Interfacial Reactions and Mechanical Properties of Bi-x wt.%Sn/Cu Joints (x = 2, 5, 10)

Bi-x wt.%Sn alloys (x = 2, 5, 10) were joined with Cu substrates and reacted at 300°C to investigate the interfacial reactions between molten high-Bi alloys and Cu. Only one intermetallic compound, Cu₃Sn, was formed at the interface for all alloy compositions. The Cu₃Sn phase did not adhere to the Cu substrate but instead detached from it. A Bi-rich layer was formed in between the Cu₃Sn phase and the Cu substrate as a result of interdiffusion of Bi and Cu through the Cu₃Sn phase. The formation of only the Cu₃Sn phase in the high-Bi alloy/Cu joints and a complicated interfacial microstructure are well explained using the Bi-Sn-Cu isothermal section. Mechanical tests indicated that the shear strength of the high-Bi alloy/Cu joints was comparable to that of the Pb-5 wt.%Sn/Cu joint, and fractures occurred partly along the high-Bi alloy/Cu₃Sn interface and partly within the high-Bi alloy.