The diffusion of Ni into Al wire at the interface of ultrasonic wire bond during high temperature storage

Ultrasonic wire bonding is one of the most important techniques for die interconnection in microelectronic packaging. It is widely used for power devices, microwave devices and photoelectron devices packaging. At room temperature, ultrasonic energy and plastic deformation energy, generated by metal wire plastic deformation under the wedge tool pressure, make the wire and metallization join together. The ultrasonic bonds, after bonding and aging, of Al+1%Si wire with 25μm diameter bonded on the Au/Ni/Cu pad, are analyzed by scanning electronic microscopy (SEM) with energy dispersive x-ray spectrometer (EDX). The joints begin at the bond periphery where it is the location of the greatest plastic flow. It is found that the mechanism of ultrasonic bonding is, both the plastic flow of metal wire generated by wedge tool pressure which results in the diffusion of Ni into Al wire; and the effect of ultrasound is that, on the one hand, ultrasonic vibration enhances the metal wire ability of plastic flow and, on the other hand, it generates many defects inside the metal wire which are the fast diffusion channels. The diffusion type is likely the short-circuit diffusion, which is more prominent than the crystal diffusion when the temperature is low. After high temperature storage at 170 °C for 10 days, there is evident diffusion of Ni into Al wire, but the microstructure is the same with the bonds after bonding, there is no evident change. Aged for 30 days, the bond interface forms a cloud-like structure, and the major composition is Al and Ni with weight percent of 78.82% and 15.55% respectively. However, the diffusion is not even and some parts of the bond interface are absence of Ni diffusion. When the aging time is 40 days, the cloud-like structure transforms into rectangular island-like structure and there are many cavities inside the bond wire, which are different from the Kirkendall voids because of the shape and dimension.