A transmission electron microscopy study of ultrasonic wire bonding

Transmission electron microscopy (TEM) was used to examine ultrasonic bonding of aluminum wire to various metal substrates. A special technique for preparing cross-sectional TEM specimens that involves embedding the sample in epoxy, dimpling, and electropolishing is described. The materials systems examined were Al wire bonded to Al foil, and Al wire bonded to Al foil onto which copper or gold was evaporated. Examination of the bond interface in the TEM revealed that in some areas the surface oxides and contaminants were dispersed, allowing the bare metals to contact, whereas in nearby areas the bond interface was filled with debris. The debris could be oxides, surface contaminants, or sheared metal particles. It also appears that the debris is 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. Therefore, it is concluded that ultrasonic wire bonding does not involve significant heating along the bonding interface. However, it is shown that dynamic annealing of aluminum wire can occur during bonding. The experimental results are compared with previously proposed theories on the mechanism of bonding in ultrasonic welding