Heat conduction analysis is performed on a model system to survey the effects of geometric and thermal parameters on the temperature of an embedded heat source. A box model has the geometric characteristics of hand-held devices and has a laminar organization composed of a printed circuit board (PCB), air gap, and a system casing. Exploiting the laminar organization and thinness of the laminate an approximate solution is derived. The solution is used to produce a guide for thermal design analysts, which is concerned with the sensitivity of the heat source temperature to the effective thermal conductivity of the PCB. Numerical examples are shown for the models that have typical dimensions and material properties of actual equipment and PCB.