Effects of Sn addition on the microstructure and properties of Bi–11Ag high-temperature solder

High-temperature lead-free Bi–11Ag solder with a Sn addition of 1, 3, and 5 wt% was investigated in this paper. The results show that the melting temperature of Bi–11Ag solders dropped from 265 to 255 °C after the addition of Sn, which is suitable for the high-temperature reflow process. When the addition of Sn was 3 wt% or less, dendrites in the microstructure would be refined, however, further additions of Sn show an opposite effect. The contact angle of Bi–11Ag–xSn (x = 0, 1, 3, 5 wt%) solders on the Cu substrate was measured, and the value decreased from 62° to 41° with the increasing of Sn content. The tensile strength of the Cu/Bi–11Ag–xSn solder/Cu sandwich structured microscale solder joint continues to increase from 43.3 to 53.9 MPa and the fracture position evolves from the solder/Cu interface to the middle of solder. In addition, according to EDS analysis of the solder/Cu interface, a Cu₃Sn intermetallic compound (IMC) layer can be identified. The combination of this IMC layer and grain boundary grooving at the solder/Cu interface can effectively improve the strength of the Bi–11Ag–xSn solder joint.