Evolution of the Mechanical and Microstructure Properties of SAC Alloys During Thermal Cycling
Type of DegreePhD Dissertation
Industrial and Systems Engineering
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In real-time applications, solder joints are exposed to thermal and mechanical stresses. As a result, the microstructure evolves, and mechanical characteristics deteriorate, resulting in component failure. Solders have progressed from the typical SnPb (tinlead) alloys to lead-free alloys containing dopants such as bismuth (Bi), antimony (Sb), and nickel (Ni). Alloying with these metals was found to improve the thermal and mechanical properties of the solder; however, multiple factors dictate the rate of damage accumulation and microstructure evolution of the solder joints. Also, the literature did not report the effects of homogenous SAC-Bi alloys on the reliability, coarsening kinetics of IMC particles, and evolution of creep deformation properties during thermal cycling. This dissertation investigates the effect of thermal cycling on the evolution of the mechanical and microstructure properties of newly developed lead-free alloys. The first study investigated the impact of Bi content on the creep behavior of solder joints under different aging conditions. Three types of lead-free solder alloys (SAC305, SAC-3Bi, and SAC-6Bi) were tested at room temperature, and three stress levels were defined for each alloy using preliminary microindentation tests. The strain vs. time data obtained from the tests were analyzed, and a new approach based on an empirical model was developed to systematically study the creep strain rate. A power dependency prediction model was also developed to investigate the creep mechanisms during different cycles. The creep mechanism associated with each strain rate vs. stress curve was identified. Microstructural analysis was carried out using SEM with EDX to examine the evolution of creep strain rate. Power law creep was found to be the dominating creep mechanism for all the solder alloys tested at predefined stress levels. The creep strain rate of Bi-based alloys is significantly lower than that of SAC305 due to the presence of Bi in the solid 3 solution. A slight change in the creep rate was noticed for SAC-6Bi compared to SAC-3Bi after 1000 hrs of aging. The second study assessed the reliability performance of the lead-free solder joints compared to SnPb on CABGA208 and SMR2512 packages. Five homogeneous leadfree solder alloys, including SAC305, SAC-3.3Bi, SAC-0.5Bi-6In, SAC-0.5Bi-1.4Sb0.15Ni, and Sn63Pb37 and two components (CABGA208 and SMR2512) were considered in this study. The test board was FR-4 with an organic solderability preservative (OSP) surface finish. After reflowing, the boards were thermally cycled at room temperature after 10 days of storage. Scanning electron microscopy (SEM) integrated with energy-dispersive spectroscopy (EDS) was performed to study the microstructural development at different cycle counts. The failure data were fitted to two-parameter Weibull curves. Representative cross-sections were used to investigate the IMC particles and the failure mode. Among all CABGA208 components, cracks were localized in the bulk solder on the component side. Also, the components containing 3% Bi alloys outperformed SAC305 and SnPb. The third study investigated the impact of Bi content on the microstructure and IMC layer evolution under various thermal cycling conditions. Five lead-free solder alloys, including SAC305, SAC-3.3Bi, SAC-0.5Bi-6In, SAC-0.5Bi-1.4Sb-0.15Ni, and Sn63Pb37, were used to assemble test boards and subjected to thermal cycling according to the JESD22-A104E standard. The IMC layer growth and Ag3Sn particle coarsening were analyzed using optical and scanning electron microscopy equipped with EBSD at various cycle intervals. Backscatter electron imaging was used to produce electron micrographs of the specimens for each test combination. Four high strain-localized regions were identified to characterize the Ag3Sn particles at each cycle interval. The multiple threshold approach is used to analyze particle size, number, and density of 4 the IMC particles using the surface imaging and analysis software Mountains® 9. In terms of microstructure, the impact of adding Bi was prominent, as solder joints containing 3% Bi exhibited less coarsening after 250 cycles with more evenly distributed precipitates. The IMC layer growth rate for Bi-based alloys was relatively low compared to SAC305 and SnPb. The fourth study analyzed the effect of newly developed solder alloys on the evolution of mechanical properties during ATC. Five lead-free solder alloys, including SAC305, SAC-3.3Bi, SAC-0.5Bi-6In, SAC-0.5Bi1.4Sb-0.15Ni, and Sn63Pb37, subjected to thermal cycling according to the JESD22- A104E standard. The microstructure, steady-state creep, and hardness properties were studied for 0, 100, 250, 625, 1560, and 3900 cycles. It was found that the creep deformation rate increased with the stress levels for all solder alloys. In addition, micro-alloying with Bi improved the creep deformation resistance through the solid solution strengthening and hardening mechanisms, and the creep rate variation was less noticeable after 250 cycles. A strong correlation between the thermal fatigue life, the creep deformation/hardness, and the particle density was established, providing insights into the behavior of SAC-based solders under realistic conditions. Overall these studies contribute to developing more reliable electronic components by advancing the understanding of creep behavior in solder alloys.