This paper describes a method for calculating the temperature of a semi-infinite heat sink plate of a given thickness, subjected to transient heating by a D2Pak power IC. Accurate prediction of the heat sink temperature over time then allows for more accurate calculation of the IC junction temperature. A set of curves have been developed for the time variation of heat sink plate temperature. This has been achieved by the use of finite element methods, and modeling a large range of configurations. The system variables were put into dimensionless form, and the model results plotted. The resulting plot indicates an effective thermal resistance of a given heat sink plate at a given point in time. A curve fit has also performed on the results. The results of the finite element model have been compared with laboratory data.