Grain effect, creep analysis, and machine learning of Sn-3Ag-0.5Cu solder joint of BGA package under thermal shock test

• The role of high-angle grain boundaries is in accelerating dislocation accumulation. • The creep stress index (n) significantly decreases during thermal shock . • The Dorn creep model indicates the creep mechanism changes. With the miniaturization trend in microelectronics packaging, solder joints in Ball Grid Array (BGA) are facing escalating reliability challenges under thermal shock. The coupled effects of microstructural evolution and multi-physics interactions on creep behavior remain poorly understood. This study investigated the degradation mechanism and creep behavior of the Sn-3Ag-0.5Cu (SAC305) solder joint in the BGA package under a thermal shock test for high reliability. The results show that the thermal shock test induces significant changes and degradation in microstructure and mechanical performance: the intermetallic compound (IMC) layer progressively thickens with increasing cycles. Recrystallization, which becomes noticeable by 600 numbers of thermal shock, initiates at the solder pad interface and propagates toward the center of the solder joint. Cu 3 Sn precipitates accumulate in recrystallized regions, exacerbating stress concentrations and impairing creep resistance. After 1200 numbers of thermal shock, cracks predominantly form along the IMC/β-Sn interface, driven by stress concentrations resulting from mismatched thermal expansion coefficients and elastic moduli. Nanoindentation tests reveal that creep displacement increases with thermal cycling, and the creep stress index (n) decreases from 7.2 for uncycled joints to 1 after 1200 cycles, indicating a shift from stress-dependent dislocation creep to stress-independent, diffusion-controlled creep.