Footprint Study of Ultrasonic Wedge-Bonding with Aluminum Wire on Copper Substrate

The effects of the process parameters of ultrasonic power and normal bonding force on bond formation at ambient temperatures have been investigated with scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis. A model was developed based on classical microslip theory to explain the general phenomena observed in the evolution of bond footprints left on the substrate. Modifications to the model are made due to the inherent differences in geometry between ball-bonding and wedge-bonding. Classical microslip theory describes circular contacts undergoing elastic deformation. It is shown in this work that a similar microslip phenomenon occurs for elliptical wireto-flat contacts with plastically deformed wire. It is shown that relative motion exists at the bonding interface as peripheral microslip at lower powers, transitioning into gross sliding at higher powers. With increased normal bonding forces, the transition point into gross sliding occurs at higher ultrasonic bonding powers. These results indicate that the bonding mechanisms in aluminum wire wedge-bonding are very similar to those of gold ball-bonding, both on copper substrate. In ultrasonic wedge-bonding onto copper substrates, the ultrasonic energy is essential in forming bonding by creating relative interfacial motion, which removes the surface oxides.