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Traditional thermal conductivity measurement such as laser-flash method for thermal-conductivity measurement is limited to homogenous materials or limited geometry, therefore, such method is not suitable for characterizing real semiconductor packages which have complex heat flow structure. In this paper, we are going to introduce structure function based evaluation of the thermal transient measurements, which is most appropriate for the characterization of heat flow path in complex semiconductor packages. The structure functions are “graphical” representations of the RC-model of thermal systems. With the help of structure function one can determine partial thermal resistances and partial thermal capacitances in a heat flow path not only inside the device package like die attach, but also outside electronic components like PCB board, surface-air boundary layer, contact thermal resistance etc., with high accuracy.