Study on Microstructure and Joint Strength of Sn-0.7Cu-0.8Zn/Cu Solder Joints by Bi Addition Modification

We have elucidated the roles of Bi addition in the melting characteristics, wettability, microstructure evolution, and mechanical properties of Sn-0.7Cu-0.8Zn/Cu solder joints. Melting properties analysis demonstrated that the melting point can be lowered by Bi alloying, while enhancing the melting range of the alloy. The wettability of the Sn-0.7Cu-0.8Zn solder first increased by a minor Bi addition and then decreased with the further increase of Bi content, and the wettability of the solder containing 1% Bi was the best. Adding Bi to Sn-0.7Cu-0.8Zn solder inhibited the development of the Cu₆Sn₅ phase, and promoted the formation of fine γ-Cu₅Zn₈. The interface intermetallic compound (IMC) layer distributed at the solder joint interface was the γ-Cu₅Zn₈ phase regardless of whether or not Bi was added. However, the IMC thickness first increased and then decreased as the content of Bi element increased. The shear strength of the Sn-0.7Cu-0.8Zn/Cu solder joint can be elevated as the amount of Bi addition increased, which was due to the combination of second-phase reinforcement, solution-strengthening and the reduction of IMC thickness. Adding Bi also decreased the toughness of the solder joint.