The thermal and hydrodynamic performance of plate-fin heat sinks is investigated numerically using the Lattice Boltzmann Method (LBM). The effect of the spacing between the fins (i.e. flow blockage) and the flow rate (i.e. Reynolds number) on both the thermal and hydrodynamic performance is studied for the low flow rate range. The Thermal Performance Factor TPF is used to weigh the gain in the thermal performance against the associated increase in the pressure drop which helps in selecting the optimum configuration for a specific application. It is found that the best fin configuration depends on the required flow rate range of the thermal management device.