The junction-to-case thermal resistance: A boundary condition dependent thermal metric

Contrary to popular belief the junction-to-case thermal resistance (Rth-JC) of a power semiconductor is not an intrinsic property of the device but depends to some extend on the cooling condition at the case surface intended for heat sinking. In addition to this the Rth-JC can be measured only with quite limited accuracy by the methods existing today. This paper investigates the dependence of the Rth-JC of power packages on different cooling conditions and tries to give an assessment of the accuracy of two measurement methods: the traditional method using a thermocouple to measure the case temperature versus the recently proposed transient dual interface method. Consequences and limits for the use of the junction-to-case thermal resistance in engineering applications as well as for standardization issues are discussed.