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Effect of cooling rate, silver composition, dwell time and solder joint size on the reliability of tin-silver-copper solder joints

Dissertation
Author: Mulugeta Abtew
Abstract:
Thermo-mechanical fatigue, as a consequence of the coefficient of thermal expansion mismatch (CTE), is considered a significant wear-out failure mode for solder joints in electronics applications. For years, accelerated temperature cycling (ATC) has been the preferred technique for evaluating the fatigue performance of eutectic Sn-Pb solder joints and a considerable amount of test data and relevant field experience exists for eutectic Sn-Pb solder. Experimental investigation was performed to evaluate the effect of cooling rate, dwell time during ATC, solder joint size and Ag composition on the microstructure and consequently on the fatigue life of ceramic ball grid array (CBGA) lead free solder joints. The key experimental variables that define the initial microstructure are the solder joint cooling rate during surface mount (SMT) assembly and the Ag content of the CBGA solder balls. Two Pb-free alloys were considered in this research, Sn3.0Ag0.5Cu, commonly called SAC305 with Ag composition of 3.0 wt. % and Sn1.0Ag0.5Cu, commonly called SAC105, with Ag composition of 1.0 wt. %. The time-temperature reflow profile was configured with a cooling rate of 1.0°C/s, and classified as "low cooling" and a cooling rate of 3.0°C/s, and classified as "fast cooling". The range of cooling rates considered were representative of the reflow soldering process for moderately complex printed circuit assembly. Furthermore, two solder joint sizes with diameters of 0.381 mm and 0.203 mm, and classified as "large" and "small" size respectively, were evaluated to study the effect of solder joint size on the solder joint microstructure. ATC was performed from 0°C to 100°C in accordance with the IPC 9701 guideline using both 10 minute dwell time and an extended 60 minute dwell time that had been shown to promote microstructural coarsening in SAC alloys. All cells were tested to 63% or greater failure rate with a test duration exceeding 1500 cycles. Baseline characterization was performed on representative board-level assemblies to document the microstructures before ATC and to enable comparisons to samples removed from temperature cycling for failure analysis. Solder joint wetting conditions and fillet formations were evaluated using an optical microscope and two dimensional X-ray imaging. The solder joints were then cross sectioned and fine polished. The main techniques used for microstructural characterization by failure analysis were (i) optical microscopy under both bright field and cross polarized imaging conditions, (ii) scanning electron microscopy (SEM) using backscattered electron imaging (BEI), and (iii) energy dispersive spectroscopy (EDS) for elemental analysis. The ATC test data and failure analysis results were discussed in terms of the relationship to the evolving microstructure and fatigue behavior that resulted from temperature cycling. The experimental results revealed that, at a 95% confidence level, dwell time at extreme temperatures during ATC testing to be the most critical factor that determined the number of cycles to fail, which corresponds to the fatigue resistance of the solder joints. Effects of cooling rate and Ag composition on fatigue life of SAC105 and SAC305 solder joints were found to be not significant. While the difference in microstructure as a consequence of cooling rate both for SAC305 and SAC105 solder joints was not significant, slow cooled SAC solder joints exhibited slightly better fatigue life than fast cooled SAC solder joints. At the 10 minute dwell time, the fatigue resistance of SAC305 was found to be slightly better than SAC105 solder joints. However, at the 60 minute dwell time, the fatigue resistance of both SAC105 and SAC305 was found to be nearly identical suggesting that the reliability of the solder joints might be insensitive to Ag content when the dwell time during ATC was longer. Furthermore, both SAC305 and SAC105 solder joints exhibited superior fatigue resistance than the eutectic Sn-Pb solder joints independent of cooling rate and dwell time. The fatigue resistance of the eutectic Sn-Pb solder joints was found to be significantly affected by both cooling rate and dwell time. At higher cooling rate and 10 minute dwell time, the solder joints exhibited better fatigue life while at slow cooling rate and 60 minute dwell time, the fatigue life of the eutectic Sn-Pb solder joints were found to have significantly inferior fatigue resistance. Solder joint size was found to have no effect on the reliability of solder joints. (Abstract shortened by UMI.)

xi TABLE OF CONTENTS

LIST OF TABLES...........................................................................................................xiv LIST OF FIGURES..........................................................................................................xv CHAPTER 1: SOLDERING AND PRINTED CIRCUIT BOARD ASSEMBLY.............1 1.1 Introduction...........................................................................................................1 1.2 Printed Circuit Board (PCB) Assembly................................................................2 1.3 Motivation.............................................................................................................4 1.4 Problem Description..............................................................................................8 1.4.1 Changing Requirements of Soldering.........................................................8 1.5 Solder Joint Microstructure.................................................................................12 1.5.1 Interconnection Microstructures and Their Evolution..............................14 1.5.2 Solidification Process................................................................................15 1.5.3 Nucleation and Growth.............................................................................15 1.5.4 Homogeneous Nucleation.........................................................................17 1.5.5 Heterogeneous Nucleation........................................................................21 1.5.6 The Growth of Nuclei...............................................................................24 1.5.7 Growth Rate of Sn Dendrites....................................................................26 1.5.8 Solidification of the Sn-Ag-Cu (SAC) System.........................................28 1.5.9 Effect of Interface Metallization on Solidification Structure...................33 1.6 Problem Statement and Research Objectives......................................................35 1.6.1 Summary...................................................................................................38 CHAPTER 2: LITERATURE REVIEW..........................................................................40 2.1 Introduction.........................................................................................................40

xii 2.2 The Growth of Cu 6 Sn 5 , Ag 3 Sn and β-Sn Dendrites............................................50 2.3 Effect of Ag Content on the Fatigue Properties of Pb-free Solder Alloys..........58 2.4 Effect of Sample Size on the Undercooling of the Pb-free Solder Alloys..........60 2.5 Anisotropic Properties of Tin..............................................................................66 2.6 Dwell Time During ATC Testing and Solder Joint Fatigue Life........................69 2.7 Summary.............................................................................................................70 CHAPTER 3: EXPERIMENTAL APPROACH..............................................................73 3.1 Introduction.........................................................................................................73 3.2 Experimental Approach.......................................................................................76 3.3 Design of Experiments........................................................................................76 3.3.1 Experimental Procedure............................................................................77 3.3.2 Test Vehicle Description...........................................................................78 3.3.3 Assembly Process.....................................................................................80 3.3.4 Post Assembly Inspection.........................................................................80 3.3.5 Statistical Analysis of Reliability Test Data.............................................85 3.3.6 Summary...................................................................................................89 CHAPTER 4: RESULTS AND ANALYSIS...................................................................90 4.1 Introduction.........................................................................................................90 4.2 Statistical Analysis..............................................................................................91 4.3 ATC Test Results................................................................................................95 4.3.1 Effect of Cooling Rate on Fatigue Resistance..........................................95 4.3.2 Effect of Dwell Time during ATC on Fatigue Resistance......................116 4.4 Summary...........................................................................................................121

xiii CHAPTER 5: CONCLUSIONS AND FUTURE WORK..............................................123 5.1 Introduction.......................................................................................................123 5.2 Research Methodology......................................................................................124 5.3 Conclusions.......................................................................................................125 5.3.1 Effect of Cooling Rate............................................................................126 5.3.2 Effect of Ag Composition.......................................................................126 5.3.3 Effect of Dwell Time..............................................................................127 5.4 Research Significance.......................................................................................127 5.5 Future Work......................................................................................................129 APPENDIX I: PCB ASSEMBLY AND SOLDERING PROCESSES..........................131 APPENDIX II: SOLDER JOINT RELIABILITY AND DEFORMATION..................134 REFERENCES...............................................................................................................138

xiv

LIST OF TABLES Table 1: Physical Properties of the SAC305 Pb-free Solder [42].....................................47 Table 2: Effect of Cooling Rate on the Secondary Dendrite Arm Spacing and Size [58] 57 Table 3: Experimental Matrix...........................................................................................79 Table 4: Test Vehicle Lay-out and Description................................................................81 Table 5: Analysis of Variance Table for ATC Results.....................................................92 Table 6: Cycles to Failure Results for Fast Cooling Rate at 10 Minute Dwell................96 Table 7: Cycles to Failure Results for Slow Cooling Rate at 10 Minute Dwell...............96 Table 8: Cycles to Failure Results for Fast Cooling at 60 Minute Dwell.........................96 Table 9: Cycles to Failure Results for Slow Cooling at 60 Minute Dwell.......................96 Table 10: Solubility of Ag, Cu and Ni in Sn [91]...........................................................109 Table 11: Summary of Reflow Profiles Parameters.......................................................133

xv LIST OF FIGURES Figure 1: Surface Mount Attachment of PCB Assembly [13]............................................3 Figure 2: Pin in Hole Attachment of PCB Assembly [13]..................................................3 Figure 3: Solder Ball Terminations in a BGA [13].............................................................3 Figure 4: Wave Soldering of a PCB Assembly [5].............................................................3 Figure 5: A sketch of (a) Heterogeneous and (b) Homogeneous Nucleation [18]...........16 Figure 6: The Variation of the Free Energy with the Radius r of the Nucleus [18].........20 Figure 7: Free Energy as a Function of Temperature for Solid and Liquid Phases [18]..20 Figure 8: Heterogeneous Nucleation of a Spherical Cap on a Substrate [13]..................22 Figure 9: A Comparison of the Free Energy of Solid Clusters [18].................................25 Figure 10: Temperature Variation on (a) Planer and (b) Spiked Solidification [18]........25 Figure 11: Typical Dendritic Microstructure showing the Side Branching Sheets [25]...27 Figure 12: Average Separation of the Dendritic Rows of Pb Crystals [25]......................27 Figure 13: Effect of Increased Growth Velocity on Lamellar Spacing of Sn-37Pb [27].29 Figure 14: Optical Micrographs of Near-eutectic Sn-Ag-Cu Microstructure [29]...........29 Figure 15: Sn-rich Region of Sn-Ag-Cu Ternary Phase Diagram [10]............................41 Figure 16: Typical Stress – Strain Curve [40]..................................................................44 Figure 17: Relationship Describing Petch Equation [41].................................................47 Figure 18: Conventional Steady Strain Rate Creep Curve [41]........................................49 Figure 19: Creep Properties of 3 Different Solders [39]...................................................49 Figure 20: Sn-Ag Binary Phase Diagram [43]..................................................................52 Figure 21: Sn-Cu Phase Diagram [43]..............................................................................52 Figure 22: Ag-Cu Phase Diagram [43].............................................................................53

xvi Figure 23: Sn-Pb Phase Diagram [45]..............................................................................53 Figure 24: Sn-2.5wt.%Ag-0.9wt.%Cu Heated to 240 o C and Cooled [61].......................55 Figure 25: The Microstructure of Sn-3.8wt.%Ag–0.7wt.%Cu Solder Alloy [61]............55 Figure 26: Etched Sn-3.5wt.%Ag Solder Showing Sn-rich Dendrites [58].....................55 Figure 27: Diffusivities of Ag, Au, Cu, and Ni in Single Crystal Sn [60]........................57 Figure 28: Effect of Ag Content on Stress-Strain Curves of Solder Joint [65]................59 Figure 29: Effect of Ag Content on the Drop Test of Sn-Ag-Cu Solder Alloys [9].........61 Figure 30: SAC105 vs. SAC305 Solder Joints under Drop Shock Test [67]...................61 Figure 31: Microstructures at Undercoolings of ∆T=a: 35K, b: 139K, c: 187K [76]......65 Figure 32: Calculated Undercooling Versus Droplet Diameter [77]................................65 Figure 33: Anisotropic elastic and thermal expansion properties of the tin [31]..............68 Figure 34: Test Vehicle Configuration.............................................................................79 Figure 35: Flow of Evaluation Processes..........................................................................81 Figure 36: Daisy Chain Configuration for ATC Testing..................................................83 Figure 37: The Weibull Distribution [90].........................................................................87 Figure 38 (a): Weibull Shape Parameters [90].................................................................87 Figure 39: Main Effects Plot for Cycles to Fail................................................................92 Figure 40: Interaction Plot of Cooling Rate and Ag composition....................................93 Figure 41: Interaction Plot of Cooling Rate and Dwell Time...........................................93 Figure 42: Interaction Plot of Dwell Time and Alloy Type Showing Interaction............94 Figure 43: Reliability Comparison of SAC305 and SAC105 at 10 Min. Dwell...............98 Figure 44: Reliability Comparison of SAC305 and SAC105 at 60 Min..........................98 Figure 45: SAC305 vs. SAC105 Fast Cooling for 10 Min. Dwell.................................100

xvii Figure 46: SAC305 vs. SAC105 Fast Cooling for 60 Min. Dwell.................................100 Figure 47: Single Grain Cross-polarized Image of a Fast Cooled SAC305 Solder Joint101 Figure 48: Cross-polarized Image of a Fast Cooled SAC105 Solder Joint....................101 Figure 49: Cross-polarized Image of a Slow Cooled SAC305 Solder Joint...................102 Figure 50: Cross-polarized Image of a Slow Cooled SAC105 Solder Joint...................102 Figure 51: Bright-field Optical Images of Slow and Fast Cooled SAC Solder Joints....104 Figure 52 (a): SAC105 With Dendrites of Primary Sn (2500X)....................................105 Figure 53: Backscattered Electron Image of IMC Found in Fast Cooled SAC105........107 Figure 54 (a): SEM Image Showing Spalled (Cu, Ni) 6 Sn 5 in Fast Cooled SAC305......107 Figure 55: Elemental Analysis for the Slow Cooled SAC105 Solder Joints..................110 Figure 56: SAC305 and SAC105 Solder Joint Failure Mode After ATC (400X)..........112 Figure 57: Better Fatigue Resistance by Fast cooled Eutectic Sn-Pb.............................112 Figure 58: Characteristic Life of SAC305 at 10 and 60 Minute Dwell Time.................117 Figure 59: Characteristic Life of SAC105 at 10 and 60 Minute Dwell Time.................117 Figure 60: Characteristic Life of Eutectic Sn-Pb Solder Joints at 60 Minute Dwell......119 Figure 61: Characteristic Life of Fast Cooled Eutectic Sn-Pb Solder Joints..................119 Figure 62: PCB Assembly Process Flow........................................................................131 Figure 63: Time-temperature Reflow Profile for SAC105 CBGA Solder Joints...........132 Figure 64: Time-temperature Reflow Profile for SAC305 CBGA Solder Joints...........132

1 CHAPTER 1: SOLDERING AND PRINTED CIRCUIT BOARD ASSEMBLY 1.1 Introduction Soldering and solder materials have been used for micro-electronic interconnects in a wide range of applications for many years [1]. Solder interconnections provide electrical integrity and mechanical support for various levels of the electronic packaging hierarchy, including components and printed circuit board assemblies. Solder has been an integral part of electronic packages, which typically consist of various materials with different properties. Because of its excellent solderability, low cost, well-known physical properties and application behaviors, Pb-based solder has been a primary interconnect material for electronics [1].

Soldering is a metallurgical joining method that utilizes a filler metal, the solder, with a melting temperature below 425 o C [1,2]. In electronics packaging, solder is one of the primary interconnect materials that enable metal joining at the various levels of packaging. It provides electrical, thermal and mechanical interconnection in electronics assembly. It is used to connect the silicon die (or chip) to the substrate in a Flip Chip (FC) configuration using solder bumps. Solder is also the primary means of interconnect in Level 2 packaging where the component, an encapsulated silicon die, is mounted on a

2 Printed Circuit Board (PCB). The electronic component can be attached to the PCB either in a Surface Mount Technology (SMT) or Pin in Hole (PIH) methods as illustrated in Figure 1 and 2, respectively. Depending on the device application requirements, surface mount components can either have leaded terminations, as shown in Figure 1, or have solder balls referred to as Ball Grid Arrays (BGAs), as shown in Figure 3.

1.2 Printed Circuit Board (PCB) Assembly PCB assembly is a Level 2 packaging technique that consists of surface mount or mixed technology assembly where SMT and PIH components are used, either in a single or double-sided PCB assembly and soldering configuration. Soldering of surface mount components is accomplished via reflow soldering, typically in a forced convection oven. Solder paste is applied on the soldering pads of the PCB and then the component is placed on top of the solder paste. The assembly is then subjected to reflow where the paste melts and forms the solder joint upon solidification. Solder paste is a thixotropic mixture of solder spheres with varying diameters, flux and additives [2]. The additives in the solder paste include surfactants to promote wetting and other additives to impart physical properties such as tackiness, slump, viscosity, etc. [3]. Soldering of PIH components, on the other hand, is performed by wave soldering, where the assembly is transported over a molten solder bath, as shown in Figure 4. The soldering process takes place when molten solder rises in the holes by capillary action and forms solder joints.

3

Figure 1: Surface Mount Attachment of PCB Assembly [13]

Figure 2: Pin in Hole Attachment of PCB Assembly [13]

Figure 3: Solder Ball Terminations in a BGA [13]

Figure 4: Wave Soldering of a PCB Assembly [5]

PCBA Liquid

4 1.3 Motivation Recent developments in environmental initiatives to limit or ban the use of Pb in solder, introduction of new solder materials and soldering technologies that offer equal or superior interconnect reliability than that of Pb-based solders and increasing interconnect miniaturization have been the focus of research subjects in both academia and industry at large [2]. The research efforts are directed at finding ways to improve or sustain the reliability of electronic products despite changes in the use environment, interconnect size miniaturization and removal of Pb from solder materials. Change in the use environment of electronic products is mainly related to increased power cycling frequency during service or use conditions. As a result, solder materials used in many applications such as consumer electronics, military, avionics, aerospace, and automotive (under-the-hood) are subjected to increasingly severe thermomechanical loads during service. These thermomechanical loads generally lead to microstructural damage evolution and strain localization consequently impacting the reliability of microelectronic devices.

The second factor of change pertains to the continued trend of smaller and denser packaging, and the corresponding miniaturization of the solder interconnect, which has resulted in decreased characteristic dimensions of solder joints. In many instances, solder joints near 50 um in size are used, and the microelectronics industry is expected to gradually move to a 35 um solder joint size technology [2]. As the size scale of the solder joint diminishes, the relative stress on the solder joints as well as the volume ratio of solder-substrate interaction products increases resulting in the formation of complex

5 heterogeneous micro-structures within the solder joints. Consequently, understanding the microstructure and mechanical behavior of solder joints with diminishing size have become critical in determining the overall reliability of interconnections [2].

The third factor of change is related to environmental initiatives. Because of the toxicity of Pb and its adverse effect on health and the environment, legislations that ban or limit the use of Pb in soldering have been in place since 2006 [3,4]. As a result, the electronics industry is challenged with finding a suitable soldering alloy to replace Sn-Pb solders [4, 5,6]. Some Pb-free alloys such as Sn-Ag, Sn-Bi, Sn-Zn, Sn-Cu binary eutectic and Sn- Ag-Cu, Sn-Ag-Bi, and Sn-Zn-Bi ternary eutectic solder materials have been proposed as substitutes for the Pb-based alloy [4,5]. All the potential Pb-free alternatives are primarily Sn-rich alloys and present distinct microstructure, with different evolving characteristics and thermomechanical properties [5]. Therefore, to select a suitable Pb-free replacement among the suggested alternatives, a thorough understanding of their behavior is required. The main difficulty of selecting a particular Sn-rich alloy arises from the fact that the microstructures of these Sn-rich alloys are significantly different than that of Sn-Pb solder and therefore their overall response under thermomechanical load is likely to be different.

One of the notable differences is the known anisotropic behavior in the elastic and thermal expansion properties of Sn, which is expected to induce a significant amount of stress at Sn-grain boundaries during thermal cycling [5,7,8]. Secondly, contrary to Sn-Pb, the microstructures of Sn-rich alloys do not contain a ‘soft’ phase that might

6 accommodate strain incompatibilities resulting from this anisotropy [8]. Last, but not the least, some of the alternative alloys contain distinct intermetallic structures with distinct mechanical and thermal properties distributed in the bulk solder matrix. This can potentially increase the overall stress level in the solder as a result of the CTE mismatch during service, which ultimately affect the mechanical and fatigue properties of these alloys [3]. Therefore, it is important to investigate the micromechanical effects resulting from the thermal anisotropy of Sn and their influence on the evolution of damage during thermomechanical cycling.

Among the many suggested Pb-free alloys as alternatives, Sn-rich, Sn-Ag-Cu (SAC) solder alloy have gained widespread acceptance in the electronics industry as a viable Pb- free solder replacement for Sn-Pb solder based on their reported acceptable thermomechanical properties [5,7]. As a result, SAC alloys are the primary Pb-free solder materials investigated in this study. Specifically, the study will focus on Sn-3.0Ag-0.5Cu (SAC305) and Sn-1.0Ag-0.5Cu (SAC105) solder alloys, the most common solder alloys used for PCB assembly soldering applications [7].

In electronic applications, solder is always exposed to thermo-mechanical fatigue originating from the CTE mismatch. In Printed Circuit Board (PCB) assembly, global mismatch exists between the components and the printed circuit board. Within the solder joints, local CTE mismatch exist among various microstructural constituents as well as between grains of thermally anisotropic Sn in a single polycrystalline phase [8]. A complete description of damage mechanisms under thermomechanical fatigue requires a

7 detailed analysis of these two loading types, namely global and local, at the microstructural level, including their interactions [6].

Several studies [5-8] have addressed the thermomechanical fatigue of Pb-free solders. However, these studies have limited insight on the effects of cooling rate on the microstructure and the intrinsic anisotropy of Sn, which play an important role in fatigue damage initiation as discussed in the latter chapters of this dissertation. Different cooling rates are likely to generate different microstructures that will exhibit varying responses to thermomechanical fatigue [3,8]. With intrinsic anisotropy and multiple grain orientations, Sn-rich solder materials are prone to exhibit heterogeneous plastic deformation at the grain level during fatigue. Localized plastic strains can act as a source of crack initiation and damage due to fatigue. This may result in the failure of solder interconnects by propagation of the crack during its normal use. Recent studies [6] have suggested that, compared to the eutectic Sn-Pb, SAC305 has relatively inferior reliability when subjected to ATC testing. Similar studies have also shown that the drop resistance of SAC alloy varies with Ag content [7,8,9]. Furthermore, the microstructure, fatigue life and thermomechanical behavior of Sn-Ag-Cu solder alloys were also reported to vary with solder joint size [7,10] and the magnitude of applied strain, which corresponds to dwell time during ATC [5,11]. Therefore, a fundamental study on the understanding of the Sn- Ag-Cu solder alloy system including the effects of solder joint size, cooling rate and Ag composition on the initial microstructure of the solder as well as effects of dwell time during ATC on the subsequent plastic strain field evolution during cyclic fatigue, is of

8 great importance vis-à-vis reliability concerns pertaining to this solder interconnection system. 1.4 Problem Description 1.4.1 Changing Requirements of Soldering Solders, in general, can be defined as low melting alloys with room temperature falling in the range of 0.4-0.5 of their melting temperature [2,12]. With increasing interconnect density and reduced package size, solder alloys are being deployed into structural applications under demanding temperature (0.5-0.8 T m ) ∗ requirements and total strain ranges exceeding 10%, which are considered aggressive even for advanced high temperature structural alloys like steel [12]. In board level packaging, the solder used is primarily 63Sn-37Pb, a eutectic composition, or 60Sn-40Pb, a near eutectic composition. With a melting temperature of 183 o C, the Sn-Pb binary system allows soldering conditions that are compatible with most substrate materials and devices. As one of the primary components of the Sn-Pb solder system, Pb provides multiple technical advantages for soldering that include the following: • Pb reduces the surface tension of pure Sn, which is 550 mN/m at 232 o C, and the lower surface tension of 63Sn-37Pb solder (470 mN/m at 280 o C) facilitates wetting [2]. • As an impurity in tin, even at levels, as low as 0.1 wt.%, Pb prevents the transformation of white or (β) Sn to gray or alpha (α) Sn upon cooling past 13 o C.

∗ T m : Melting Temperature

9 The transformation, if it occurs, results in a 26% increase in the volume of the crystal structure and causes a loss of structural integrity in Sn [13]. • Pb serves as a solvent metal, enabling the other solder joint constituents such as Sn and Cu to form intermetallic bonds rapidly by diffusing in the liquid state. • Sn and Pb have mutual solubility in both liquid and solid states.

These factors, combined with Pb being readily available and a low cost metal, makes it an ideal alloying element with Sn. The board level soldering system that is mainly based on eutectic and near eutectic Sn-Pb solders has been well developed and refined with many years of experience [12,13]. A relatively well established knowledge-base about the physical metallurgy, mechanical properties, flux chemistries, manufacturing processes and reliability of Sn-Pb solder system exists. Board level assembly and soldering equipment are almost exclusively designed with eutectic and near eutectic Sn- Pb solder in mind. This extensive knowledge and understanding of the behavior of the Sn-Pb solder system has enabled current board level technology to form miniature geometry solder joints approaching 100 µm in size with high levels of reliability [2]. However, there are legal, environmental and technological factors that require alternatives for soldering material and processing. This has challenged and stretched the requirements for soldering.

I. Environmental Compliance The implementation of Reduction of Hazardous Substances (ROHS) initiatives effectively banned Pb from board level electronic soldering. The new Pb-free solder alloy

10 alternative, primarily the SAC alloy system, is expected to meet the manufacturing and reliability requirements of board level soldering. The composition of the SAC Pb-free solder system is predominantly Sn. Consequently, the dominating properties of the SAC solder exhibit the property of Sn. With the absence of Pb in the soldering system, all the benefits that Pb provided as the primary component of the soldering system are not available, creating challenges for processing and reliability issues including but not limited to tin whiskers, tin pests, anisotropic behavior, undercooling issue, etc [13].

II. Miniature Electronics Packaging and Interconnection The increasing demand towards packaging and interconnect miniaturization in SMT is stretching the physical limits of solder to provide sound and reliable solder joints. The natural radius of curvature of molten solder, R, as determined by surface tension (γ), density (ρ) and acceleration of gravity, (g), (R=γ/ρg) 1/2 = 2.2 mm) [2], is already larger than the size of the solder joints of the majority of SMT devices with less than 0.5 mm pitch. This means during the formation of solder joints with a smaller radius of curvature, the molten solder will be subjected to high internal liquid pressure that can potentially result in the runaway of solder from soldering pad locations creating a starved solder joint. This solder joint defect is commonly called wicking.

For PCB assembly applications, solder joint miniaturization can further have critical manufacturing and reliability challenges. • The size reduction of solder joints effectively reduces both the cross-sectional area and the stand-off height of the solder joint from the PCB resulting in a considerable increase in stress and strain that the solder joint experiences

Full document contains 164 pages
Abstract: Thermo-mechanical fatigue, as a consequence of the coefficient of thermal expansion mismatch (CTE), is considered a significant wear-out failure mode for solder joints in electronics applications. For years, accelerated temperature cycling (ATC) has been the preferred technique for evaluating the fatigue performance of eutectic Sn-Pb solder joints and a considerable amount of test data and relevant field experience exists for eutectic Sn-Pb solder. Experimental investigation was performed to evaluate the effect of cooling rate, dwell time during ATC, solder joint size and Ag composition on the microstructure and consequently on the fatigue life of ceramic ball grid array (CBGA) lead free solder joints. The key experimental variables that define the initial microstructure are the solder joint cooling rate during surface mount (SMT) assembly and the Ag content of the CBGA solder balls. Two Pb-free alloys were considered in this research, Sn3.0Ag0.5Cu, commonly called SAC305 with Ag composition of 3.0 wt. % and Sn1.0Ag0.5Cu, commonly called SAC105, with Ag composition of 1.0 wt. %. The time-temperature reflow profile was configured with a cooling rate of 1.0°C/s, and classified as "low cooling" and a cooling rate of 3.0°C/s, and classified as "fast cooling". The range of cooling rates considered were representative of the reflow soldering process for moderately complex printed circuit assembly. Furthermore, two solder joint sizes with diameters of 0.381 mm and 0.203 mm, and classified as "large" and "small" size respectively, were evaluated to study the effect of solder joint size on the solder joint microstructure. ATC was performed from 0°C to 100°C in accordance with the IPC 9701 guideline using both 10 minute dwell time and an extended 60 minute dwell time that had been shown to promote microstructural coarsening in SAC alloys. All cells were tested to 63% or greater failure rate with a test duration exceeding 1500 cycles. Baseline characterization was performed on representative board-level assemblies to document the microstructures before ATC and to enable comparisons to samples removed from temperature cycling for failure analysis. Solder joint wetting conditions and fillet formations were evaluated using an optical microscope and two dimensional X-ray imaging. The solder joints were then cross sectioned and fine polished. The main techniques used for microstructural characterization by failure analysis were (i) optical microscopy under both bright field and cross polarized imaging conditions, (ii) scanning electron microscopy (SEM) using backscattered electron imaging (BEI), and (iii) energy dispersive spectroscopy (EDS) for elemental analysis. The ATC test data and failure analysis results were discussed in terms of the relationship to the evolving microstructure and fatigue behavior that resulted from temperature cycling. The experimental results revealed that, at a 95% confidence level, dwell time at extreme temperatures during ATC testing to be the most critical factor that determined the number of cycles to fail, which corresponds to the fatigue resistance of the solder joints. Effects of cooling rate and Ag composition on fatigue life of SAC105 and SAC305 solder joints were found to be not significant. While the difference in microstructure as a consequence of cooling rate both for SAC305 and SAC105 solder joints was not significant, slow cooled SAC solder joints exhibited slightly better fatigue life than fast cooled SAC solder joints. At the 10 minute dwell time, the fatigue resistance of SAC305 was found to be slightly better than SAC105 solder joints. However, at the 60 minute dwell time, the fatigue resistance of both SAC105 and SAC305 was found to be nearly identical suggesting that the reliability of the solder joints might be insensitive to Ag content when the dwell time during ATC was longer. Furthermore, both SAC305 and SAC105 solder joints exhibited superior fatigue resistance than the eutectic Sn-Pb solder joints independent of cooling rate and dwell time. The fatigue resistance of the eutectic Sn-Pb solder joints was found to be significantly affected by both cooling rate and dwell time. At higher cooling rate and 10 minute dwell time, the solder joints exhibited better fatigue life while at slow cooling rate and 60 minute dwell time, the fatigue life of the eutectic Sn-Pb solder joints were found to have significantly inferior fatigue resistance. Solder joint size was found to have no effect on the reliability of solder joints. (Abstract shortened by UMI.)