On the Reproducibility of Thermal Measurements and of Related Thermal Metrics in Static and Transient Tests of Power Devices

Traditionally the thermal behavior of power devices is characterized by temperature measurements at the junction and at accessible external points. In large modules composed of thin chips and materials of high thermal conductivity the shape and distribution of the heat trajectories are influenced by the external boundary represented by the cooling mount. This causes mediocre repeatability of the characteristic R thJC junction to case thermal resistance even in measurements at the same laboratory and causes very poor reproducibility among sites using dissimilar instrumentation. The Transient Dual Interface Methodology (TDIM) is based on the comparison of measured structure functions. With this method high repeatability can be achieved although introducing severe changes into the measurement environment is the essence of this test scheme. There is a systematic difference between thermal data measured with TDIM method and that measured with temperature probes, but we found that this difference was smaller than the scatter of the latter method. For checking production stability, we propose the use of a structure function-based R th@Cth thermal metric, which is the thermal resistance value reached at the thermal capacitance belonging to the mass of the package base. This metric condenses the consistency of internal structural elements into a single number.