Wire Flexure Fatigue Model for Asymmetric Bond Height

This paper presents the first physics-of-failure based life prediction model for flexural failure of wires ultrasonically wedge bonded to pads at different heights. The life prediction model consists of a load transformation model and a damage model. The load transformation model determines the cyclic strain at the heel of the wire during temperature cycling. This cyclic strain is created by a change in wire curvature at the heel of the wire resulting from expansion of the wire and displacement of the frame. The damage model calculates the life based on the strain cycle magnitude and the elastic-plastic fatigue response of the wire. The model supports virtual qualification of power modules where wire flexural fatigue is a dominant failure mechanism. The model has been validated using temperature cycling test results, and can be used to derive design guidelines and establish a relation between accelerated test results and field life.