Minor Ge Addition to Suppress the IMC Growth in Interfacial Reactions of Co with Sn-Ag-Ge and Sn-Ag-Cu-Ge Solders

This study investigated the effects of minor Ge addition (0.01–0.1 wt%) on the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders (in wt%) with a Co substrate. At the Sn-3.5Ag/Co solder joint, the major reaction phase was CoSn3, which exhibited a high growth rate. In the liquid-state Sn-3.5Ag-Ge/Co reactions at 250°C, the CoSn3 growth was significantly suppressed with only 0.01 wt% Ge addition. When the Ge content exceeded 0.05 wt%, an unknown X phase was initially formed, and then the CoSn3 phase become dominant and grew thicker. With a 0.1 wt% Ge addition, the thin X phase layer stably existed at the interface and was suggested to be a ternary Co-Ge-Sn intermetallic compound (IMC). Similar interfacial results were also observed in the liquid-state Sn-3.5Ag-0.5Cu-Ge/Co reactions. Owing to the Ge and Cu additions, the suppression effect of the CoSn3 growth was more obvious. Similar solid-state reactions were examined at 180°C. For the Sn-3.5Ag-Ge/Co reactions, the minor Ge addition also greatly inhibited the CoSn3 growth. However, different from the liquid-state reactions, the X phase was not formed in the solid-state reactions. In the Sn-3.5Ag-0.5Cu-Ge/Co reactions, the addition of 0.01 wt% Ge completely inhibited the CoSn3 formation, and only the thin (Cu,Co)6Sn5 layer was observed even after a long-term reaction. This indicated that the Cu and Ge had a synergistic effect and strongly suppressed the CoSn3 growth.