Preparation of Sn42Bi58 and Sn64.7Bi35Ag0.3 soldering pastes for Cu–Al direct soldering and study of their performance

Cu–Al soft soldering plays a crucial role in the fabrication of Cu–Al composites. In this study, a new rosin-type flux for Cu–Al direct soldering was synthesized, and G-1 (Sn42Bi58 paste) and G-2 (Sn64.7Bi35Ag0.3 paste) soft soldering pastes were prepared. The results of flux physicochemical tests are as follows: density 1.11 g/cm³, pH 5.2, viscosity 32 Pa s, and non-corrosive to the copper plate. These are in line with the requirements for soldering applications. The wettability test results show that G-1 and G-2 displayed good wettability on the Cu and Al plates. This is in line with the standard Cu–Al soft soldering paste. The mechanical property test showed that G-2 had better mechanical properties than G-1, with a tensile strength of 112.57 MPa. SEM revealed that the IMC (intermetallic compound) layer of G-1 was flatter than that of G-2. This is closely related to the fact that the Ag in Sn64.7Bi35Ag0.3 powders can reduce the enrichment of metallic Bi and refine the grains.