Role of bismuth in the mechanical and corrosion properties of Cu/Sn-3.0Ag-0.5Cu/Cu solder lap joints

Cu/Sn-3.0Ag-0.5Cu-xBi/Cu solder lap joints were prepared using Sn-3.0Ag-0.5Cu (SAC305) solder paste doped with 0, 1 wt%, 3 wt% and 7 wt% Bi powder, respectively. The different samples were immersed in 3.5 wt% NaCl solution for a maximum of 28 days, and the corrosion products were SnO, SnO₂ and Sn₃O(OH)₂Cl₂. The samples were observed and tested at different periods of the study, and the shear strength, corrosion morphology, XRD phase diagrams and weight changes of the joints were compared with those of the joints with different doping concentrations. The results showed that the shear strength of SAC305 solder joints increased and then decreased with the increase of Bi doping concentration. Although the 7 wt% Bi doped lap joint had the lowest shear strength and was still the lowest at the end of the 28 day immersion corrosion tests, the decrease in shear strength with increasing immersion time was the slowest compared to the joints with other Bi doping concentrations. The results suggest that high concentrations of Bi doping in SAC305 solder joints have a corrosion-blocking effect.