Asymmetric growth of Cu₆Sn₅ intermetallic compound in Cu/Sn/Cu interconnection system under the vertical ultrasonic vibration

The asymmetric growth of Cu₆Sn₅ intermetallic compound (IMC) in a Cu/Sn/Cu interconnection system was studied during the transient liquid phase (TLP) soldering process with the assistance of the vertical ultrasonic vibration. Being different from the symmetrical growth during isothermal aging without ultrasonic waves (USW), highly asymmetrical growth of Cu₆Sn₅ at the upper and down Sn/Cu interfaces was observed with the vertical USW, i.e., Cu₆Sn₅ grains exhibited scallop-type morphology, and were discrete at the upper Sn/Cu interface (the side of ultrasonic action); while that at the lower Sn/Cu interface (the opposite side of ultrasonic action) exhibited column-type morphology, and were conterminous. Under the assistance of USW, the Cu₆Sn₅ grains were remarkably refined, the shear strength of joint was increased, and the fracture mode was changed from transgranular fracture to transgranular and intergranular fracture. This anomalous behavior can be completely ascribed to the asymmetrical ultrasonic effects across the entire Sn the intermediate layer of Sn.