Abstract Steady-state convective heat-transfer and pressure-drop characteristics for laminar air-flows over a horizontally-orientated simulated printed-circuit board (PCB) assembly have been measured experimentally and predicted numerically. The considered assembly consisted of a plate with uniformly-spaced identical electrically-heated rectangular uniform ribs mounted orthogonal to the mean air-flow. Mathematical correlations were determined between the cavity-height to ribs' protrusion and width-to-protrusion ratios, namely ( H B ) and ( L B ), respectively, and the Reynolds number (Rec) of the air-flow with the steady-state Nusselt number (Nuc) and friction factor (fc), both of these latter parameters being highly dependent on H B . When H B ≥8 , natural convection provided a significant portion of the total rate of heat transfer, and mixed (i.e. forced plus natural) convection ensued. For assemblies with ribs each of the same volume, the assembly with the ribs having a larger top surface-area has the higher heat-transfer coefficient and smaller pressure drop under otherwise identical conditions.