Localized recrystallization and cracking behavior of lead-free solder interconnections under thermal cycling

The failure mechanism of lead-free solder interconnections under thermal cycling has been studied by cross-polarized light microscopy, scanning electronic microscopy (SEM), and nanoindentation test. From the results of finite element modeling (FEM), it was found that the critical solder interconnection was located at the chip corner, and the stress was concentrated at the outer neck region beneath the ball grid arrays (BGA) component. The FEM results were in good agreement with the experimental observation. Two failure modes of the interconnections were identified: one is the intergranular or transgranular cracking through many small equiaxed recrystallized grains and the other is the transgranular cracking in few large irregularly shaped recrystallized grains. The results show that the localized recrystallization makes the Ag₃Sn intermetallic compounds (IMC) coalesce and distribute sparsely, which leads to the degradation of the recrystallized microstructure and easy propagation of the cracks.