The coupled conduction and forced convection transport from substrate-mounted modules in a channel is numerically investigated to identify the effects of the substrate conductivity. The results presented apply to air with Pr = 0.7 and two-dimensional laminar flow conditions. It was found that recirculating cells as well as streamwise conduction through the substrate play an important role in predicting convective heat transfer from the printed circuit board (PCB) and modules and in determining the temperature distributions in the PCB, modules, and fluid. The dimensionless temperature and the local Nusselt number along the interface between the fluid and the module or PCB are rather complicated, and therefore, predetermined simple boundary conditions along the solid surface may be inappropriate in many conjugate heat transfer problems. Overall, the results show that the maximum temperature within the heat sources is greatly reduced by increasing the conductivity of the PCB. The results shed some light on the development and practical use of high conductivity substrates for enhanced thermal management.