Microalloying Impact and Damage Mechanisms in Sn-Bi and Sn-Ag-Cu Based Solder Alloys: Constitutive Behavior and Fatigue Reliability

Abstract

Solder interconnects are critical mechanical and electrical elements in electronic packages, and their reliability depends strongly on alloy composition, processing condition, use temperature, strain rate, creep deformation, and fatigue loading. As electronic packaging continues to move toward finer pitch, larger package size, heterogeneous integration, and lower thermal budgets, both low-temperature SnBi solder alloys and high-reliability SnAgCu-based solder alloys have received increasing attention. However, these solder systems exhibit complex joint-scale deformation behavior, including rate-dependent viscoplasticity, creep, strain softening, aging-induced microstructural evolution, and fatigue degradation. Therefore, systematic joint-scale mechanical characterization and constitutive modeling are required to connect measured solder joint behavior with reliability-oriented package simulation. This thesis investigates the mechanical behavior, microstructural damage mechanisms, and constitutive response of lead-free solder alloys using a custom micro-precision mechanical tester. The tester was designed to perform joint-scale monotonic, creep, and fatigue tests under controlled displacement, strain rate, load, and temperature conditions. Validation experiments, repeatability tests, PID control tuning, and specimen-design studies were conducted to establish the reliability of the test platform and quantify measurement uncertainty. These efforts provide the experimental foundation for the subsequent mechanical characterization and model calibration studies. The first part of this thesis focuses on low-temperature near-eutectic SnBi solder alloys with Ag, Sb, and combined Ag+Sb microalloying additions. Joint-scale monotonic and creep tests were conducted on solder joints assembled on OSP-finished Cu pads to preserve device-relevant interfacial metallurgy. The results show that Ag additions increase peak stress and creep resistance but tend to promote more rapid post-peak softening and reduced ductility. In contrast, Sb additions improve ductility by promoting more distributed damage evolution and shorter, more tortuous crack paths. Combined Ag+Sb additions provide a more balanced mechanical response by retaining much of the Ag-provided strengthening while moderating the loss of ductility. Post-test scanning electron microscopy further reveals that the macroscopic strain-softening response of SnBi joints is associated with microcrack initiation, crack coalescence, and the transition from distributed damage to dominant macrocrack propagation. The effects of isothermal aging on SnBi solder joints are also examined. Aging at 85$^\circ$C for 240 h produces diffusion-driven microstructural evolution, including coarsening of the Sn/Bi eutectic structure and interfacial changes near the Cu pad. After aging, the SnBi alloys generally exhibit higher peak stress and improved creep resistance, while retaining their characteristic post-peak strain-softening behavior. However, the alloys experience decreased ductility that depends strongly on alloy chemistry. Ag-containing alloys show high strength but more severe localization of damage, whereas Sb-containing alloys better preserve ductility by delaying the transition from distributed microcracking to dominant macrocrack growth. These observations demonstrate that aged SnBi joint reliability is governed not only by strength and creep resistance, but also by the stability of the damage evolution process. The second part of this thesis examines high-temperature and high-reliability solder alloys for applications involving wide temperature ranges and severe cyclic loading. Monotonic, creep, and fatigue tests were conducted on SAC305 and candidate solder alloys such as QSAC20, Innolot, and Alloy 10. Although the candidate alloys exhibit higher peak stress and improved creep resistance relative to SAC305, they generally show reduced fracture strain under monotonic loading. Strain-controlled fatigue tests conducted over multiple strain ranges further demonstrate that fatigue life is strongly controlled by ductility and imposed strain amplitude. At high strain ranges, alloys with lower strain-to-fracture degrade more rapidly, whereas SAC305 maintains better fatigue resistance due to its higher ductility. Numerical simulations are used to relate the experimentally measured fatigue behavior to package-level thermal cycling and package-size effects. To support simulations that enable reliability assessment, this thesis develops constitutive modeling frameworks for solder joint behavior. Anand viscoplastic model as well as the time-hardening creep model are calibrated using the measured monotonic and creep data. The fitted parameters clarify how microalloying modifies rate-dependent flow, hardening evolution, primary creep, and secondary creep response. In addition, a strain-based constitutive damage model is developed to represent the post-peak softening behavior of SnBi solder joints. In this framework, damage evolves as the progressive activation of local failure events governed by a critical-strain threshold distribution. The resulting model provides physically interpretable damage parameters, captures the smooth transition from intact response to post-peak degradation, and enables alloy-dependent comparison of damage activation and softening localization. Overall, this thesis establishes a joint-scale experimental and modeling framework for evaluating lead-free solder alloys across monotonic, creep, fatigue, aging, and damage conditions. The results show that solder reliability cannot be assessed from strength alone; instead, the combined balance among peak stress, creep resistance, ductility, microstructural stability, and damage evolution must be considered. The findings provide guidance for designing SnBi low-temperature solders and high-reliability solder alloys for advanced electronic packaging, and provide calibrated constitutive models suitable for future package-level reliability simulation.