Effects of Minor Addition of Cu and Ni on the Microstructure, Thermal and Mechanical Properties, and Wettability of Sn-58Bi Solder Alloys

Small amounts of Cu and Ni were added to Sn-58Bi solder alloy to improve the reliability of solder joints on Cu substrates. The effects of these additions on the microstructure, thermal and mechanical properties, and wetting behavior of Sn-58Bi solder alloys were investigated. The results indicated that Cu refined the microstructure and induced the formation of a Cu₆Sn₅ intermetallic compound phase in the solder matrix. Meanwhile, Ni promoted the formation of a Ni₃Sn₄ intermetallic compound phase in the solder matrix, and the microstructure became coarser. The ultimate tensile strength and hardness of the solder alloy increased with the addition of Cu, while the effects of Ni on these properties were only slight. The achieved strength improvement was due to microstructural refinement and the formation of intermetallic compound phases in the solder matrix. Fracture surface analysis indicated that the joints of Sn-58Bi solder alloys containing Cu and Ni exhibited signs of a brittle fracture mode. The shear test results indicated the advantage of adding Cu for the refinement of the Bi-rich phase, which resulted in enhanced shear strength. However, Ni increased the thickness of the intermetallic layer at the solder–substrate interface and formed coarsening Bi-rich phases that reduced shear strength. The additions of Cu and Ni both contributed positively to the wetting behavior, melting point, and undercooling of the Sn-58Bi solder alloys, whereas there was no significant improvement in the pasty range of the solder alloy.