Interfacial reactions with and without current stressing at Sn–Co/Ag and Sn–Co/Cu solder joints

Sn–Co alloys are promising Pb-free solders, while plating layers and substrates of Ag and Cu are commonly encountered in electronic products. This study examines the interfacial reactions between Sn–0.25 wt% Co/Ag and Sn–0.25 wt% Co/Cu at 180 and 210 °C, with and without current stressing. CoSn₃ precipitates are found in the solder matrix in the as-prepared condition. In Sn–0.25 wt% Co/Ag couples, a continuous Ag₃Sn reaction phase layer is observed at the interface and Ag₃Sn phase particles are dispersed in the matrix, with and without current stressing. When there is a 500 A/cm² electrical current, the growth rate of the Ag₃Sn phase is not affected at either the cathode side or the anode side. However, the passage of an electrical current leads to the formation of needle-like Ag₃Sn phase particles in the solder matrix. In Sn–0.25 wt% Co/Cu couples, both Cu₆Sn₅ and Cu₃Sn reaction phases are formed at the interface, with and without current stressing. Cu₆Sn₅ precipitates, with a higher Co content, are found in the matrix, mostly nucleated on CoSn₃ precipitates. When there is a 500 A/cm² electrical current stressing, all the reaction phase layers are thicker and the anode interfaces are nonplanar. It is observed that there is cracking and that there are discontinuous Cu₆Sn₅ layers at the interface and that a significant amount of Cu₆Sn₅ phase in the matrix accompanies 500 A/cm² electrical current stressing.