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The standard procedure to measure the Rth-JC of semiconductor devices requires a thermocouple measurement of the case temperature while the IC-package is in contact with a water-cooled heat-sink. This method often produces wrong results since the measurement of the case temperature is quite prone to errors. Transient dual interface (TDI) measurements have been suggested as an alternative to overcome the problems of the thermocouple method: Two Zth-curves of the device are measured, each with a different interface material between package and heat-sink. The diverging Zth-curves can be evaluated in different ways to determine the Rth-JC of the device. Herein we present a detailed investigation of the correlation between the Rth-JC and the point of separation of the Zth-curves. Based on these findings we suggest evaluation procedures for the determination of the Rth-JC of power packages. Complementary methods apply to solder die attach and glue die attach devices. The paper concludes with a discussion of the expected accuracy and reproducibility of the evaluation methods presented herein.