Transient Measurement of the Junction-To-Case Thermal Resistance Using Structure Functions: Chances and Limits

The accurate and reproducible measurement of the junction-to-case thermal resistance Rth-JC of power semiconductor devices is far from trivial. In the recent time several new approaches to measure the Rth-JC have been suggested, among them transient measurements with different interface materials between the package and a heat-sink which allow identifying the Rth-IC in the structure function of the heat flow path. This paper shows that numerical effects during the calculation of the structure function as well as 3D heat spreading have a big influence on the structure function which makes it often difficult to determine the Rth-IC. Finite element simulations can provide a clue to identify this value in the structure function. The theoretical findings are applied to and demonstrated for actual measurements and the new approach is compared to the traditional method (involving a thermo-couple measurement of the case temperature) with respect to accuracy and reproducibility. Finally an alternative approach to determining the Rth-IC from transient dual- interface measurements, which is not based on structure functions, is presented.