In situ Electromigration and Reliability of Pb-Free Solders
At Extremely Small Length Scales
by
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
i
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.
ii
DEDICATION
To my parents, Simon Kirubanandham and Josephine Flora Mary, who have inspired and
encouraged me to dream big throughout my life.
iii
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.
iv
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
v
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
vi
CHAPTER Page
In situ Thermal Cycling Behavior ....................................................................... 89
Summary............................................................................................................... 96
7. CONCLUSIONS ........................................................................................................ 97
Summary of Research Findings ........................................................................... 97
Future Work ......................................................................................................... 98
REFERENCES ..................................................................................................................... 100
vii
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
viii
LIST OF FIGURES
Figure Page
1. A) SEM Image of Cross-Sections of the 250M 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
ix
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
x
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
xi
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 0.4m. ............................................................................................. 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 1.2m. ............................................................................................................... 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 1.2m. ................................................................................................................ 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
xii
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
xiii
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
1
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
2
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,
3
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
4
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
5
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.
6
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.
7
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).
8
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
9
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
10
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.
11
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.
12
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
13
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).
14
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).
15
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:
𝐹𝑛𝑒𝑡 = 𝐹𝑒𝑤 − 𝐹𝑒𝑓 = 𝑞𝐸𝑍∗ = 𝑞𝑍∗𝑗𝜌
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.
16
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
17
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.
18
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.
19
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.
20
Fig.1: a) SEM image of cross-sections of a 250m solder joint used in this study. b) High
magnification image showing distribution of fine Cu6Sn5 precipitates throughout the
solder volume.
21
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).
22
Fig.3: Reflow profile followed for fabricating solder joints for this study.
23
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 2m 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.
24
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.
25
3.2 Investigation of the Cu6Sn5 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
26
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
27
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.
28
Fig.5: CAD model of the sample fixture. Key design features and their descriptions are
mentioned (Mertens et al. 2015).
29
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.
30
Fig.6: CAD model of the sample fixture with the Watlow ceramic heater attached to it
(Mertens et al. 2015)
31
Fig.7: JEOL JSM 6100 Scanning Electron Microscope with EBSD, in situ nanoindenter
and in situ electromigration testing capabilities.
32
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).
33
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.
34
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.
35
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.
36
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.
37
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
38
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
39
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.
40
Fig.9: a) Secondary electron image b) Backscattered electron image c) OIM map of 250
m pure Sn solder joint.
41
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.
42
43
44
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.
45
46
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.
47
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).
48
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.
49
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.
50
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.
51
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.
52
53
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
54
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.
55
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 290m x 130m, 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.
56
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
57
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.
58
Fig.15: a) Secondary electron image and b) backscattered electron image of the solder
joint.
59
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.
60
Fig.16: Circuit diagram of the in situ electromigration testing setup.
61
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.
62
Fig.17: Schematic diagram of the test sample for electromigration showing current
direction.
63
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.
64
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.
65
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.
66
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.
67
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.
68
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.
69
Fig. 21: Backscattered electron images (a) Before electromigration testing (b) after 144
hours testing.
70
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.
71
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].
72
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].
73
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.
74
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.
75
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 0.4m.
76
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 1.2m.
77
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
1.2m.
78
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.
79
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.
80
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
81
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.
82
Fig.28: a) Optical micrograph of 500 m solder joint before interrupted thermal aging.
b) Optical micrograph with OIM image overlay.
83
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.
84
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.
85
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.
86
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.
87
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.
88
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.
89
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
90
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.
91
Fig.34: Temperature profile for 10 cycles of thermal cycling.
92
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.
93
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.
94
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.
95
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
96
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.
97
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 250m 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.
98
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, 250m,
150m 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.
99
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.
100
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