Accurate Theta jc measurement for high power packages

Measuring the case temperature is one of the most challenging measurements for determining the junction-to-case thermal resistance (Theta jc) in high power packages. This is especially true for low Theta jc measurement, in which high power is necessary to control accuracy. Inaccurate case temperature measurement would lead to an inaccurate Theta jc value. This study explores different methods for measuring case temperature and quantifies their impact on Theta jc. A new method of cold-plate protruded thermocouple is proposed and compared with commonly adopted method of lid embedded thermistor both experimentally and numerically. It is found correction is not negligible for low Theta jc measurement in both methods due to the temperature difference between the case surface and the thermal probe location. A standard test jig is also proposed to determine the correction for the cold-plate protruded thermocouple experimentally.