Effective cooling of electronic components plays an important role in system design and efficiency. In this study, the effects of using the flow routing plate in cooling printed circuit boards have been investigated. For this purpose, effects of the flow routing plate on the laminar mixed convection heat transfer from protruded heat sources at the side walls of the horizontal channel, were investigated numerically. The air was used as cooling fluid, and protruded heat sources were equipped as rows into the rectangular channel with insulated walls. Numerical investigations were carried out for different plate inclination angles at different Reynolds and modified Grashof numbers. It is observed that the using of flow routing plate increases the heat transfer at different ratios by comparison to the case without plate and enhances the cooling conditions for all values of parameters in the analyses. The highest heat transfer enhancement occurred at values where Reynolds number (Re) was Re = 1000 and plate inclination angle (α) was α = 60 °.The results obtained during the numerical analyses are presented in detail in the form of graphics for the row averaged Nusselt number, heater temperatures, velocity vectors, and temperature contours.