Prediction of gap in QFP with unattached heat spreader

Due to cost and process considerations, sometimes the heat spreader of a quad flat package (QFP) is not attached to the die pad. Consequently, due to mismatch of thermal expansions of the components, gapping is inevitable on the unattached interface during temperature change. Presence of the gap would potentially decrease the reliability and thermal performance of QFP. In this paper, contact methodologies were incorporated into finite element models to calculate the distribution of gap on the unattached interface. Both plane two-dimensional and general three-dimensional models were built up and compared for a multi-chip module QFP with a drop-in heat spreader.