The Impact of Bi Content on the Coarsening Kinetics of IMC Particles and Creep Deformation Under Thermal Cycling

This work investigates the impact of Bi content on the microstructure of solder joints under various thermal cycling conditions. Five lead-free solder alloys were used to assemble test boards and subjected to thermal cycling according to JEDEC standards. After reflowing, the boards were thermally cycled after 10 days of storage at room temperature. The thermal profile JESD22-A104E was designed for cycling from −40°C to +125°C with ramp and dwell times of 16.5 and 15 minutes, respectively, at extreme temperatures. The microstructural evolution was studied using scanning electron microscopy and energy-dispersive spectroscopy 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 examine the Ag₃Sn particles for each test condition. The multiple threshold approach was used to analyze particle size, number, and density using the surface imaging and analysis software Mountains® 9. The results revealed that the effect of adding Bi on the microstructure was instantly apparent, as solder joints with 3% Bi had more uniformly dispersed precipitates. During the accelerated temperature cycling process, the Ag₃Sn precipitates underwent strain-enhanced coarsening and localized re-crystallization. SAC-3Bi exhibited the slowest growth of Ag₃Sn particle size compared to other SAC-based alloys. In addition, the growth rate of the intermetallic compound layer for SAC with Bi alloys was slower compared to SAC305. This was due to the solid solution hardening processes, in which the presence of micro-alloys plays a vital role in strengthening solder joints.