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The QFN (quad flat non-leaded) package is a new type of chip scale package. In this study, both the FEM solid model and CFD model of QFN package were generated to predict the junction-to-ambient thermal resistance of the package under natural and forced convection. The solid model uses flat plate surface convection correlations to simulate the package and board cooling. The CFD model uses computational fluid dynamics software to simulate surface convective loads. In comparison with experimental measurements, the results show that the CFD model is more accurate than the solid model to predict the thermal resistance of the package. The convection heat transfer coefficients obtained from CFD model were also applied to the surface correlations of FEM solid model. It shows that the accuracy of simulation results by FEM solid model can be improved with the surface convection correlations of CFD results. Furthermore, the distribution of heat transfer over external faces of the package is calculated. The results show that most of the heat is dissipated to the board through the bottom surface of the lead frame.