A transient three-dimensional analysis was performed for passive thermal control of a plastic quad flat package by incorporating an organic phase change material (PCM) under the printed wiring board. Governing conservation equations for mass, momentum, and energy were solved by an implicit finite volume numerical technique. The effects of phase change were modeled by a single-domain enthalpy-porosity technique. To study the effects of thermal conductivity of the board, a total of six cases were considered with two different board materials. It was found that passive cooling with PCM can arrest the temperature rise for substantial time, for the power levels considered. A higher board thermal conductivity resulted in a reduction in temperature levels. The melt region for a lower thermal conductivity of the board was found to be localized near the package footprint, while for a higher board conductivity, the melt region extends along the board.