The ultrasonic wire bonding process is widely used for making interconnections between IC chips and package lead frames, yet the relationships between the wire/substrate materials properties and the bond formation processes are not yet well understood. In this study, we have conducted experiments to understand the mechanical response in thin film substrates subject to ultrasonic wire bonding. Atomic force microscopy (AFM) has been used to examine the morphology of thin film surfaces before and after bonding. Al wires were bonded to thin films of Cu, Cu-Ni and Ag, after which the aluminum wire was removed by etching. AFM examination of the Cu and Cu-Ni bonded surfaces revealed the extent of surface smoothing increased with bonding power level. However, at power levels typically employed in practice almost no morphological surface changes were observed, indicating surface smoothing is not required to obtain good bonds. TEM and SEM cross-sections were also used to examine Al wedge bonds and Au ball bonds to Al films on Si. A wide variety of features were observed, including surface smoothing, microcracking and void formation.