Evaluation of the characteristics of 305SAC lead-free solder joints between a chip electrode and a Cu-pad in automotive electronics

Abstract

The joint characteristics of 96.5Sn-3.0Ag-0.5Cu lead-free solder were confirmed by thermal life test, shear test, and the finite elements method (FEM). The thermal life test was performed as a reliability test considering the vehicle environment, and the stress concentration in the shear test was calculated using FEM. After the thermal test, shear strength decreased 8.2% in the R 1608 chips and 24.1% in the C 1608 chips. To grasp the cause of the shear strength reduction, we examined the fracture mode and analyzed the fracture surface and detected elements. Metal compounds, including Cu3Sn, Cu6Sn5, Ag3Sn, and Ni3Sn4 as intermetallic compounds, were formed by diffusion between materials as could be expected with aging at high temperature. The change in the thickness of metal compounds at the early stage followed by thermal diffusion-induced grain boundary migration (DIGM) of the Ni plating layer and the chip electrode (Ag, Cu) caused the reduction in shear strength. Thus, controlling the kinds and thicknesses of metal compounds under the high temperatures common in the operating environments of vehicles is necessary in the future.