A high-Cu-content-mixed solder paste comprising Sn–3.0Ag–0.5Cu (SAC305) + 20 wt% Sn–xCu (x = 3, 5, 8, and 10) was prepared in this study. The melting characteristics, wetting properties, liquid solder joint fluidity, melt viscosity, and surface tension of the mixed solder paste as well as the interfacial reaction mechanism and the shear properties of the mixed solder joints were investigated. The findings indicate that during the initial reflow, the solderability and wettability of the mixed solder paste are comparable to those of SAC305. However, during the second reflow, the overflow capability of the solder joint is restrained. Additionally, after the initial reflow, the mixed solder joint becomes a high Cu alloy, its composition deviates from the Sn–Ag–Cu eutectic point, and the liquidus temperature increases when remelted. For example, the liquidus temperature of SAC305 + 20-wt% Sn–10Cu is 71 °C higher than that of SAC305 (225.7 °C) during the second heating. Furthermore, during the second reflow process, the increased liquidus temperature of the mixed solder joints leads to higher melt viscosity and surface tension. The surface tension of the SAC305 + 20-wt% Sn–10Cu liquid alloy is 235.73 times that of SAC305, while the fluidity of the solder joints is restrained. In addition, a thin intermetallic compound (IMC) layer with larger grain is formed at the Cu/solder interface in the mixed solder joints. The thickness of the IMC layers at the Cu/solder interface in the mixed solder joints are as follows: 2.63 μm for SAC305 + 20-wt% Sn–3Cu, 1.87 μm for SAC305 + 20-wt% Sn–5Cu, 2.19 μm for SAC305 + 20-wt% Sn–8Cu, and 2.21 μm for SAC305 + 20-wt% Sn–10Cu. All of these values are lower than the thickness of the IMC layer in SAC305 (3.39 μm). In addition, we found that the addition of reinforced particles did not reduce the shear strength of the solder joints after multiple reflows. The shear strength of SAC305, SAC305 + 20-wt% Sn–3Cu, SAC305 + 20-wt% Sn–5Cu, SAC305 + 20-wt% Sn–8Cu, and SAC305 + 20-wt% Sn–10Cu were 43.04, 42.69, 43.10, 43.19, and 43.77 MPa, respectively, and all solder joints exhibited ductile fracture after shear tests.
