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

Abstract

Sn–4.0Ag–0.5Cu (SAC) and Sn–4.0Ag–0.5Cu–0.05Ni–0.01Ge (SACNG) lead-free solders reacting with the Au/Ni/Cu multi-layer substrate were investigated in this study. All reaction couples were reflowed at 240 and 255 °C for a few minutes and then aged at 150 °C for 100–500 h. The (Cu, Ni, Au) 6Sn 5 phase was formed by reflowing for 3 min at the interface. If the reflowing time was increased to 10 min, both (Cu, Ni, Au) 6Sn 5 and (Ni, Cu, Au) 3Sn 4 phases formed at the interface. The AuSn 4 phase was found in the solder for all reaction couples. An addition of Ni and Ge to the solder does not significantly affect the IMC formation. After a long period of heat-treatment, the thickness of the (Cu, Ni, Au) 6Sn 5 and (Ni, Cu, Au) 3Sn 4 phases increased and the intermetallic compounds (IMCs) growth mechanism obeyed the parabolic law and the IMC growth mechanism was diffusion-controlled. The mechanical strengths for both the soldered joints decreased with increasing thermal aging time. The SACNG/Au/Ni/Cu couple had better mechanical strength than that in the SAC/Au/Ni/Cu couple.