Effect of intermetallic compounds on mechanical properties of copper joints ultrasonic-soldered with Sn-Zn alloy

The transformation of the intermetallic compounds (IMC) at the interface of Sn-Zn solder and Cu substrate by an ultrasonic-assisted soldering method with the variation of Zn content was investigated. The formation of IMC at the solder/Cu interface is controlled by the Zn content in the filler metals. The IMC layer of Cu₆Sn₅ forms if the mass fraction of Zn is lower than 0.5 wt.%. The IMC becomes a single-layered Cu₅Zn₈ if the mass fraction of Zn exceeds 1 wt.%. The layer of Cu₅Zn₈ thickens with the increase of Zn content. Joints with an IMC layer of Cu₅Zn₈ exhibit a higher strength than that with Cu₆Sn₅. In the joints soldered with the Sn-0.5Zn filler metal, fractures occur inside the Cu₆Sn₅ IMC grains. In the joints with the Zn content higher than 1 wt.%, fractures propagate along the Cu₅Zn₈ IMC/solder interface. The locations of the fracturing are associated with the micro-cracks that existed at the IMC grains.