Thermal characterization of semiconductor devices is an ever increasing challenge for device producers. Heat flow modeling and ways to estimate thermal behaviour, especially in term of junction temperature for semiconductor devices, are key factors. The thermal resistance junction to case is one of the most important characteristics. However, the evaluation of this parameter is not trivial at all: the accuracy and the repeatability of its measurements are key issues. Moreover, the three-dimensional heat spreading, typical of a considerable slice of devices, in addition to the description of thermal properties by the mean of a unique element with the limit of the definition itself, make the determination of this parameter influenced by the operating conditions and by the measurement set-up.