A transmission electron microscopy study of ultrasonic wire bonding

Transmission electron microscopy (TEM) is used to examine ultrasonic bonding of Al wire to Al foil and to foil onto which copper or gold was evaporated. Examination of the bond interface in the TEM reveals that in some areas, the surface oxides and contaminants were dispersed, allowing the bare metals to contact, whereas in other nearby areas, the bond interface was filled with debris. The debris could be oxides, surface contaminants, or sheared metal particles. It also appeared that the debris was dispersed to low-lying regions between asperities. In studies of the Al/Au/Al and Al/Cu/Al interfaces, microchemical analysis showed no evidence for diffusion or melting along the interface, indicating that ultrasonic wire bonding does not involve significant heating along the bonding interface. However, dynamic annealing of aluminum wire can occur during bonding. The experimental results are compared with the previously proposed theories on the mechanism of bonding in ultrasonic welding