A correlative approach to observing the thermomechanically driven microstructural evolution of ultrasonically bonded copper wires

Abstract:
In this paper, the microstructural evolution of pre-annealed heavy copper wires ultrasonically bonded onto Cu-AlN substrates is investigated. Bonds are subjected to thermal cycling between -55 and 150 °C. The bond interfaces are nondestructively imaged prior to and after thermal cycling using 3D X-ray tomography. They are then examined using electron backscattered diffraction and nanoindentation. Tomography datasets show negligible change after 1500 cycles, however, modest crack growth is observed after 3000 cycles at the bond peripheries. The microstructures, however, are perceptibly altered by cycling after 1500 cycles, including loss of subgrains and some continued recrystallisation at the interfaces, as well as softening of the bulk wire, interfacial region and copper substrate. Nonetheless, after 3000 cycles, microstructures remain fine and retain strong textures.