Microstructure evolution and growth kinetics of Cu6Sn5 intermetallic compound at liquid-solid interfaces in Cu/Sn/Cu interconnects under ultrasonic waves

The microstructure evolution and growth kinetics of Cu6Sn5 intermetallic compound (IMC) in a Cu/Sn/Cu interconnection during ultrasonic-assisted transient liquid phase (TLP) soldering were studied. Being different from the symmetrical growth during isothermal aging without ultrasonic waves (USW), highly asymmetrical growth of Cu6Sn5 at the upper and lower Sn/Cu interfaces was observed with USW. Cu6Sn5 grains exhibited scallop-type morphology and were discrete at the upper interface; while that at the lower Sn/Cu interface exhibited column-type morphology and were conterminous at the lower interface. USW induced Cu atom migration from the upper Cu surface to the lower interface, providing a sufficient atomic flux for rapid Cu6Sn5 formation at the lower interface and creating cavitation pits on the upper Cu surface. A growth model was developed to explain the Cu6Sn5 IMC kinetics, suggesting a reaction-controlled growth mechanism.