Accurate prediction of PQFP warpage

This paper addresses the use of finite element techniques to predict warpage in plastic encapsulated IC's. A basic modeling assumption adopted in such analyses is that after ejection from the mold, warpage occurs due to the contraction of the molding compound as the package cools to room temperature. It is shown that this basic starting assumption can lead to incorrect predictions of the package warpage. Measurements of the warpage of a plastic power package with temperature indicate that the package is significantly deformed at the molding temperature. This is attributed to chemical shrinkage of the molding compound. A new F.E. model of package warpage after encapsulation is proposed which considers both the TCE shrinkage and chemical shrinkage of the molding compound. This leads to accurate predictions of warpage, particularly where heat spreaders make the package asymmetric. Measurements of a 208 lead power PQFP are used to verify this improved model.