Deformation analysis of QFN packages for validation of thermo-mechanical finite element simulations

Abstract

Electronic package deformation is a possible cause for reliability issues. The deformations and warpage are caused by the in-plane coefficient of thermal expansion (CTE) mismatch between epoxy molding compound (EMC) and other materials inside package. The objective of this study is to investigate the warpage in Quad flat nolead (QFN) packages and establish a correlation between projection moiré experiments and finite element simulations. FE simulations demonstrates the importance of temperature dependent material models for the warpage simulations and shows how it affect the warpage results and shape of the warping package. With the viscoelastic material model for an EMC and temperature dependent CTE, simulations illustrate the true shape of the QFN package warpage, but simulations overestimate the absolute warpage values specifically at peak temperatures. It has been identified that the dominant factor behind package warpage is the CTE difference between EMC, copper lead frame and silicon die. This paper also compares CTE measurements from experiments with FE simulations and FEA predictions are within tolerable limits of ± 1 ppm$/ ^{\circ}\mathrm{C}$ which validates the FE model.