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Estimating junction temperature (Tj) of the die encapsulated in a plastic package becomes critical especially in early stage of electronic product development. Confirming the surface temperature (Tc) of LSI packages by measurement is allowed for users while measuring Tj directly is quite difficult. Estimating Tj from Tc using thetasjc is commonly used even for the cases where quite big error is generated. Based on the nature where temperature deference between Tj and Tc is depending on the heat flux flowing through the molding compound, a new approach for estimating accurate deltaT (Tj-Tc) is developed and described in this paper. In addition to the accuracy, this method simplifies the Tj estimation for both LSI users and suppliers.