Wire fatigue model for power electronic modules

This paper presents two, semi-analytical, physics-of-failure based life prediction model formulations 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 first formulation provides quick calculation of the time to failure for a wire of known geometry. The second formulation optimizes the wire geometry for maximum time to failure. These model formulations support virtual qualification of power modules where wire flexural fatigue is a dominant failure mechanism. The model has been validated using temperature cycling test results.