Determination of the Weighted-Average Case Temperature for a Single Chip Package

The RJC thermal characterization technique is commonly used for determining the maximum temperature a given chip will experience when operating in a specified environment. As such, RJC provides a simple, inherent figure-of-merit that is also readily adaptable to the numerical analysis of virtually any packaging design or type. Unfortunately, RJC is strictly valid only for an isothermal package surface and, when significant temperature variations are encountered, the use of the reported value of RJC can lead to substantial errors in chip temperature. The use of the junction-to-case thermal resistance can be extended to non-isothermal packages by defining an appropriately weighted, average surface temperature based on numerically derived “Thermal Influence” coefficients for each package surface (or segment) of interest. The present study applies the expanded RJC methodology to an actual PLCC package, and demonstrates its use.