Characterizing junction-to-case thermal resistance and its impact on end-use applications

High power packages require a low junction-to-case thermal resistance (Theta jc) to achieve target junction temperatures. Theta jc is a metric used to compare package-to-package relative thermal performance. It is also used in two-resistor models to predict thermal performance under system level conditions. This study shows that the commonly used Theta jc definition does not correctly track package junction temperatures in actual end-use applications nor does it serve as a reliable metric for comparing package-to-package performance. When spreading resistance in the lid is taken into account, the modified Theta jc definition overcomes some of the shortfalls found with the standard Theta jc calculation. Lastly, a simplified thermal interface resistance model is presented as a more accurate alternative to the one resistor Theta jc and the multi-resistor Delphi models.