Relationship between crack propagation trends and grains in SnAgCu interconnects

Thermal shock test was performed on the SnAg3.8Cu0.7 (SAC387) solder bump interconnecting AlN and Cu substrate. Finite element modeling (FEM), cross-polarized light microscopy, scanning electronic microscopy (SEM) and EBSD were used to analyze the failure mechanism of them. It was realized that during thermal shock, the areas with higher stress would suffer from more serious IMC coarsening and then lead to easier propagation for the cracks. The recrystallization phenomena and crack could also be observed after 1500 cycles. The coefficient of thermal expansion (CTE) mismatch in thermal shock also caused serious cracks and was transgranular fracture along the diagonal the same direction with the stress. The energy bring by the crack tip was the necessary condition of the recrystallization and sliding and rolling of refined grains. The radius of the influenced area was only about 10 μm. The recrystallization could consume some of the crack energy but then allowing the crack propagates in a more plastic mode.