Mechanical properties of SnBi-SnAgCu composition mixed solder joints using bending test

A comparative analysis between the microstructure and tensile properties of eutectic Sn-58Bi, Sn-3.0Ag-0.5Cu and SnBi-SnAgCu composition mixed solders including 20, 25 or 50 wt% Sn-Ag-Cu in Sn-Bi solder was conducted, and the three point flexural bending properties of the solders as Cu/solder/Cu joints were evaluated with the effect of loading rate. With Sn-Ag-Cu added into Sn-Bi solder, the content of eutectic Sn-Bi phase decreased while Cu6Sn5 IMC and Sn-Ag-Bi eutectic structure were produced in the microstructure. Tensile results on the solder alloys indicated that SB-20SAC and SB-25SAC mixed solders showed an increased ductility with higher elongation and lower tensile strength compared with Sn-Bi solder, while SB-50SAC showed a brittle characteristic with the greatest strength but lowest elongation among all the mixed solders. The bending tests on the solder joints illustrated that composition mixing was helpful to increase the toughness of Sn-Bi solder, and the bending fracture energy of joints was dependent on the loading rate. A relationship between strain rate, crack propagation and fracture toughness was developed. Sn-Bi solder always had a brittle fracture mode. With Sn-Ag-Cu mixed into Sn-Bi solder, the fracture toughness of joints increased. The crack propagation was then decided by the strain rate, in which ductile failure in the solder matrix was occurred in the bending with a lower strain rate while brittle failure along IMC/solder interface was produced in a higher strain rate.