Stress Analysis and Thermal Characterization of a High Pin Count PQFP

A three-dimensional finite element model of a high pin count plastic-quad-flat-pack (PQFP) has been developed by using ANSYS™ finite element simulation code [1]. The model has been used for both thermo-mechanical stress analysis during temperature cycling and thermal characterization of the package under forced air cooling. Parametric studies have been performed on two different molding compounds with and without a drop-in heat spreader. In addition, the model has been simplified by substituting the leadframe and molding compound with a single homogeneous material, reflecting both molding compound and the leadframe thermo-physical properties. Results from the molding compound parametric studies indicate a lower package stress if the molding compound with low thermal expansion coefficient is used. Comparisons of principal and von Mises stresses show that the simplified model, overall, underpredicts the stresses. Although both the simplified and detailed models predict almost the same value for junction-to-case resistance (θ_JC), calculated values are significantly lower than the measured θ_JC. In contrast to θ_JC, the predicted junction-to-ambient resistances (θ_JA) are in good agreement with the measured data.