The operation of power semiconductor modules creates thermal stresses that grow voids in the solder die-attach layer. These voids reduce the ability of the die-attach solder layer to conduct heat from the silicon junction to the heat spreader. This results in increased thermal impedance. The effect of accelerated aging on solder bond voiding and on thermal transient behavior is investigated. Commercially packaged TO-247 style MOSFETs are power cycled, imaged, and thermally analyzed to generate a correlation between void percentage and thermal impedance.