Formation and evolution of intermetallic compounds at interfaces of Cu/Al joints by ultrasonic-assisted soldering

Abstract The formation and evolution of Cu–Zn intermetallic compounds (IMCs) and Al–Cu–Zn ternary IMCs of Cu/Sn–9Zn/Al joint interfaces by ultrasonic-assisted soldering without flux in air were investigated. Aluminum particulates dissolved from the base metal and migrated into the solder by ultrasonic action. As the duration of ultrasonic action increased to 60 s, the contents of Al in the solders reached 7.85 wt%, which is significantly higher than the equilibrium solubility limit. The formation of Cu5Zn8 was prevented, and the IMC layers of the copper–solder surface changed from Cu5Zn8 to the Al4.2Cu3.2Zn0.7 (T′), because of the additional aluminum present in the solder. The Al–Cu–Zn ternary IMCs exhibited a slower growth rate and considerably thinner thickness than the Cu–Zn IMCs. The effect of the ultrasonic action on the thickness of the IMC layers and tensile strength of the joints was investigated. As duration of ultrasonic action increased from 2 s to 60 s, the thickness of the IMC layer increased from 1 μm to 20 μm, and the tensile strength of the joints decreased from 77 MPa to 39 MPa. The critical thickness of T′ was approximately 5 μm.