Due to cost and process considerations, sometimes a drop-in heat spreader is incorporated in a quad flat package, which is not attached to the die pad. Consequently, due to mismatch of thermal expansions of the components, gapping is inevitable on the unattached interface between drop-in heat spreader and die pad during temperature change. The presence of the gap would potentially decrease the reliability and thermal performance of the package. In this paper, gap distributions on the unattached interface and thermal characteristics of a quad flat package implemented with an unattached drop-in heat spreader are examined through the thermal-mechanical coupling analysis. A numerical procedure is proposed to specify film coefficients as functions of gap openings on separated surfaces. Effects of mesh density and interfacial heat transfer conditions on the numerical solutions are then investigated.