Influence of superimposed ultrasound on deformability of Cu

The effects of superimposed ultrasonic vibration on the plastic deformation of 99.99% pure polycrystalline Cu are studied both during (temporary) and after (residual) the application of ultrasound (US) using deformability measurements acquired from an automated wire bonding machine and microhardness testing. It is found that if ultrasonic irradiation is applied during the deformation of the 100 μm diameter Cu free air balls (FABs) the Cu becomes softer with increasing US power compared to Cu FABs that are deformed without US. When comparing this temporary acoustic softening of Cu to that of Au, it is found that the amount of softening is similar between the two materials. After the US is turned off, a residual acoustic softening remains. This residual softening effect increases with increasing the US power. With residual acoustic softening, a maximum increase of 13% in deformability is measured for Cu during wire bonding compared to a maximum increase of 8% for Au during wire bonding. Stronger residual acoustic softening effects are obtained in Cu than in Au with a maximum decrease in microhardness of 19% and 9%, respectively. Dynamic annealing and dislocation theory are used to explain both the temporary and residual effects of US on the deformation of Cu and Au.