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

•Cu/Al can be successfully joined by ultrasonic-assisted soldering using Sn–9Zn filler.•Ultrasound could promote the dissolution of the base Al and migrating to the solder dramatically.•Cu5Zn8 was transferred to Al4.2Cu3.2Zn0.7 at the Cu-side interface due to the dissolved Al in solder.•The thickness of the Al4.2Cu3.2Zn0.7 layer has an important effect on the joint strength. 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 60s, the contents of Al in the solders reached 7.85wt%, 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 2s to 60s, the thickness of the IMC layer increased from 1μm to 20μm, and the tensile strength of the joints decreased from 77MPa to 39MPa. The critical thickness of T′ was approximately 5μm.