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Thermal resistance data were collected using two different style flip chip ball grid array (FCBGA) packages; one with an exposed molded die and a second with a lid. Eleven different heat sink designs and two different thermal interface materials (TIM) were tested to quantify the thermal interaction between heat sink size, base material and TIM resistance as a function of package style. Package style and TIM material did not appreciably change the total thermal resistance (less than 10%) for small heat sinks 50 mm × 50 mm smaller. The exposed molded die package thermal resistance was 14% smaller than the lidded package when tested with a heat pipe heat sink. An understanding of the long term performance impact of TIM II degradation was investigated using conduction based models. Lidded style packages may increase safety margin when TIM II materials experience pump-out, dry-out or voiding.