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 A degrees 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/cm(2) 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/cm(2) 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/cm(2) electrical current stressing.