Coupling Effect of the Interfacial Reaction in Co/Sn/Cu Diffusion Couples

Co/Sn/Cu sandwich couples formed by electroplating were examined to investigate the interaction between Cu and Co across the Sn layer for various Sn thicknesses from 75 μm to 580 μm. At the Sn/Cu interface, both Cu6Sn5 and Cu3Sn are formed. Unlike in a binary Sn/Cu couple, Cu6Sn5 has a spiked structure for couples with a thinner Sn layer. At the Co/Sn interface, two phases, CoSn3 and (Cu,Co)6Sn5, were simultaneously observed after reaction at 200°C. Remarkably, the CoSn3 reaction layer was much thinner than that in the binary Sn/Co couple. Furthermore, only the (Cu,Co)6Sn5 phase was formed at 150°C. This finding indicates that CoSn3 growth is significantly inhibited in Co/Sn/Cu sandwich couples due to the Cu substrate.