Co Solubility in Sn and Interfacial Reactions in Sn-Co/Ni Couples

It has been reported that minute Co additions to Sn-based solders are very effective for reducing undercooling, probably due to low Co solubility in Sn. In this study, Co solubility in molten Sn was determined experimentally. According to results of metallographic analysis, Co solubility in molten Sn is as low as 0.04 wt.% at 250°C. Interfacial reactions in Sn-Co/Ni couples at 250°C were examined for Co contents from 0.01 wt.% to 0.4 wt.%. The Ni3Sn4 phase was the only interfacial reaction phase in almost the entire Sn-0.01 wt.%Co/Ni couple. For Sn-Co/Ni couples with a Co content higher than 0.01 wt.%, a thin, continuous Ni3Sn4 layer and a discontinuous decahedron (Ni,Co)Sn4 phase were formed in the initial stage of reaction. The reaction products evolved with time. With longer reaction time, the Sn content in the decahedron (Ni,Co)Sn4 phase decreased, and the (Ni,Co)Sn4 phase transformed into the (Ni,Co)Sn2 phase and cleaved into a sheet, which then detached from the interface, after which Ni3Sn4 began to grow significantly with longer reaction times.