Interfacial reactions and mechanical properties between Sn-4.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge lead-free solders with the Au/Ni/Cu substrate

► The complete study of the interfacial reactions and mechanical property of the solder joints in the SAC/Au/Ni/Cu and SACNG/Au/Ni/Cu couples have been investigated in this paper. ► The (Cu, Ni, Au)6Sn5 phase was formed by reflowing for 3 min at the interface. If the reflowing time was increased to 10 min, both (Cu, Ni, Au)6Sn5 and (Ni, Cu, Au)3Sn4 phases formed at the interface. ► The intermetallic compound growth mechanism was diffusion-controlled and the IMC formation was not significantly affected by the addition of Ni and Ge to the solder. ► The mechanical strength in the SACNG/Au/Ni/Cu couple is better mechanical strength than that in the SAC/Au/Ni/Cu couple.