Failure Analysis of Thick Wire Bonds

Abstract

In the last decade, reliability problems have become a critical subject in power modules. Understanding design weakness and failure mechanisms of thick wire bond are two critical steps in managing the risk of wire bond heel crack which is the topic of this thesis. Although this thesis does not target a specific type of power modules, we note that thick wire bond heel crack failures occur in Insulated Gate Bipolar Transistors (IGBTs). In fact, our aim is to understand failure mechanism in 300μm thick wire bonds with different geometries and materials. Since these wires experience harsh environmental conditions and high load transients, the wires undergo repetitive flexural movement which causes heel crack due to fatigue. For the purpose of understanding this failure mechanism, two experimental setups are built and utilized. The first experimental setup loads the wires using constant currents and observes the response using a scanning laser vibrometer to measure the displacement. The second experimental setup applies repetitive prescribed displacement to the first foot of the wire and detects fatigue failure using a Wheatstone bridge. It is realized that wires have different displacement property depending on their geometry and material. Maximum displacements are observed for Al-H11 instead of CuCorAl and PowerCu.