Transient thermomechanical study of a thick-wire bond with particular attention to the interfacial shearing stress

A high-power IGBT (Isolated Gate Bipolar Transistor) module has been thermomechanically analyzed in the moment of switching on short circuit. We studied the extremely rapidly changing temperature distribution within a wire bond by a transient thermal simulation on the basis of an experimentally determined power loss as function of time, a finite element simulation of thermomechanically induced stress and strain distribution within the bond-wire wedge and the silicon chip, and a simple but meaningful analytical model of the same arrangement, with special attention to the interfacial shearing stress as function of time. The analyses revealed a surprising result: during the first few microseconds the extremely high power loss density in the silicon chip causes a rapid temperature increase and expansion of the silicon chip which leads to a tensile stress in the aluminum bond wire, whereas in the consecutive phase the temperature of the aluminum wire approaches the one of the chip, whereby the aluminum wire turns to compressive stress which means a reversal of shear stress at the wire/chip interface. This could be a new explanation for the field failures known.