Interfacial reactions in the Sn–xBi/Au couples

The interfacial reactions between Sn–50 wt% Bi (Sn50Bi), Sn–57 wt% Bi (Sn57Bi) and Sn–65 wt% Bi (Sn65Bi) alloys with the Au substrate aged at 80, 100 and 120 °C for 12–350 h are investigated in this study. The Au–Sn binary intermetallic compounds (IMCs) were observed at the interface, and the Au–Sn binary IMCs and ternary Au–Bi–Sn metastable (T) phase in the solder when the Sn–50Bi and Sn–65Bi couples. The Au–Sn binary IMCs and T phases were formed in the Sn–50Bi/Au couple aged at 100 °C and Sn–57Bi/Au couple aged at 120 °C for 100 h. However, this T phase was transferred into the AuSn4 phase when the reaction was extended over 72 h. The increase in the Bi content in the Sn–Bi alloys could make the large (Bi) phase segregate in the solder/Au interface and decrease IMC growth. The IMC growth mechanism in all reaction couples is diffusion-controlled. The lowest reaction activity energy value could be found in the Sn–57Bi/Au couple due to its lowest liquidus temperature.