The effect of cyclic hardening in copper heat-sinks on die fracture probability

The use of pure copper heat-sinks (also termed flange) is a significant step forward to improve the thermal performance of flange mounted RF devices. One of the major risks however, is the large CTE mismatch between copper and silicon, which may result in die fracture. In order to asses the probability of die fracture during temperature cycling, a FE modeling study, combined with experiments to extract material properties, validate the models and get material strength data, has been performed and will be presented here. In order to take into account nonuniform and changing stress distributions and the probabilistic nature of surface defects that determine the die fracture strength, Weibull statistics and a surface area stress integral have been used to calculate the probability of die fracture. With incorporation of the cyclic hardening of the copper, by a combined kinematic and isotropic constitutive model, the predicted failure probability matched experimentally observed failure probabilities and provides a physical explanation for early failures.