Microstructural Evolution of SAC-Based Solder Joints Exposed to Thermal Cycling: Effect of Paste Alloy, Paste Volume, and Surface Finish

SAC (Sn-Ag-Cu) solder joints are frequently exposed to thermal cycling, leading to microstructural evolutions that accelerate thermal fatigue. During thermal cycling, the coarsening of Ag₃Sn precipitates and recrystallization occur, impacting the reliability and microstructure of solder joints. To enhance the high-temperature performance of lead-free alloys, dopants such as Bismuth (Bi), Antimony (Sb), and Indium (In) have been proposed by leveraging the solid solution and dispersion-strengthening mechanisms. However, limited studies have investigated the impact of varying paste volumes of third-generation alloys on thermal cycling reliability and the resulting microstructural evolution. This study examines the thermal cycling reliability of three CABGA208 components mounted on a printed circuit board (PCB) using three different paste alloys, namely SAC305, SAC-Bi, and SAC-Bi-Sb, while maintaining fixed 18-mil SAC305 solder balls. Three paste volumes were printed to achieve paste-to-sphere-volume ratios of 0.1, 0.3, and 0.5. The PCB was plated with Electroless Nickel Immersion Gold (ENIG) and Organic Solderability Preservative (OSP) surface finishes. Thermal cycling tests were conducted between -40 °C and +125 °C, with a dwell time of 15 minutes and a ramp rate of 10°C/min. Weibull analysis revealed that doped alloys outperformed SAC305, with SAC-Bi exhibiting the highest reliability. A higher paste-to-sphere volume ratio generally improved reliability, except for SAC-Bi-Sb, where larger voids formed at higher paste volumes. The ENIG surface finish provided better performance than OSP, likely due to the beneficial effect of the Ni layer. Microstructural analysis showed that higher paste volume in SAC-Bi and SAC-Bi-Sb significantly reduced Ag₃Sn coarsening. Additionally, increased paste volume resulted in a thinner intermetallic compound (IMC) layer and less evolution.