Cyclic loading and fatigue in power packages

The author studied the behavior of power packages exposed to repeatedly high current loads. Self-heating of the power transistors can cause a tremendous temperature rise in the vicinity of the active device. The resulting temperature gradient and the thermal mismatch of materials induce mechanical strains and stresses in the power package. The stress level can in some cases exceed the yield limit of the metallization layer on the die. Repetitive current peaks will then cause a fatigue phenomenon that can cause a device failure. Coupled thermal and mechanical finite element simulations were performed for fault conditions of an analog power semiconductor. Transient temperature fields of the chip surface are analyzed experimentally with a high speed infrared camera system. Test results and transient simulation are compared for heat pulses in the microseconds time range. The validated transient temperature fields are applied to a subsequent thermo-mechanical analysis. Furthermore, maximum stresses and stress amplitudes are calculated for the selected metal layers. The risk for material fatigue will be assessed.