Microstructural characteristics of Au/Al bonded interfaces

Fracture characteristics at the interface of ultrasonic bonds between Au and Al were characterized by SEM following pull-testing to effect separation of the bonded joints. Vertical sections at the bonding point were produced by ion-sputter thinning, and were examined by TEM. Results show that the thickness of the Au/Al atomic diffusion interface was about 500 nm due to combined effects of ultrasonic and thermal energy. Ultrasonic vibration activates dislocations in the crystalline lattice and increases atomic diffusion. The fracture morphology on the lift-off interface was dimpled rupture. Tensile fracture occurred during the pull-test not at the bonded interface but in the base material; the bond strength at the interface was enhanced by the diffusion reactions that occurred across the interface due to the combined ultrasonic and thermal energy.