Characterizing the Anisotropic Hardening Behavior of Aluminum Bonding Wires

Abstract

In power electronic devices the electrical connections of different components are mainly realized by heavy aluminum wire bonding. When a device heats up or cools down during use, there is a relative displacement between the first and the second contact because of differences in thermal expansion coefficients of the components and the housing of the device. This cyclic thermo mechanical loading can lead to fatigue failure of the bonding wire. Especially when placed near mechanical working components (e.g. automotive engine) additional vibrations can increase damage evolution and heating can accelerate ageing effects of the bonding wire. In the last few years there have been multiple publications presenting experimental and numerical results for high and low cycle fatigue of wire bonded devices. But all of these publications did not consider the mechanical properties of the wire in an adequate way. We present results of a micro-compression test that allows the determination of the hardening behavior parallel and perpendicular to the wire axis at moderate and large strains of small wire cylinders. The hardening behavior in compression parallel to the wire axis correlates very well to the hardening behavior determined by tensile tests at moderate strains. The hardening behavior perpendicular to the wire axis shows an anisotropic behavior of the aluminum wires depending on the drawing texture which was also analyzed by electron backscatter diffraction methods. The results for different wire materials show a dependence of the yield stress on the grain size. With the determined hardening parameters it is possible to consider the hardening of the material during the bonding process.