Modeling Study on the Impact of Mold Thickness on Strip Warpage of a Molded Leadframe Package

Abstract

Strip warpage is a common problem in molded leadframe packages. When warpage becomes excessive, the strip could not be processed as it would result in the strip being stuck or damaged during loading to the handling machine loader. This study focuses on the impact of mold cap thickness on strip warpage of a molded Quad Flat No Lead (QFN) package to provide guidance in reducing the strip warpage to an acceptable level. Different mold thickness values were considered in the modeling using finite element analysis (FEA). Results showed that there is an optimum mold thickness, which is around 1.0 mm for 0.20 mm leadframe thickness and 0.65 mm for 0.125 mm leadframe thickness, that gives the lowest warpage in the case considered. The optimum mold cap thickness is lower for thinner leadframe. At mold thickness lower than the optimum value, warpage is in “frowning” mode and increases as the package gets thinner and this agrees with actual observations. This study shows that mold cap thickness has significant impact on molded strip warpage and could be assessed using FEA. Therefore, controlling the mold cap thickness could now be considered an option to reduce strip warpage in molded semiconductor packages as supported by modeling.