Cross-Interaction in Cu/Sn/Co/Sn/Ni and Cu/Sn-Co/Co/Sn- Co/Ni Couples

In this study we examined cross-interactions of Cu/Co and Co/Ni by use of Cu/Sn/Co/Sn/Ni and Cu/Sn-0.04 wt.% Co/Co/Sn-0.04 wt.% Co/Ni couples. In the Cu/Sn/Co couple, Cu can diffuse through the pure Sn solder, and the (Cu, Co)(6)Sn-5 phase is formed at the Co side. The rate of consumption of Co decreases in the presence of Cu. Adding 0.04 wt.% Co to the pure Sn solder blocks diffusion of Cu; the (Cu, Co)(6)Sn-5 phase at the Co side is not observed, and the rate of Co consumption is increased. In the Co/Sn/Ni couple, no noticeable Co and Ni diffusion is observed. When 0.04 wt.% Co is added, the ternary (Ni, Co)Sn-4 phase is formed at the Ni side. These results indicate Co is an effective barrier to diffusion of Cu which can be used in flip chip packaging of Cu/low-k chips.