Segmented A) Cu6Sn5 Nano-Imcs And B) Cu6Sn5 Hexagonal

Needle Imcs With A Magnified Cross-Section Showing The Hollow Needle

Morphology...... 50

14. A) Ion Channeling Image Of A 250 M Pure Sn Solder Joint Showing Tin Grain

Contrast. Two Dotted Boxes, One Close To The Interface And The Other At The

Center, Mark The Areas From Where FIB Serial Sectioning Was Done B) FIB Trench

is Milled In Front Of The Region Of Interest After Thin Platinum Layer Deposition C)

Cu6Sn5 IMC Distribution At Region 1 (Closer To Cu Interface) And D) Magnified

Region Within Region 1 Showing Nearly Spherical Morphology Of Cu6Sn Nano-Imcs

E) Cu6Sn5 IMC Distribution At Region 2 (Closer To The Center Of The Solder Joint).53

Figure Page

15. A) Secondary Electron Image and B) Backscattered Electron Image Of The Solder

Joint…………………………………………………………………………………..58

16. Circuit Diagram Of The In Situ Electromigration Testing Setup...... 60

17. Schematic Diagram Of The Test Sample For Electromigration Showing Current

Direction ...... 62

18. Resistance Vs Time Plot For A Total Period Of 456 H Showing No Significant Change

In Resistance. Small Drops/Fluctuations Are Points Where The Current Was Turned

Off For EBSD Analysis...... 64

19. Orientation Image Maps A) Before Electromigration Testing B) After 24 Hours Testing

C) After 48 Hours Testing D) After 72 Hours Testing E) After 96 Hours Testing And

F) After 144 Hours Testing…………………………………………………………....66

20. Modified Orientation Image Map Highlighting Orientation Along the Electron Flow

Direction. Large Crystal Lattice Indicates C-Axis Nearly Normal to The Electron

Flow Direction. Grains Within the Red Circle in The Top Left Indicate C-Axis

Orientation Parallel to Electron Flow Direction……………………………………..68

21. Backscattered Electron Images (A) Before Electromigration Testing (B) After 144

Hours Testing. ……………………………………………………………………….69

22. Backscattered Electron Images Of The Cathode Interface A) Before Electromigration

And B) After 144 Hours Testing. The Images Show That A Significant Amount Of

IMC Dissolution Has Occurred At The Cathode Marked By [1]...... 71

Figure Page

23. Backscattered Electron Images Of The Anode Interface A) Before Electromigration

And B) After 144 Hours Testing. The Images Show That Apart From The Increase In

IMC, There Seems To Be Some Failure Features Marked By [1], [2] And [3]...... 72

24. Backscattered Electron Images Of The Anode Interface A) After 24 Hours B) After 96

Hours And C) After 144 Hours Testing. The Images Show Crack Like Features That

Seem To Increase Progressively With Time. D) SEM Image Of The Sample Placed On

An Inclined Stage...... 74

25. A) Backscattered Electron Image Of Cathode Interface B) Thresholded IMC Layer

Before Electromigration And C) After 144 Hours Testing Showing A Decrease In

Thickness Of About 0.4m...... 75

26. A) Backscattered Electron Image Of Anode Interface B) Segmented IMC Layer Before

Electromigration And C) After 144 Hours Testing Showing An Increase In Thickness

Of About 1.2m...... 76

27. A) Backscattered Electron Image Of Cathode Interface B) OIM Of IMC Layer Before

Electromigration And C) After 144 Hours Testing Showing An Increase In Thickness

of About 1.2m...... 77

28. A) Optical Micrograph Of 500 m Solder Joint Before Interrupted Thermal Aging.

B) Optical Micrograph With OIM Image Overlay...... 82

29. BSE Images Of The 500 m Solder Joint Showing Evolution Of Banded Grain

Boundary Ledge Formation. A) 24 Hours Aging, B) 120 Hours Aging……………....84

Figure Page

30. A) SE Image Of The 500 m Solder Joint Showing Banded Grain Boundary Ledge

Formation. B) Oriemtatiom Map Of The Same Region With Misorientation Angles

Between Grains 1 Thhrough 6 Indicating All High Angle Boundaries. C) SE Image At

Higher Magnification Showing The Banded Grain Boundary Ledges Pinned By The

Cu6Sn5 IMC Particles ...... 85

31. A) Secondary Electron Image Of The 500 M Pure Solder Joint Used For The High

Temperature Observation Test. B) EBSD Map Of The Corresponding Cross-Section . 87

32. SE Images Of The 500 M Solder Joint Showing No Grain Boundary Ledge

Formation. A) Before Aging, B) After 180 Hours Aging While Sample Is Still

Maintained At 100ºc ...... 88

33. A) Secondary Electron Image Of The 500 M Solder Joint Prepared For Thermal

Cycling. B) Backscattered Electron Image, And C) OIM Map Of The Corresponding

Cross-Section ...... 90

34. Temperature Profile For 10 Cycles Of Thermal Cycling ...... 91

35. A) SE Image Of A Region Close To The Cu/Solder Interface After 20 Cycles. B) SE

Image Of The Same Location After 60 Cycles, Showing Severe Surface Relief At The

Cu/Solder Interface. C) BSE Image Of A Different Location After 20 Cycles, D) BSE

Image Of The Same Location After 60 Cycles, Showing Extensive Grain Boundary

Ledges And Surface Relief ...... 93

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Figure Page

36. A) Region Within (001) Grains Where Surface Mismatch Between Solder And IMC Is

Large. B) OIM Map Of The Sample Cross-Section. C) Region Within (100) Grains

Where No Surface Mismatch Is Seen Between Solder And IMC. D) And E) Schematics

Showing Cu8Sn5 Needles Within The Solder Aligned Normal To The Polished

Surface. The Arrows Indicate Directions Where Maximum Expansion Is Seen With Sn

Grains And IMC Needles……………………………………………………………94

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CHAPTER 1

INTRODUCTION

Solder is a low melting temperature interconnect material that has been used in a variety of applications for several decades. Due to their ease of processing (Hobart 1906) and relatively low cost (Harrison et al. 2001), tin based solder alloys are the unmatched choice for soldering applications with benefits such as excellent wetting characteristics, superior electrical and mechanical performance, and formation of high density bonding through the formation of intermetallic compounds (Wood and Nimmo 1994, Abtew and

Selvaduray 2000). An electronic package is a complex hierarchical structure with solder interconnects of different sizes ranging from the ball grid array or BGAs which connect the package to a printed circuit board and a subsequent smaller solder interconnection called the C4 joint or the controlled-collapse-chip-connection which are used within a chip’s package (Tu and Zeng 2001, Gilleo 2004). Over the past several years, the density of integrated circuits has been increasing at a very fast rate, following Moore’s law

(Moore 2006). The number of devices per chip area has been doubling almost every two years. Three dimensional (3D) packaging technologies enable the increase in density of integrated circuits without necessarily shrinking the dimensions of the device (Sakuma et al. 2008). Under such constraints, the solder volume necessary to join the various layers of the package is also extremely small. At smaller scales, the local cooling rates are higher, so the microstructures are much finer than that obtained in larger joints (BGA,

C4). Preliminary results show that, a larger fraction of intermetallic compounds (IMCs) may be present in solder joints in these volumes (Khan et al. 2010). The Cu6Sn5 precipitate size and spacing, and Sn grain structure and crystallography will be different

1 at very small volumes. These factors will certainly affect the performance of the solder.

Examining the mechanical behavior and reliability of Pb-free solders at these small volumes is difficult, primarily because a methodology to characterize the microstructure and the mechanics of deformation at these extremely small length scales is yet to be developed.

It is important to understand the microstructure, crystallography, and mechanisms of deformation, especially under real-time processes such as electromigration and thermal fatigue conditions, on the microstructural evolution and reliability of Pb-free solder joints at very small length scales. Because of the tetragonal crystal structure, tin exhibits a highly anisotropic behavior, where properties in mechanical, thermal, electrical and diffusive behavior are different along different directions (Dyson 1966, Dyson et al.

1967, Yeh and Huntington 1984, Bieler et al. 2006, Lu et al. 2008, Sylvestre and Blander

2008). The interstitial diffusion of copper in tin at 25oC is 500 times more along the c- axis than along a or b-axis and is also about 1012 times the self-diffusion of tin (Yeh and

Huntington 1984). The diffusivity of nickel at 120oC is 7 × 104 times more along the c- axis than along a or b-axis (Dyson et al. 1967). Thus the importance of crystallography of tin in the reliability of Sn-based solders is evident.

Electron backscatter Diffraction (EBSD) analysis can be used to obtain a variety of crystallographic information from a 2D cross-section of a sample. The tin grain orientation, grain boundary misorientation, orientation of intermetallic particles and the substrate are just a few among the several possible analyses that could be done. However,

2 the drawback of 2D analysis is that it limits us from understanding the behavior of the complete solder volume. A study on 2D vs 3D characterization of Ag3Sn needles using computer aided reconstruction of optical micrographs shows the accuracy of the size and aspect ratio of the intermetallic needles obtained through 3D processing (Sidhu and

Chawla 2004). A similar approach can be utilized for obtaining three dimensional crystallographic data on solder joints by serial sectioning / polishing (Dudek and Chawla

2008). The recent emergence of integrated focused ion beam and scanning electron microscopy (FIB-SEMs), commercially known as Dual-Beam instruments, have pushed the limits in terms of rapid acquisition times while allowing serial sectioning in the nanometer scale. The immense potential of such 3D characterization methods can be utilized for a more detailed study on the deformation behavior of solder joints.

Research on electromigration damage became of interest with studies pertaining to migration of aluminum (Black 1969, Hu and Huntington 1982, Hu and Huntington 1985,

Ding 2007), gold (Huntington and Grone 1961, Hartman and Blair 1969) and silver (Ho and Huntington 1966). In microelectronic packages, the solder operates at much severe conditions due to its low melting temperature and anisotropic behavior. The influence of crystallographic orientation of Sn on the electromigration reliability of solders has been of particular interest to researchers (Wu et al. 2004, Lu et al. 2008, Lu et al. 2009, Lara

2013). Due to the decreasing size of solder joints, the microstructure often fits into a few large randomly oriented grains after the reflow process (Telang et al. 2004). Since the number of grains is limited, the homogeneous nature of bulk polycrystalline solders is no longer valid and the properties of individual grains becomes more dominant. A vast

3 majority of the literature on electromigration damage pertains to failure analysis after testing. Literature survey shows that studies on evolution of electromigration damage using an in situ testing capability is very limited (Meyer et al. 2002, Vairagar et al. 2004,

Vanstreels et al. 2014). Moreover, in situ studies pertaining to the evolution of electromigration damage due to the influence of Sn crystallographic orientation is even more limited and not fully understood (Buerke et al. 1999).

The research work in this dissertation primarily focusses on characterization of Sn grain orientation and Cu6Sn5 intermetallic particle distribution in the solder volume. Solder samples prepared in lab were mechanically polished in a sequence of polishing steps by serial sectioning technique to obtain information in 3D. The three dimensional datasets were segmented and analyzed for grain shape, size and crystal orientation. Other microstructural features such as grain boundary misorientation, specifically the distribution of high angle and low angle boundaries were analyzed in detail. The distribution of Cu6Sn5 intermetallic particles was also obtained as a three-dimensional dataset. The 3D characterization technique can also be utilized for the investigation of the

‘volume effect’, wherein evolution of solder microstructure with decreasing solder volume can be studied with samples of different sizes. The latter half of the dissertation focusses on in situ characterization of electromigration damage inside a scanning electron microscope by studying the evolution of microstructural features such as Sn grain orientation, solder migration, under-bump-metallization (UBM) consumption, intermetallic growth, and void formation. For the quantification of these features, instruments such as the EBSD Orientation Image Mapping (OIM) and Focused Ion Beam

Tomography (FIB) system were be used. This research is novel in terms of providing a multi-scale / multi-characterization approach that can be used for a more detailed understanding of damage evolution in real-time processes like electromigration and thermal fatigue on industrial scale solder joints.

CHAPTER 2

LITERATURE REVIEW

Solder joints in microelectronics packaging industry are critical components that serve as electrical connections between devices, mechanical connection between the different layers and thermal pathways for heat dissipation (Choudhury and Ladani 2014). Eutectic and near eutectic Sn-Pb alloys have been used widely used as solder alloys for a very long time (Quan et al. 1987, Abtew and Selvaduray 2000, Tu and Zeng 2001, Yan et al.

2006), until 2006 when the European Union Waste Electrical and Electronic Equipment

Directive (WEEE) and Restriction of Hazardous Substance (RoHS) prohibited the addition of lead in products in Europe, due to its toxicity and influence on environment and health (Wood and Nimmo 1994). Following this, most manufacturers have moved to the electronics industry and research focus in developing environmentally benign Pb-free solders.

The newest Pb-free candidate alloys, contain tin, copper and silver as primary components. Several binary, ternary and higher order alloys were developed by altering the compositions of the primary components, while experimenting with other minor alloying additions to replace Sn-Pb alloys (P. Vianco 2004). With the development of new alloys, significant research has been demonstrated in characterizing the microstructures and predicting lifetimes in real-time processes like electromigration and thermal cycling. Crystallographic analysis using EBSD has proven to be vital in the understanding of microstructural evolution, especially in small volume solder joints.

2.1 Solder joints types in microelectronics packages

Solder joints are used in a variety of levels within a microelectronics package to serve as bonding between the silicon chip, substrate and the printed circuit board (PCB). The size and number density of solder bumps varies depending upon the package architecture.

Finer pitch size bumps (C4 joints) are used to bond the silicon die to the package substrate and hence a larger number of interconnections are needed. The substrate is mounted on the PCB using larger bumps and hence fewer number of bumps are used (Tu and Zeng 2001). Solder joints that connect directly to the silicon die are classified as level

1 interconnects. These could be either C4 solder bumps which are about 100 m in diameter (Gilleo 2004) or wire bonds, which are usually copper leads connecting the silicon die and the package substrate (Abtew and Selvaduray 2000). Solder joints bonding the substrate to the PCBs are classified as level 2 interconnections, which are commonly through-hole and ball grid array (BGA) bumps. These solder bumps are larger at about 760 m in diameter (Gilleo 2004). To increase package density, one of the most recent advancements is 3D packaging technology using through silicon via (TSV) interconnects (Gilleo 2004).

2.2 Microstructural characterization of pure Sn lead-free solder joint

Considering Pb-free solder alloys, the majority phase is β-Sn grains which contain a dispersion of Cu6Sn5 intermetallic particles that form due to diffusion from the copper

UBM. The reflow temperature, reflow time, cooling rate strongly influence the amount of

IMC formation within the solder volume (Ochoa et al. 2003, Ochoa et al. 2004). Steady state thermal aging is also seen to play a critical role in determining the amount of IMC

(Zou et al. 2008). During reflow, the interaction between molten Sn and the Cu UBM results in the formation of intermetallic layers (Cu6Sn5 and Cu3Sn) and these are essential for a good metallurgical bond. However, the IMC layers are brittle in nature and excessive IMC layer formation could affect the mechanical properties of the solder bump and reliability of electronic packages.

The growing trend in reducing pitch size for high density interconnects has increased the volume fraction of IMCs. In certain cases, extremely small solder joints could completely transform to IMCs after reflow. Reducing the volume of solder joints has also seen to increase the mean IMC thickness in solder joints. (Islam et al. 2005). Abdelhadi et al.

(Abdelhadi and Ladani 2013) obtained solder joints composed of entirely of IMC phase when the soldering time and temperature were increased, in 15 -25 m thick joints. This is expected to be huge reliability concern in solder joints under 10 m bond thickness.

As the solder volume is reduced, the size of features in the microstructure, such as the grain size of β-Sn or the IMC particle size, approach the size of the solder bump. This

8 causes the microstructure to be very heterogeneous and anisotropic effects start to have more significant roles. The properties of the solder bump as obtained using conventional techniques on bulk solders will no longer be comparable to small size solder joints

(Xiong et al. 2014). Moreover, microstructure evolution due the size and geometry of small interconnects could create new reliability issues (Kwon et al. 2005, Huang et al.

2006, K. Khoo 2007, Josell et al. 2009, Rao et al. 2010, Chen et al. 2012). Studies on size and geometry effects will now be more complicated since the composition of the various phases will be different (Huang et al. 2006, Rao et al. 2010, Chen et al. 2012). Modifying the geometry of solder joints was seen to drastically change fatigue lifetime. Opting for an hourglass shaped solder bump, instead of the conventional barrel shaped joints, improved the fatigue lifetime of Sn37Pb solder joints by approximately 60% (Xingsheng and Guo-Quan 2003).

At small solder volumes, there is a strong influence of crystallography of tin on the observed properties. Due to its body centered tetragonal structure, tin is highly anisotropic, where properties vary along different axes and the effects on reliability is exacerbated in joints with fewer number of grains. Several studies on grain orientation of tin, show its influence on the performance and reliability of Pb-free solder interconnects under thermomechanical and electromigration conditions (Park et al. 2008, Chen et al.

2011, Chen et al. 2012, Chen et al. 2012). Interconnects with c-axis of tin grain aligned parallel to the solder/substrate interface failed prematurely due to easy crack propagation

(Zhou et al. 2010). In case of electromigration testing, grain orientation strongly influences the observed failure mechanism. C-axis of tin grains favor faster diffusion of

9 solute atoms like Cu, compared to the more tightly packed a-axis. In solder bumps where the c-axis is parallel to the electron flow, the under bump metallization is consumed faster and this creates voids that result in failure of Pb-free solder interconnects (Lu et al.

2008) (Hu et al. 2003).

Due to the complex structure of tin grains and IMC phases, characterizing and visualizing the microstructure are critical to understand and correlate structure – property relationships. Most microstructural analyses are done in two dimensions due to the opacity of the microstructure. Although 2D stereological techniques are helpful in providing insightful information on understanding microstructural evolution, uncertainties in topology of microstructural features cannot be clarified with these 2D techniques. Considering to the anisotropy and heterogeneity in Pb-free solder microstructures, measurements like grain size, morphology and curvature of boundaries require the knowledge of 3D microstructure. Thus 3D reconstruction and visualization of microstructural features is essential. Various 3D microstructural analysis techniques are currently available, such as serial sectioning, X-Ray tomography, and magnetic resonance imaging. Determining the appropriate technique depends on the length scale of the microstructural features for each application (Ullah et al. 2014).

Serial sectioning is a versatile technique where thin layers of material are removed by polishing after every analysis step. This technique can be used in conjunction with any type of surface analysis to obtain a stack of 2D datasets which can then be rendered and reconstructed in 3D.

This technique is well established and has been widely used for numerous important findings (Rhines et al. 1976, DeHoff 1983, Li et al. 1999, Yokomizo et al. 2003, Zhang et al. 2004). Among the various advancements over the years are ultra-milling (Alkemper and Voorhees 2001), automated polishing (Dinnis et al. 2005), and dual beam focused ion beam scanning electron microscopy (FIB-SEM) (Groeber et al. 2006). These advancements in serial polishing improve the ease of data collection and allowed capability of large cross-sections to be analyzed and segmented in 3D (Groeber et al.

2006, Ghosh et al. 2008, Groeber et al. 2008, Rowenhorst et al. 2010).

However, despite the advances in 3D microstructural studies, the evolution of microstructure in small length scale solder joints is not fully understood. Performing a 3D characterization study on the Sn crystallography and the size and distribution of the intermetallic particles will give a more detailed understanding of the deformation mechanics of the solder volume.

2.3 Volume effect on microstructure of a pure Sn lead-free solder joint

Following Moore’s law, the pitch size of level 1 and level 2 interconnects in electronic packages are decreasing every year. The International Technology Roadmap for

Semiconductors (ITRS) predicts that as technology nodes reduce in size, the I/O pad pitch size reduce as low as 20 μm (Aggarwal et al. 2005). The package size is also constantly decreasing with growing demand for high density interconnections, which will result in different microstructures and performance.

There have been numerous attempts to study the effect of miniaturization of solder volume on the microstructure Sn-rich interconnects. In Sn-rich Pb-free solder alloys, the undercooling becomes larger as the solder volume is reduced. In SAC solder joints, it was more common to observe regions of ‘interlaced’ areas where the grains are small and their orientations are random (Lehman et al. 2005). Effect of solder volume on its mechanical performance, specifically creep of level 1 and level 2 interconnects were compared with bulk samples (Wiese et al. 2006), The results of all these investigations lead us to conclude that there is a clear effect on material behavior as solder size changes.

Undercooling plays an important role in all small scale solder joints.

Experimental investigations (Zimprich et al. 2008, Yin et al. 2009) and numerical models

(Cugnoni et al. 2006) have been conducted to study the effect of dimensional constraints on tensile strength of solders. Decreasing the gap size resulted in an increase in ultimate tensile strength and yield strength. The fracture strain was also lowered due to brittle

12 failure at the IMC/solder interface. Experimental and modeling work done on lap-shear joints by Chawla et al. (Chawla et al. 2004, Shen et al. 2005) showed that there is a variation between applied shear strain and actual shear strain when the solder gap is reduced, and to get a true shear response, thicker joints are preferred. Very few studies have been conducted on the volume effect on microstructure, focusing on the crystallography of tin. In addition, the differences in local cooling rates and their influence on microstructure need to be elucidated.

2.4 Electromigration in lead-free solder joints

With increasing package density, the small solder bumps experience high current densities that facilitate electromigration failure (Tu 2003). Due to the migration of atoms, we can expect the solder joint to fail by either formation of voids at the cathode or by excessive consumption of UBM (Liu et al. 2000, Huynh et al. 2001, Minhua et al. 2009).

These two mechanisms differ by the type of migrating atoms – the voiding at cathode interface occurs due to self-diffusion of tin (dependent on the orientation of Sn), whereas the UBM consumption is due to fast diffusion of Cu, Ni into the solder volume

(temperature dependent process) (Ke et al. 2011), (Lu et al. 2008). For solder bumps that experience a change of current direction, current crowding effect can be expected (Yeh et al. 2002). In these cases, the damage is due to local heating and the lifetime is greatly affected (Chiang et al. 2006), (Alam et al. 2006, Chiu et al. 2006), (Gan and Tu 2002, Ou and Tu 2005).

To obtain a fundamental understanding of how damage initiates and propagates over time, a real time observation of damage evolution is vital. For characterizing electromigration damage evolution in real time, several in situ characterization techniques have been utilized over the past few decades. Scanning electron microscopy is the most common technique which gives high spatial resolution, phase and topographical information (Vanstreels et al. 2014). However, SEM analysis requires a polished face and is a destructive technique. In situ transmission electron microscopy (TEM) (Liao et al.

2005) and in situ transmission X-ray microscopy (TXM) studies (Schneider et al. 2002,

Schneider et al. 2003, Schneider et al. 2006) have also been conducted as a non- destructive approach.

Vanstreels et al. were one of the first to observe electromigration damage in 30nm Cu interconnects in real time. In this investigation, it was observed that the grain boundary between a large grain and a cluster of small grains in a Cu line, acts as a diverging point for diffusion of Cu atoms. And it was also found that a thicker barrier reduced the interface diffusivity of Cu atoms, thereby mitigating void growth during EM (Vanstreels et al. 2014). In situ experiments conducted by Vairagar et al. study the electromigration- induced failure in dual-damascene Cu test structures. The in situ experiments were key in finding that void migration occurred through the Cu/dielectric cap interface during electromigration (Vairagar et al. 2004).

In situ SEM electromigration experiments conducted by Buerke et al. on Al interconnects study electromigration damage with orientation mapping by EBSD. The ability to combine in situ electromigration testing with EBSD technique proved vital in studying void formation in relation to specific grain boundaries. In this case, it was observed that the end of a blocking grain interrupts the diffusion path along the current direction and favors void formation (Buerke et al. 1999).

2.4.1 Fundamentals of Electromigration

When electric current is passed through a conductor, forces due to the electric field and the charge carriers, i.e. electrons, interact with atoms in the conductor and cause mass transport.

The transport of atoms due to the momentum transfer of metal atoms and electron ‘wind force’ is called Electromigration. The electrostatic force Fef acts in the direction of the electric field, while the force due to the electron wind, Few, acts in the opposite direction.

The net force, Fnet, on the metal atoms in the conductor can be expressed by the following relation:

∗ ∗ 퐹푛푒푡 = 퐹푒푤 − 퐹푒푓 = 푞퐸푍 = 푞푍 푗휌

where, q is the charge of the conducting species, qZ* is the effective charge, E is the electric field, j is the current density and ρ is the resistivity. For conductors, the electron wind force is often 10 orders of magnitude higher than the direct electric field force.

Hence, the net mass transport of atoms due to electromigration is in the same direction as the electron flow (Tu 2007).

Over extended periods of current stressing a conductor, the momentum transfer from the electron force causes a significant number of metal atoms to be displaced from the cathode towards the anode. This results in a steady buildup of material at the anode and creation of vacancies/voids at the cathode. Formation of these voids at the cathode results in a reduced cross-sectional area at the solder-substrate interface thereby increasing the local current density. This phenomenon of local high current density, (which often occurs when electric current turns or converges at an edge of a solder bump) is termed current crowding. This in turn accelerates electromigration damage and the solder joints forms an open circuit due to a break or gap the interface. At the anode interface, metal atoms are accumulated and are piled up, causing unintended growth of tin hillocks and whiskers

(Tu 2007).

2.4.2 Thermal effects of Electromigration

Conventional accelerated electromigration testing involved the application of high current density and high operating temperature to facilitate accelerated degradation. Thus we get influence from both thermal and electrical effects in the mass transport of atoms.

Influence of thermal effect can be restricted by maintaining low temperature of the conducting species, which restricts the atomic mobility of diffusion. Since the primary component in Pb-free solders is tin, (melting point 232oC), even at room temperature, tin

16 experiences a high homologous temperature, and mechanisms such as lattice diffusion, surface diffusion and grain boundary diffusion can occur. As it is very common for devices to operate at 100oC, for pure Sn solder joints, the homologous temperature is over

0.73. This also means that thermally activated mechanisms like creep, play an important role in deciding the structural integrity of solder joints. These effects are exacerbated in solder bumps experiencing high resistance values where Joule heating (proportional to square of current times the resistance) starts to play a significant role.

2.5 Influence of Thermal Cycling

Due to the different types of materials used in an electronic package, the coefficients of thermal expansion often causes thermal-mechanical stresses. Frequent cycling of current and/or temperature results in a low cycle fatigue in solder bumps. For reliability, electronic packages are required to survive cyclic thermal stresses, where the applied temperature is cycled between high and low temperatures for several cycles. The differences in thermal expansion coefficients of the various components in and electronic packages strongly influences lifetime of solder joints, especially in flip chip joints on organic substrates. The bending between layers, introduces shear stresses in the joints and in extreme cases fracture at the solder-chip interface. While technologies such as epoxy under-fill aims to mitigate thermal mechanical stresses, thermal cycling reliability remains a serious issue in the electronic industry.

CHAPTER 3

RESEARCH OBJECTIVES AND APPROACH

Previous studies on electromigration lack information on the evolution of the solder crystallography. Even in studies where the initial grain orientation is considered, the final damage features are correlated to the initial crystal orientation which might raise questions since the grain structure changes significantly during the experiment.

With these critical issues in mind, this research work proposes to achieve the following objectives.

1) Obtain 3D datasets of Sn grain orientation and Cu6Sn5 intermetallic particle

distribution in industrially relevant length scale pure Sn solder joints using

Electron Backscatter Diffraction and Serial sectioning techniques.

2) Quantify the microstructural features such as Sn grain size and orientation, IMC

particle size and morphology in 3D, to understand the influence of microstructure

on the volume effect.

3) Develop an in situ electromigration setup inside a scanning electron microscope

and utilize it to study the evolution of damage (solder migration, intermetallic

growth and void formation) due to electromigration on pure Sn solder joints.

4) Understand the influence of thermal component on grain growth due to aging

during electromigration on pure Sn solder joints. 18

3.1 Investigation of Sn grain orientation in 3D

Solder joints with butt joint geometry, were be used for this study. 99.99% purity Sn solder spheres (Indium, Ithica N.Y.) and high purity oxygen free high conductivity

(OFHC) copper wires of diameters 500 m and 250 m diameters were used as precursors. Fig.1 shows SEM images of the solder joint prepared with a butt joint geometry and its typical microstructure. The solder joint was reflowed on a standard hot plate with the sample held in place by a Si wafer with a V-groove. A multi groove silicon wafer was fabricated for making the solder joints. Shown in Fig.2, the silicon v-groove stage consists of three grooves of different sizes. The v-groove, which was prepared in a clean room at ASU through a sequence of photolithography processes, (Xie et al. 2013),

(Fig 2.c.) is really beneficial because it helps maintain identical thermal gradient which is critical for reproducible microstructures. And also, since there are three grooves of multiple sizes, this allows preparation of samples of multiple sizes simultaneously under identical reflow conditions. Cooling rate after reflow was maintained at 1.25o/C by controlled cooling in a furnace. The reflow profile used for fabricating the joints is shown in Fig.3. The prepared joints were then mounted in epoxy and polished for microstructural analysis.

Fig.1: a) SEM image of cross-sections of a 250m solder joint used in this study. b) High magnification image showing distribution of fine Cu6Sn5 precipitates throughout the solder volume.

Fig.2: Silicon V-Groove stage used to prepare solder joints a) Schematic of setup and b) optical image of silicon wafer stage taped on to a copper block c) Flowchart describing the preparation method for the multi-groove silicon wafer stage (Xie et al. 2013).

Fig.3: Reflow profile followed for fabricating solder joints for this study.

For obtaining the OIM data, TSL OIM Data Collection and Analysis software was used.

Crystallographic orientation information at different depths was analyzed by combining serial sectioning technique and EBSD. The samples were polished on a semi-automatic polisher which was standardized to polish approximately 2m every cycle, represented in

Fig.4.

By performing mechanical polishing and EBSD alternatively, a stack of OIM images was obtained. Avizo Fire software was used to import these images as a stack and by the use of a proper interpolation technique, voxels were defined in 3D thereby rendering the entire microstructure in 3D.

Fig.4: a) Optical images of fiducial marks, which helps monitor the polishing rate. b) Plot showing standardized polishing rate as thickness loss vs polishing cycle. c) Vickers indent geometry using which the change in depth is calculated. 24

3.2 Investigation of the Cu 6Sn5 IMC distribution

Presence of Intermetallic compounds (IMCs) greatly influences the mechanical stability of the solder joint. The pure Sn solder sample used in this study has a wide distribution of

Cu6Sn5 IMC formation along the interface between Cu and Sn, and also within the Sn phase. The microstructure consists of a few, relatively large Cu6Sn5 needles and a large number of homogeneously distributed Cu6Sn5 precipitates (~ 300-500 nm) within the Sn phase.

Analysis of the Cu6Sn5 nanoparticles for size and distribution was done by segmentation using ImageJ software. The Cu6Sn5 nanoparticles are clearly visible in an SEM image.

By converting the image to grayscale, the IMC particles that have a specific range of gray values, can be identified and segmented for separate analysis. By the use of ImageJ software, it is possible to calculate the fraction of IMC particles in the region of interest, size, aspect ratio, etc.

3.3 Study of the Volume effect

An extension of this study can be done by analyzing the crystallographic orientation and the IMC distribution on solder volumes of different sizes. Researchers have noted that there is a significant influence of the volume change on the microstructure. Because of the local cooling rates differences, it is difficult to estimate the microstructural changes that occur in extremely small volumes. By performing the 3D analysis on solder joints of

smaller sizes, we would be able to get an idea of the volume effect which is a key to the understanding the microstructural evolution.

3.4 In situ electromigration testing jig

For in situ electromigration in an SEM, a custom sample fixture was designed and fabricated for the study of electromigration damage in butt joint solder samples at elevated temperatures (Mertens et al. 2015). The sample jig was designed taking into considerations some key aspects such as electrical and thermal conductivity, thermal expansion coefficients, size, specimen constraint, auxiliary ports for attachment to a programmable ceramic heater and x-ray transparency for x-ray tomography scanning.

The highlight of this sample jig is that it is designed to be complementary with the in situ

XCT system in our lab. Thus the same sample fixture can be used for both in situ SEM studies as well as in situ x-ray tomography experiments.

Fig.5 shows a CAD model of the sample fixture. The sample is composed of two aluminum blocks that houses butt joint geometry solder joints between 150 m and 500

m in diameter. The two aluminum blocks are held together by a Zerodor glass rod which is electrically insulating and x-ray transparent. To provide heating during electromigration, a programmable ceramic heater (Watlow Inc., St. Louis, Missouri) is mounted to the fixture.

Aluminum has a good combination of stiffness, strength, electrical and thermal conductivity and hence was suited best for making the blocks. Cylindrical sample through holes were made using 500 m EDM hole-punch. Set screws on each aluminum 26

block hold the sample in place between the two blocks. The Zerodor glass rod, 1.5 mm in diameter, is similarly fixed between the two Al blocks using set screws. The glass rod provides a mechanical bridge between the Al blocks, but is electrically insulating and thus current passes only through the solder sample. The Zerodor glass rod also has near zero thermal expansion and thus does not impart any external stresses on the sample especially at elevated temperatures during electromigration testing. Even though the glass rod is brittle, it possesses sufficient strength for supporting the fixture. Each Al block has ports where positive and negative lead wires from the power supply can be attached using set screws. And finally, an extra port of available for attaching an external thermocouple to measure sample temperature during testing.

Fig.5: CAD model of the sample fixture. Key design features and their descriptions are mentioned (Mertens et al. 2015).

The programmable ceramic resistive heater is an 8mm x 8mm square, which easily sits the Al sample jig. The setup with the heater attached to the sample jig is shown in Fig.6.

The heater is capable of going up to 200°C. The heat transfer from the ceramic heater to the sample jig is purely conductive, and thus using a small amount of thermal grease is recommended for reducing any contact heat loss.

Fig.7 shows the Scanning Electron Microscope used for this study. Apart from the EBSD capability of the system and the newly installed electromigration setup, the SEM also has an in situ nanoindenter for mechanical property analysis. This multi-function capability makes this SEM truly one of a kind for in situ studies.

Fig.6: CAD model of the sample fixture with the Watlow ceramic heater attached to it

(Mertens et al. 2015)

Fig.7: JEOL JSM 6100 Scanning Electron Microscope with EBSD, in situ nanoindenter and in situ electromigration testing capabilities.

CHAPTER 4

3D CHARACTERIZATION OF TIN CRYSTALLOGRAPHY AND Cu6Sn5

INTERMETALLICS IN SOLDER JOINTS BY MULTISCALE TOMOGRAPHY

4.1. Introduction

Solder alloys have been used extensively in the electronics industry for several decades.

The transition to Pb-free interconnects has increased the demand for Sn-rich solder alloys

(Wood and Nimmo 1994). The development of three dimensional (3D) packaging technologies and decreasing pitch size has significantly reduced the volume of solder spheres and interconnects in the package. While the Sn crystallography and distribution of intermetallic compounds (IMCs) of common alloys have been widely investigated

(Rollett et al. 1992, Fan et al. 1998, Holm and Battaile 2001), the evolution of solder microstructure is not well understood, especially with decreasing length scale. This is largely due to the two dimensional (2D) nature of microstructural analysis. In addition, at small length scales, the microstructure is highly heterogeneous and a statistical representative volume cannot be obtained to represent the properties of the solder joint.

In solder systems, Sn is the primary constituent and its body centered tetragonal (BCT) structure is strongly anisotropic making crystallographic orientation analysis extremely important. Indeed, Sn grain size, shape, orientation and texture have an important role in influencing solder reliability (Lu et al. 2008).

Lehman et al. (Lehman et al. 2005) studied the effect of miniaturization and showed that decreasing the solder volume increases the undercooling behavior. Randomly oriented

“interlaced” areas were seen to appear more frequently. Dimensional constraints in such small volume solder joints influence the mechanical properties, and several experimental

(Zimprich et al. 2008, Yin et al. 2009) and numerical (Cugnoni et al. 2006) studies have been conducted to investigate this effect. As the solder volume decreases, the bulk behavior is gradually lost, leading to a more orientation dependent structure. As the solder volumes get smaller, it is expected that there will be a fewer number of grains.

Thus, a particular grain orientation may be more influential in controlling the deformation behavior of that volume. (Lu et al. 2008)

A significant amount of work has been done on characterizing the crystallography of tin and the intermetallics in 2D, especially under conditions of electromigration and thermal fatigue. Recent work done by Mertens et al. (Mertens et al. 2015, Mertens et al. 2016) shows the volumetric evolution of Cu6Sn5 IMCs within the solder microstructure using

X-ray tomography and EBSD, and discusses the strong influence of orientation in estimating the effective diffusivity of Cu with respect to a-axis and c-axis of tin. Based on the X-ray tomography data, a large chunk of Cu6Sn5 IMC was seen to have formed within the solder volume during electromigration. This is a serious reliability issue and it would not have been identified unless a 3D characterization technique is used. In such cases, to understand the performance and true behavior of a solder joint, 3D reconstruction, visualization, and quantification of the different microstructural features becomes necessary.

Among the several 3D characterization capabilities currently available, serial sectioning is a well-established technique to characterize and visualize the microstructure. Serial sectioning work done by Sidhu and Chawla (Sidhu and Chawla 2004) on Ag3Sn intermetallic particle distribution showed the large deviation in IMC needle aspect ratios between 2D and a 3D measurements. As the size of these IMCs were of the order of several microns, fine mechanical polishing and optical microscopy techniques were sufficient for reconstruction and 3D analysis. In order to resolve finer particles measuring less than a micron in diameter, a focused ion beam serial sectioning process can be utilized. Recent advancements in dual beam focused ion beam scanning electron microscopy (FIB-SEM) have greatly increased the rate of data collection and allow large datasets to be analyzed and visualized in 3D. A study on the influence of oxidation of trace amount of rare earth intermetallics (RESn3 particles) in favoring whisker growth using a FIB serial sectioning approach revealed how large IMCs tend to grow whiskers and smaller IMC particles favor growth of hillock-type whiskers (Dudek and Chawla

2009).

Considering the complex structures of Sn grains and the Cu6Sn5 intermetallic distributed in the material, a methodology to perform 3D characterization study on both Sn crystallography and the size and distribution of the intermetallic particles has yet to be developed. The challenge here is that the Sn grains and Cu6Sn5 particles are present at different length scales. The present work utilizes crystallographic characterization by

EBSD in conjunction with serial sectioning technique using mechanical and FIB polishing to obtain 3D experimental volumes of the different microstructural features.

With these techniques we were able to elucidate the microstructure of both the Sn grains and Cu6Sn5 intermetallic needles and nanoparticles as a function of solidification on a copper substrate (Kirubanandham et al. 2016).

4.2. Materials and Experimental Procedure

Cu/pure-Sn/Cu sandwich joints were prepared in this study. Sn solder spheres (99.99% purity, Indium, Ithica N.Y.) with a diameter of 250 m were reflowed between two polished high purity oxygen free high conductivity (OFHC) copper wires. A silicon v- groove assembly was used for solder reflow on a programmable hot plate. The sample stage is a multi-groove silicon wafer consisting of 3 grooves of different sizes capable of holding solder joints of sizes anywhere between 500 m and 100 m. The wafer stage was fabricated using photolithography and anisotropic KOH etching techniques, similar to the wafer stage described elsewhere (Xie et al. 2013). These grooves enable us to maximize the alignment of the sample during reflow and also allow multiple samples to be reflowed simultaneously under identical conditions, thereby allowing us to maintain the same applied cooling rate for all specimens for a given reflow. The polished copper wire and solder sphere preforms were coated with a rosin-mildly-activated (RMA) flux

(Indium, Ithica N.Y.) and reflowed to a peak temperature of 250oC (approximately 20oC above the melting point of Sn) with a soak time of 40s. Samples were cooled at approximately 1.2oC/s after reflow by controlled cooling in a furnace. The fabricated butt joint solder samples were then cleaned using acetone followed by isopropanol and then mounted in graphite based conductive epoxy mount.

Fig.8: Optical micrograph showing typical microstructural features in a 250 m pure Sn solder joint. The majority phase is β-Sn which contains a dispersion of Cu6Sn5 IMC particles.

Samples were metallographically polished for electron backscatter diffraction (EBSD) analysis. 0.3 m alumina and 0.05 m colloidal silica solutions were used in the final polishing step. Fig. 8 shows an optical image of the polished cross-section. To monitor the section depth after each polishing cycle of serial sectioning, fiducial marks were made by a Vickers indenter on the copper wires. The depth of material removal was calculated by measuring the change in indentation diameter, as described elsewhere (Sidhu and

Chawla 2004). Determining the distance between slices in serial sectioning depends on the size of the smallest feature being visualized. The solder microstructure revealed some of the smallest grains sizes to be between 6 m to 8 m. Hence a semi-automatic polisher was standardized to polish 2 m per polishing cycle which gives about 3 to 4 sections per grain.

A scanning electron microscope (JEOL JSM-6100) equipped with an EBSD (TSL™-

EDAX, Mahwah, N.J.) system was used to obtain EBSD patterns. Orientation image mapping (OIM) was conducted with a step-size of 0.6 m on the entire solder joint cross- section for every polishing step. OIM maps were analyzed using TSL OIM Data

Collection and Analysis software. To reduce isolated noise in the OIM maps, a three step cleaning process using ‘neighbor CI correlation’, ‘grain dilation,’ and ‘single average orientation per grain’ methods was followed. Repeating this process for every polished section, a stack of 2D OIM images are obtained which can be segmented and reconstructed to obtain the complete 3D experimental volume. Additionally, by performing backscattered electron imaging (BSE) at every polishing step, the IMC particle distribution can be obtained due to phase contrast and this stack of 2D images 37

can also be reconstructed into a 3D experimental volume. The segmentation of Sn grains and the IMC particles was done using a combination of grayscale processing using

ImageJ (Bethesda, MD) and 3D region grow and reconstruction using the Magic wand tool in Avizo® Fire software (VSG, Burlington, MA) which was also used for visualization and 3D quantification analysis.

To visualize and quantify the true morphology of the nanosized Cu6Sn5 IMCs, FIB serial sectioning study was done at two volumes approximately 20 m x 20 m x 10 m.

During reflow, as Cu diffuses in Sn from the interfaces, the local compositional differences may influence the size and morphology of the Cu6Sn5 nanoparticles. To examine if such differences exist, one region close to the interface spanning several low angle boundaries and the other region closer to the center of the solder joint was chosen for FIB serial sectioning. A dual beam focused ion beam scanning electron microscope

(Nova 200 NanoLab FEG FIB-SEM, FEI Co, Oregon) was used. An accelerating voltage of 30 KeV, Ga+ ion beam with 1 nA current was used to mill sections through the thickness and a final polishing with 0.3 nA current was done to sequentially obtain a stack of 2D images. A thin layer of platinum (approximately 1 m) was deposited on the surface to minimize FIB damage. A rectangular trench was milled out in front of the volume of interest, with a range of currents from 7 nA to 0.3 nA, to produce a clean cross-section through the thickness. Based on the size of the Cu6Sn5 nanoparticles

(approximately 500 – 700 nm), sections were milled approximately 300 nm apart. A total of 30 sections were obtained and after each milling step, ion channeling images were taken with the ion beam at 0.1 nA current. Ion channeling images produce good contrast 38

with the Cu6Sn5 IMCs and between certain grain orientations. These image were then used for segmentation, 3D reconstruction, and quantification.

4.3. Results and Discussion

4.3.1. 3D Sn grain orientation

The majority phase in the joint is β-Sn rich with a dispersion of fine Cu6Sn5 IMC nanoparticles and Cu6Sn5 needles with hexagonal cross-section. Secondary electron (SE) and backscattered electron (BSE) images with the corresponding OIM map of a single cross-section of a 250 m pure Sn solder joint are shown in Fig.9. The EBSD scan dimensions are 320 m by 290 m with a lateral step size of 0.6 m and a total of 65

OIM slices. The EBSD maps indicate the grain orientation normal (ND) to the solder sample surface. A dispersion of Cu6Sn5 nanoscale IMCs, about 300 nm in diameter, segregated both at the grain boundaries and within the Sn grains. Apart from the nanoparticles, several Cu6Sn5 IMC needles were also observed within the solder volume indicated by red arrows in Fig. 9b. These IMC needles were seen to have hexagonal cross-section and often had hollow cores. Work done by Frear et al. (Frear et al. 1987) explains that these IMC needles form when molten Sn comes in contact with Cu, forming hexagonal rods along a screw dislocation using a ledge mechanism. Due to the turbulence in molten Sn during reflow, these rods break from the Cu interface and drift into the solder volume, where a local deficiency of Cu tends to dissolve the rods from its high energy sites i.e., the screw dislocation core. Molten Sn flows into this hollow tube and eventually solidifies. 39

Fig.9: a) Secondary electron image b) Backscattered electron image c) OIM map of 250

m pure Sn solder joint. 40

OIM analysis showed the presence of thinner elongated grains with the grain growth direction normal to the Cu interface on either side. This indicates a high local cooling rate influenced by the high thermal conductivity of copper. Grains closer to the center however are larger and more equiaxed indicating a limited heat flow and, thus, favoring more grain growth. The thin elongated grains were about 5-7 m in width with the average length normal to the Cu interface exceeding 50 m. The larger equiaxed grains in the center had an average grain size of 90 ± 20 m.

Fig.10: a) 2D OIM stack of the 250 m solder joint and b) 3D reconstructed volume of

Sn grain orientation c) Segmented volume from the 3D dataset showing grains colored with respect to the OIM map and rendered in 3D though the thickness.

Upon nucleation, the Sn grains grow rapidly and there could be growth competition between neighboring grains. Sn grains nucleate from the Cu/Sn interfaces and also from

Cu6Sn5 IMC needles that form during reflow within the molten solder. However, the surface area of the Cu/Sn interface is much larger compared to the IMC needles and thus provides more nucleation sites.

The extent of local cooling rate differences within the solder volume was analyzed by measuring the aspect ratio of the Sn grains which were segmented as shown in Fig. 10c.

3D aspect ratio measurement as a function of position of grains from either Cu interface was plotted. The centroids of the grains were identified and the x-coordinates were used for assigning the grain position with respect to the left interface. A bimodal type trend was clearly seen with high aspect ratio grains (> 4 – 5) present at either interface, with few large equiaxed grains in the center as shown in Fig. 11a.

Fig.11: a) Aspect ratio plot showing the extent of formation of elongated grains b)

Misorientation plot giving the distribution of grain boundary rotation angles within the solder volume and c) OIM map of a single cross-section with high angle boundaries (>

15o) highlighted in black.

The Sn grains showed extensive low angle boundary formation with 38% of the grain boundaries having misorientation angle less than 15o. The majority of these low angle grain boundaries were present close to the interface, show in in Fig. 11c, which can be explained by the high local cooling rate. Also, it can be seen from Fig. 11b, that there is a significant amount of grain boundaries with rotation angles between 55 – 70o. This is common in Sn solder systems as twinning occurs during Sn solidification with twins forming on {1 0 1} and {3 0 1} crystal planes with twinning angles 57.2o and 62.8o respectively (Lehman et al. 2004, Borgesen et al. 2007, Lehman et al. 2010).

4.3.2. Investigation of Cu6Sn5 IMCs

Fig.12: Segmentation of Cu6Sn5 nanoparticles. a) SE image of a magnified part of the solder cross section. b) Cu6Sn5 phases are selectively thresholded by their grayscale value. c) Segmented Cu6Sn5 IMCs as black features in a white matrix. 48

Table I: 2D quantification results of the Cu6Sn5 nanoparticles

Area fraction Mean area Mean maximum feret diameter Aspect ratio

2.18 % 0.235 m2 0.69 m 1.64

The mean IMC thickness at the Cu interfaces was approximately 3.7 ± 2 m. The high standard deviation can be attributed to the scallop-type morphology of these intermetallics, which can be clearly seen in Fig. 12. The Cu6Sn5 nanoparticles were segmented in all 65 serial sections and were quantified in 3D. To enhance the contrast of these nanoparticles, a two-step filtering process using Band-pass filter followed by anisotropic diffusion filter was used prior to grayscale thresholding. The volume fraction of these IMCs in the reconstructed volume was measured as 1.67% and Fig. 13 shows the reconstruction in 3D.

Fig.13: 3D Reconstruction of segmented a) Cu6Sn5 nano-IMCs and b) Cu6Sn5 hexagonal needle IMCs with a magnified cross-section showing the hollow needle morphology.

The Cu6Sn5 IMC needles were also segmented, reconstructed and quantified in the same volume from BSE images as described in (Chen et al. 2015). Analysis showed that the large IMCs had a high mean aspect ratio exceeding 3.4 and the results are given in Table

II.

Table II: 3D quantification results of the Cu6Sn5 IMC needles

Volume fraction 3.1 %

Mean aspect ratio 3.4 ± 1.8

Max length > 150 m

FIB cross-sectioning was done to study the morphology of the nano-sized Cu6Sn5 IMCs in detail. This was done at two separate volumes in a 250 m solder joint, one close to the interface and the other closer to the center, as shown in Fig. 14.

Fig.14: a) Ion channeling image of a 250 m pure Sn solder joint showing Tin grain contrast. Two dotted boxes, one close to the interface and the other at the center, mark the areas from where FIB serial sectioning was done b) FIB trench is milled in front of the region of interest after thin platinum layer deposition c) Cu6Sn5 IMC distribution at region 1 (closer to Cu interface) and d) magnified region within region 1 showing nearly

spherical morphology of Cu6Sn nano-IMCs e) Cu6Sn5 IMC distribution at region 2

(closer to the center of the solder joint).

FIB cross-sectional analysis shows the nearly spherical morphology of the Cu6Sn5 nano-

IMCs. Region 1 was extracted close to the Cu interface and it can be seen that the IMC prefer to segregate at the grain boundaries, and hence have a directional arrangement perpendicular to the interface. Region 2 however, is more non-directional but still shows selective segregation of clusters at locations which were found to be triple points in grain boundaries. The volume fraction of these two volumes were fairly close at 2.1% and

1.6% for regions 1 and 2 respectively. The mean aspect ratio was 1.8 ± 0.4, which verifies the nearly spherical morphology observed.

The extensive 3D reconstruction and quantification analysis done on the Sn grains and

IMC phases can be utilized for several computational modeling studies on predicting the time dependent evolution of microstructure under external stimuli. As an addendum to this study, statistical information on grain morphology, orientation and spatial distribution of IMC particles were extracted from single 2D EBSD and BSE micrographs and virtual 3D microstructures were stochastically reconstructed using two-point correlation functions (Chen et al. 2015). Considering the highly heterogeneous microstructure, a multi-scale computational procedure was adopted and the accuracy of reconstruction was compared with the experimental volume obtained via serial sectioning.

4.4. Conclusions

A multi-scale methodology to conduct 3D characterization of Sn grain crystallography and Cu6Sn5 intermetallics was presented by coupling EBSD imaging, SEM/FIB imaging, and serial sectioning technique. Reconstruction and visualization of true 3D morphologies of the Sn grains and Cu6Sn5 intermetallics within the solder was conducted.

A 3D EBSD reconstructed volume measuring 320 m x 290m x 130m, was quantified for the grain aspect ratio as a function of distance of the Sn grains from the Cu interfaces, and the influence of local cooling rate differences was elucidated. Grain boundary misorientation distribution showed that 38% of the boundaries were low angle

(< 15o) and that the majority of these were present near the elongated grains at the Cu interface.

3D visualization and quantification of Cu6Sn5 nanosized IMCs and hollow hexagonal needles yielded volume fractions of 1.67% and 3.1% respectively. True morphology of the nano-IMCs was further investigated using FIB-serial sectioning and a mean aspect ratio of 1.8 ± 0.4 verified the nearly spherical morphology of the nanosized particles.

While 2D analysis give us an idea of the heterogeneity of solder microstructures, performing 3D quantification of morphology and distribution prove to be very valuable for better understanding the microstructural evolution in small scale solder joints.

CHAPTER 5

EX SITU CHARACTERIZATION OF ELECTROMIGRATION DAMAGE IN PURE Sn

BASED SOLDER JOINTS

5.1. Introduction

Chapter 5 presents an investigation of electromigration induced microstructure evolution, specifically focusing on the evolution of crystal orientation of pure Sn solder joints and

IMC phases. Due to the continuing trend of downsizing the bump size, fine pitch solder joints could easily experience current densities in excess of 104 A/cm2. This is a serious reliability issue in the microelectronics packaging industry. The microstructures of small scale solder joints tend to be very heterogeneous, where solder joints could be composed of very few grains. In these cases, anisotropic effects of Sn grains play a crucial role in determining electromigration lifetime of a component.

Over the years, the crystallographic dependence of electromigration in solder joints has been studied extensively. Reports have suggested several degradation phenomena to take place, however the evolution of electromigration damage has not been understood well. It is important to study how the microstructure and damage features evolve over time, to explain dominant failure mechanisms that undergoes in solder joints during electromigration. An in situ investigation is thus required to monitor how damage initiates and simultaneously characterize the evolution of crystal orientation of tin grains due to grain growth. This investigation presents an electromigration test fixture that was

utilized to test butt joint geometry pure Sn solder joints while monitoring grain growth and electromigration damage over a period of 500 hours.

5.2. Materials and Experimental procedure

The test vehicle used for the study is a 250 m pure Sn solder joint. Fig. 15 shows secondary electron and backscattered electron images of the test sample. To study the crystallographic orientation of the Sn grain structure, we need a flat polished surface for doing EBSD. The sample was thus polished approximately half way through the thickness leaving an unencapsulated solder joint with free surface all around. This is also beneficial for accelerated electromigration testing as there will be more surface for damage to occur without constraints.

Fig.15: a) Secondary electron image and b) backscattered electron image of the solder joint. 58

The solder joint was mounted in the electromigration test fixture and electrical lead wires were connected to the power supply. Circuit diagram shown below in Fig. 16 describes the test setup and connections to the computer for programming the electromigration/thermal fatigue experiments.

Fig.16: Circuit diagram of the in situ electromigration testing setup.

To monitor the temperature of the solder during electromigration, a secondary thermocouple was fed through the chamber containing the solder joint until the joint’s copper wire was contacted with the couple. This connection picks up the temperature of the joint after any Joule heating.

The least cross section area of the sample was measured to be approximately 2.51 x 10-4 cm2. The target current density was fixed at 104 A/cm2 which would require an applied current of about 2.51A. The interface with the least cross section was connected to the circuit as the anode as shown in Fig. 17.

Fig.17: Schematic diagram of the test sample for electromigration showing current direction.

The sample jig was maintained at a temperature of 100oC with the help of the ceramic heater, for accelerated testing conditions. The test was run for a total duration of 500 hours. The initial resistance of the sample was measured to be 4.2mΩ.

The Sn grain orientation and electromigration damage to the sample was studied by performing EBSD and SEM imaging at every 24-hour time step. Fig. 18 shows the change in resistance as a function of time.

Fig.18: Resistance vs time plot for a total period of 456 h showing no significant change in resistance. Small drops/fluctuations are points where the current was turned off for

EBSD analysis. 64

5.3. Results and Discussion

The plot (Fig. 18) shows that the resistance was almost constant during the entire test duration. The small drops seen on the resistance profile correspond to the interruptions in test for conducting EBSD experiments. At about 70 hours into the test, a small jump of about 10% increase in resistance was captured. Although 10% increase in resistance sounds to be a significant jump, it should be noted that the change in resistance in still in the milli ohms range. This sudden increase in resistance was thought to be due to initiation of some sort of damage in the sample, however SEM imaging did not reveal any kind of failure feature.

Fig.19 shows the initial Sn grain crystal orientation and its evolution over a period of 144 hours. The microstructure is polycrystalline with a significant number of fine grains near the interface and a few large grains close to the center. This microstructure is expected and is as seen in the earlier study on 3D OIM characterization.

Fig.19: Orientation image maps a) Before electromigration testing b) after 24 hours testing c) after 48 hours testing d) after 72 hours testing e) after 96 hours testing and f) after 144 hours testing. 66

From the OIM images, it can be inferred that there is a steady and significant grain coarsening that is occurring throughout the sample. By the end of the 144 hours test duration, the mean grain size has significantly gone up. Whether this grain coarsening is due to thermal aging or thermal migration is still to be confirmed. What is crucial is that the heterogeneity of the microstructure is constantly increasing; meaning that the influence of orientation is becoming stronger as time progresses. As the electromigration test was done in air, the surface oxide resulted in poor EBSD patterns after 144 hours.

Hence grain growth was monitored only till 144 hours.

The orientation of the large blue grain that is preferentially growing is (-16 -12 -1) [-2 1

21]. The crystal lattice representing this orientation is shown in Fig. 20, which indicates that the c-axis is nearly normal to the electron flow direction. During electromigration and thermal aging, tin grains have been reported to favorably orient towards directions of least electrical resistance. Tin grains exhibit better electrical conductivity along a and b axes, compared to the c-axis direction. As the large grain is already oriented along low electrical resistance direction, it experiences preferential grain coarsening.

Another significant part of the microstructure that needs attention is the Cu6Sn5 IMC phase. At low magnification, there are no indications of severe electromigration damage.

The cathode is expected to form voids and the anode is expected to pile up. But since the electromigration damage is still in the early stages, there no significant damage on the sample surface as seen in Fig. 21.

Fig. 20: Modified orientation image map highlighting orientation along the electron flow direction. Large crystal lattice indicates c-axis nearly normal to the electron flow direction. Grains within the red circle in the top left indicate c-axis orientation parallel to electron flow direction.

Fig. 21: Backscattered electron images (a) Before electromigration testing (b) after 144 hours testing. 69

But taking a closer look at the IMC phase close to the interfaces, it can be seen that there is some dissolution of Cu6Sn5 at the cathode (Fig. 22) and an increase in IMC layer thickness at the anode as seen in Fig. 23. This can be quantitatively measured as shown below in Fig. 25 and Fig. 26.

Fig. 22 shows that the Cu6Sn5 IMC needle is consumed more rapidly compared to the scallop-type IMCs at the cathode interface. This can be explained by looking at orientations of the grains surrounding the IMC needle as marked within the red circle in

Fig. 20. The crystal lattice overlays of the grains show that the grains have their c-axis aligned with the direction of electron flow. And since c-axis favors faster diffusion of Cu atoms, the IMC needle is consumed faster.

Fig.22: Backscattered electron images of the cathode interface a) Before electromigration and b) after 144 hours testing. The images show that a significant amount of IMC dissolution has occurred at the cathode marked by [1]. 71

Consequently, at the anode there is a slight increase in the IMC layer thickness along with a few other failure features as seen in Fig.23.

Fig.23: Backscattered electron images of the anode interface a) Before electromigration and b) after 144 hours testing. The images show that apart from the increase in IMC, there seems to be some failure features marked by [1], [2] and [3]. 72

At the anode interface, unexpected failure features were seen close to the IMC layer.

Marked by [1], the first failure feature seen is a lot of delamination at the IMC/Sn matrix interface. The reason behind this is thought to be the presence of pre-existing porosities.

These local porosities may serve as locations for current crowding to occur. As we may know, current crowding drastically increases the chance of electromigration failure and this might be the reason that we see delamination of IMC particles.

Second, marked by [2], some of the IMC scallops at the Sn/Cu were seen to have fractured. The reason behind this might be the combination the crack formation due to [1] and the effect of thermal expansion mismatch between the matrix, IMC particle and the

Cu substrate. Already existing and growing cracks due to delamination may cut through the brittle IMC particles as they are exposed to accelerated electromigration conditions.

The third and final failure feature, marked by [3], is crack like features that seem to form along the grain boundaries. Since grain boundary diffusion is the dominant diffusion mechanism in this system, the mass flow is higher at grain boundaries. Occasionally when this mass flow is obstructed at a triple boundary or a grain boundary with high misorientation angle, the grains tend to rotate in such a way, at the surface it would look as if the surface roughness is higher. These crack like features are actually grain boundary ledges that form due to the cycling heating and cooling of the sample for every time step. This can be clearly seen in Fig.24.

Fig.24: Backscattered electron images of the anode interface a) after 24 hours b) after 96 hours and c) after 144 hours testing. The images show crack like features that seem to increase progressively with time. d) SEM image of the sample placed on an inclined stage clearly reveals the delamination, particle fracture and grain rotation that makes the surface look rougher and seem to appear like cracks on the backscattered electron image.

Fig.25: a) Backscattered electron image of cathode interface b) Thresholded IMC layer before electromigration and c) after 144 hours testing showing a decrease in thickness of about 0.4m.

Fig.26: a) Backscattered electron image of anode interface b) Segmented IMC layer before electromigration and c) after 144 hours testing showing an increase in thickness of about 1.2m.

To monitor any change in the IMC layer orientation, EBSD was performed at the cathode interface. As shown in Fig.27, apart from the dissolution of the IMC, there was no significant orientation effect at the IMC layers.

Fig.27: a) Backscattered electron image of cathode interface b) OIM of IMC layer before electromigration and c) after 144 hours testing showing an increase in thickness of about

1.2m.

5.4. Summary

With an applied temperature of 100oC and current density of 104 A/cm2, the electromigration damage and microstructural evolution in a 250 m pure Sn solder joint was studied. An in situ electromigration stage was designed and installed inside a scanning electron microscope, which facilitates crystallographic analysis using EBSD without the need to remove/handle the sample. Interrupted ex situ electromigration testing showed three main damage features – Delamination at the IMC/Sn matrix interface, fracture of IMCs at the interface and the formation of grain boundary ledges.

Over the first 144 hours, significant grain growth was seen, which was monitored by stopping the test every 24 hours. Due to the interruptions in current and temperature every 24 hours, a low frequency thermal cycling effect was induced. This cyclic heating and cooling resulted in the formation of grain boundary ledges that appeared as crack-like features. The presence of pre-existing porosities combined with the thermal expansion differences at the solder/IMC/Cu interface resulted in delamination and fracture of scallop-type Cu6Sn5 IMC particles. Even after 500 hours, the resistance was stable at 5 mΩ suggesting that these damage features are still in the early stages. This investigation provided a detailed overview of early stage of electromigration, where initiation and propagation of damage features was studied.

CHAPTER 6

AN IN SITU STUDY ON THE INFLUENCE OF TEMPERATURE ON FAILURE

MECHANISMS IN PURE Sn BASED SOLDER JOINTS

6.1. Introduction

Traditionally accelerated electromigration tests combine the use of high current density and high operating temperature to hasten failure mechanisms. High current density is known to create a large electron wind force that displaces metal atoms from the cathode to the anode interface. Considering the anisotropic nature of tin, it is also known that diffusion of atoms is faster though the c-axis as compared to the packed a-axis or b-axis.

However, due to the high operating temperature, tin experiences homologous temperatures in excess of 0.7. At these temperatures, grain growth is significant and this dynamic change in microstructure could drastically change the expected failure mechanisms during electromigration.

In the previous chapter, an in situ electromigration stage was designed and developed to fit inside the vacuum chamber of a JEOL JSM-6100 SEM. Electromigration damage evolution was studied on a 250 m pure Sn solder joint with applied current density of

104 A/cm2 and temperature of 100oC for a total test duration of 500hrs. The results revealed significant grain growth and grain boundary ledge formation along with intermetallic delamination and fracture at the solder/Cu interface. The uncertainty in preferential grain growth and the extent of influence of the thermal component in electromigration served as the basis for this investigation.

This chapter presents a systematic approach to understand grain boundary ledge formation and propagation. EBSD analysis is done to compare observed trends in ledge formation with the crystallographic orientation of the Sn grains. In order to accelerate the degradation mechanisms, a thermal cycling experiment is conducted which elucidates the influence of orientation and microstructural features on the observed damage.

6.2. Materials and Experimental procedure

Solder joints with butt joint geometry, similar to samples discussed previously, were prepared. 99.99% purity Sn solder spheres (Indium, Ithica N.Y.) and high purity Oxygen free high conductivity (OFHC) copper wires were used as precursors. The solder joint was reflowed on a hot plate with the sample held in place by a Si wafer with v-grooves.

Cooling rate after reflow was maintained at 1.25oC by controlled cooling in a furnace.

The reflow profile used is shown in Fig.3. The prepared butt joints were then superglued on a conductive epoxy mount and polished roughly about half way through the thickness for microstructural analysis. After polishing, the sample was removed from the epoxy mount by dissolving the superglue using acetone in an ultrasonic cleaning setup. The cleaned samples are then mounted on to the in situ electromigration jig as discussed previously.

To study the influence of the thermal component in an electromigration test, a 500 m pure Sn solder joint was prepared and aged in situ at 100oC for a total test duration of

180hrs. This test was performed in an interrupted mode where the in situ heater was turned off for EBSD analysis and SE imaging, after aging for specific periods of time.

This was done to replicate the interruptions in electromigration testing experienced by the sample discussed in chapter 5. Although studies on free surface damage formation in solder joints report random formation of grain boundary ledges, there is no clear understanding if these ledges actually influence solder joint failure.

The optical image and OIM map of the sample used for this interrupted aging test are shown in Fig. 28. SE-BSE images along with OIM maps were obtained every 24 hours by EBSD. The scan was performed on the entire cross-section with a step size of 1 m spacing. At the end of every 24 hours aging step, the heater was turned off for OIM and

SE imaging. A second sample, also fabricated in a similar process, was used for a continual aging test. This test was done to verify whether alternate mechanisms such as grain rotation and grain boundary sliding are active at the 100oC operating temperature.

Lastly, a similar 500 m pure Sn solder joint was used to study the thermal cycling behavior and damage development. The sample was cycled between room temperature and 150oC with a cycle time of 1 hour. EBSD and SEM imaging was done initially before the test and during the test, SEM images were taken to monitor the evolution of damage features.

Fig.28: a) Optical micrograph of 500 m solder joint before interrupted thermal aging. b) Optical micrograph with OIM image overlay. 82

6.3. Results and Discussion

6.3.1. Interrupted aging experiment

The interrupted aging test revealed that after 24hrs, the surface showed formation of crack-like features at the grain boundary which were seen in backscattered electron images – as shown in Fig. 29. On continuing the experiment, these features were seen to multiply forming banded structures, which was assumed to be due to formation of grain boundary ledges. Correlating the secondary electron images with the OIM map, it can be seen from Fig. 30 that these ledges nucleate at triple points and form at high angle grain boundaries which are fast diffusion pathways. Furthermore, Fig. 30 also shows that the presence of fine Cu6Sn5 IMCs creates a pinning effect, which also influences the formation and propagation grain boundary ledges.

Fig.29: BSE images of the 500 m solder joint showing evolution of banded grain boundary ledge formation. a) 24 hours aging, b) 120 hours aging. 84

Fig.30: a) SE image of the 500 m solder joint showing banded grain boundary ledge formation. b) Orientation map of the same region with misorientation angles between grains 1 through 6 indicating all high angle boundaries. c) SE image at higher magnification showing the banded grain boundary ledges pinned by the Cu6Sn5 IMC particles. 85

Work done on electromigration and thermal fatigue on solders by researchers show similar surface features forming, some of which have been explained the observed surface roughness as a result of grain boundary sliding and/or grain rotation to reduce the overall resistance to current flow. Since grain boundary ledges only form during the cooling cycle after operating at high temperature, it can be proved that the crack like features were indeed ledges by performing a continuous aging and monitoring of microstructure using the in situ electromigration stage.

6.3.2. Uninterrupted aging experiment

A 500 m pure Sn solder sample shown in Fig. 31, was aged at 100oC for a period of 180 hours, just as the previously tested solder joint, however now SE/BSE images were taken at high temperature without turning off the heater and the surface revealed no change

(Fig. 32). The sample discussed in chapter 5 was inadvertently thermal cycled by stopping the heat for taking EBSD images and SE/BSE images, thereby stressing the sample producing grain boundary ledges.

Fig.31: a) Secondary electron image of the 500 m pure Sn solder joint used for the high temperature observation test. b) EBSD map of the corresponding cross-section.

Fig.32: SE images of the 500 m solder joint showing no grain boundary ledge formation. a) Before aging, b) After 180 hours aging while the sample is still maintained at 100oC. 88

6.3.3. In situ thermal cycling behavior

Fig. 33 shows the secondary electron and backscattered electron images along with the

OIM map of the 500 m pure Sn solder joint used for the thermal cycling experiment.

The sample was cycled between room temperature and 150oC with a cycle period of 1 hour. Table III gives the test parameters used for thermal cycling and the applied temperature profile can be seen in Fig. 34.

Table III: Parameters used for thermal cycling test

Ramp up/down rate 25oC/minute

Ramp up/down time 5 minutes

Hold time 25 minutes

Cycle period 60 minutes

Fig.33: a) Secondary electron image of the 500 m solder joint prepared for thermal cycling. b) Backscattered electron image, and c) OIM map of the corresponding cross- section.

Fig.34: Temperature profile for 10 cycles of thermal cycling.

Grain boundary ledge formation was seen within the first 10 cycles. The experiment was conducted in situ inside the SEM and SE/BSE images were taken to monitor sample surface degradation at every 10 cycles. SEM images as shown in Fig. 35, showed ledge formation almost throughout the entire cross-section of the solder joint and there was significant stress relief at the surface indicating that anisotropy of tin grains and coefficient of thermal expansion mismatch at the Cu/Sn and Cu6Sn5/Sn interface also strongly influence the sample degradation.

Table IV: Thermal expansion coefficients for the different phases in pure Sn solder joints

Material Coefficient of Thermal Expansion (x 10-6 /K )

Sn 30.5 along a-axis, 15.5 along c-axis

Cu 17

Cu6Sn5 16.3

Cu3Sn 19

The coefficients of thermal expansion for the various phases in these solder joints are listed in Table IV. The anisotropy in thermal expansion of Sn grains is clearly seen between a-axis and c-axis orientations.

Fig.35: a) SE image of a region close to the Cu/solder interface after 20 cycles. b) SE image of the same location after 60 cycles, showing severe surface relief at the Cu/solder interface. c) BSE image of a different location after 20 cycles, d) BSE image of the same location after 60 cycles, showing extensive grain boundary ledges and surface relief. 93

Fig.36: a) Region within (001) grains where surface mismatch between solder and IMC is large. b) OIM map of the sample cross-section. c) Region within (100) grains where no surface mismatch is seen between solder and IMC. d) and e) Schematics showing Cu6Sn5 needles within the solder aligned normal to the polished surface. The arrows indicate directions where maximum expansion is seen with Sn grains and IMC needles. 94

Compared to the interrupted aging experiments, the surface degradation results in the thermal cycling test are much severe. The higher peak aging temperature, high frequency of heating and cooling, and the large number of cycles undergone by the sample, promoted grain boundary ledge formation throughout the entire polished cross-section and also resulted in intensified surface relief. There is a possibility of 3 factors influencing the observed damage phenomenon. 1) Thermal expansion/contraction of Cu wires on either side of the solder joint, 2) thermal expansion/contraction of Cu6Sn5 IMC needles within the solder volume and 3) anisotropic thermal expansion/contraction of Sn grains.

Fig. 36 presents different types of surface relief and their relation to location and the Sn grain orientation. Looking at Fig. 36a, the IMC needles normal to the surface seem to be no longer in plane with the neighboring Sn grains. Similarly, Fig. 36c, shows IMC needles normal to the surface staying in plane with the polished surface. These differences depend on the thermal expansion coefficient values of Sn grains along a-axis and c-axis. In Fig. 36a, the Sn grains have (001) orientation, which suggests that the major expansion direction is parallel to the polished surface of the solder. Since the tin grains are constrained on the sides along the direction of maximum thermal expansion, the expansion is forced upwards to the free surface and thus resulting in a height difference between the solder and the IMC needle. On the other hand, Fig. 36c, contains grains that are oriented along (100) normal to the polished face. This means that the major expansion directions of both the Sn grains and the IMC needles are in line and unconstrained by the free surface. Thus, both the Sn grains and the IMC needles

expand/contract together and hence stay in plane to the polished surface. The huge bulging seen at the left interface in Fig 36a. and at the right interface in Fig. 36c, can be attributed to the compressive forces imparted by the expanding Cu wires.

6.4. Summary

The experiments conducted in this chapter help understand the various factors through which the temperature component influences degradation phenomena observed in solder joints. Interruptions in temperature during electromigration/ aging tests results in the formation of grain boundary ledges on the free surface. The ledges preferably initiate at high angle grain boundaries closer to the Cu/solder interface. Multiple cycles of heating and cooling produce a banded ledge formation that propagate away from the interface, towards the center of the sample. The Cu6Sn5 nanoparticles distributed throughout the solder volume, pin these ledges, thereby restricting their propagation.

Thermal cycling test done between room temperature and 150oC with a cycle time of 1 hour resulted in a more aggravated damage evolution. Grain boundary ledges and surface relief were present over the entire polished sample surface. Damage observed was strongly dependent on the thermal expansion coefficients of the nearby phases.

Anisotropic expansion of Sn grains resulted in different degrees of surface relief based on the orientation of Sn grains.

CHAPTER 7

CONCLUSIONS

7.1. Summary of research findings:

 The 3D Sn grain orientation for a 250 m pure Sn solder joint was obtained using

EBSD combined with serial sectioning technique. 3D visualization showed that

the grain morphologies are strongly influenced by the differences in cooling rates.

This was elucidated by segmenting and quantifying the aspect ratio of Sn grains

as a function of position within the solder volume.

 The 3D Cu6Sn5 intermetallic particle distribution was obtained by segmentation

from backscattered electron images. Quantification was done for both the fine

intermetallic nanoparticles as well as the larger IMC needles in the solder volume.

FIB serial-sectioning and reconstruction was done and the true morphology of the

Cu6Sn5 nanoparticles was studied.

 An in situ electromigration stage was designed and installed inside a scanning

electron microscope. Interrupted electromigration testing performed ex situ,

showed the evolution of damage mechanisms on a 250m solder joint sample.

Three main damage features were seen. Delamination at the IMC/Sn matrix

interface, fracture of IMCs at the interface and the formation of grain boundary

ledges, were observed and possible reasons were discussed. 97

 To study the influence of temperature on observed damage phenomena, solder

joints were tested in interrupted and uninterrupted conditions. 500 m samples

were fabricated and tests were run in situ, which showed that grain boundary

ledges prefer to initiate at high angle grain boundaries close to the Cu/solder

interface.

 Thermal cycling test done between room temperature and 150oC with a cycle time

of 1 hour resulted in a more aggravated damage evolution. Grain boundary ledges

and surface relief were present over the entire polished sample surface. Damage

observed was strongly dependent on the thermal expansion coefficients of the

nearby phases. Anisotropic expansion of Sn grains resulted in different degrees of

surface relief based on the orientation of Sn grains.

7.2. Future work:

 Thorough the study on 3D EBSD and IMC visualization, it was seen that the

solder bump dimension strongly influences the heterogeneity in the

microstructure. A study on the volume effect of Sn-rich solder joints can be done

on solder joints of multiple sizes. By obtaining 3D datasets for say, 250m,

150m and 100 m solder joints, the Sn grain and IMC phase distribution can be

analyzed to give a better understanding of the evolution of the microstructure with

decreasing solder volume. 98

 With respect to electromigration testing, a study on more heterogeneous structures

with single/bi-crystal solder joints will reveal the influence of specific grain

boundaries and orientations. It is known that c-axis aligned with the current

direction favors electromigration better than a-axis. However, the influence of

high angle grain boundaries running parallel/ perpendicular to current direction,

the influence of twin boundaries and influence of low angle grain boundaries are

not well understood. These investigations will help tailor microstructures required

for reliable performance of electronic packages and predict the evolution of

damage over time.

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