Microstructures and shear properties of antimony- and indium-strengthened Sn5Bi/Cu joints

The effects of Sb/In (with a loading x of 1, 2, 3 wt.%) in the Sn5Bi solder alloy on the melting properties, microstructures, and the shear behavior of solder/Cu joints were investigated combined with the corresponding phase diagram. It is found that the addition of Sb reduces the melting range and supercooling with the increase of Sb content caused by SbSn formation, while the In addition can reduce the melting point resulted by In solid solution. Both Sn5BixIn/Cu and Sn5BixSb/Cu are mainly composed of β-Sn and two types of precipitates, i.e., Bi particles and Cu₆Sn₅ compound. The difference lies in that In doping only can dissolve into β-Sn and Cu₆Sn₅ while Sb doping mainly forms tiny SbSn. Increasing In content also inhibits the precipitation of Bi particles and reduces the phase fraction Cu₆(Sn, In)₅, resulting in that precipitation strengthening becomes weaker and solid solution strengthening becomes stronger. In contrast, the phase fraction of Bi particles and Cu₆Sn₅ increases with increasing the Sb content, leading to the stronger precipitation strengthening. These strengthening mechanisms cause their ultimate shear force to monotonically increase with increasing Sb and In content, and the strengthening effect of the Sb element is better. Sb and In addition also can improve the ductility of Sn5Bi/Cu solder joints, and both 2 wt.% In/Sb additions present the best shear fracture work. Thus, the optimal mechanical property was achieved by 2 wt.% Sb addition. These reinforcements will provide a strong support and specific data reference for the interconnection requirements in high-strength interconnection fields such as automotive electronics, power semiconductor, and medical electronic products.