Design Study to Prevent Mold Delamination for Overmolded Lead Frame Package

Abstract

Mold delamination is one of the critical concerns in plastic encapsulated lead frame package. During excursion to temperature and moisture, interfacial delamination between lead frame and overmold can cause mold cracking or wire bond interconnection failure. This becomes critical in case of Lead Frame Chip Scale Packages (LFCSP) with increasing volume of silicon die in a given package size. It generates increased level of deformation and stress due to low CTE of die compared to other packaging materials such as copper lead frame, mold compound, and die attach. In this paper, mold compound and lead frame interfacial stress of LFCSPs, with varying level of silicon volume, lead frame paddle size and die attach fillet size were evaluated by thermo-mechanical finite element analysis. It shows that packages consisting of high die volume, lower die attach fillet length (less die attach amount), and smallest distance from die edge to paddle edge exhibit higher equivalent stress at paddle to mold interface. To validate the simulation result, Temp Cycle −65/150°C preconditioned with Moisture Sensitivity Level1 (MSL1) has been tested. The experimental study shows that optimal package design can significantly reduce interfacial stress and prevent delamination at mold compound lead frame interfaces. However, a mold compound crack was found on the bottom side of several parts after the temperature cycle was completed. Additional reliability test has been conducted and the parts did not show any electrical failure.