A heat sink’s ability to dissipate thermal energy is determined by its thermal impedance, which is measured in degrees C per watt ( ◦ C/W). This thermal impedance rating is based upon natural “convection air flow” over the heat sink’s surface in a still air environment. Generally, the greater the exposed surface area of the heat sink, the lower its thermal impedance and the greater its ability to dissipate thermal energy. However, more surface area often means increased dimensional size as well as cost, even for finned extrusion profiles. A Heat Sink’s thermal rating can also be effectively improved by increasing air flow over its exposed surface area. Closed extrusion profiles or so called “chimney” designs can arguably improve convection air flow verses “open” profiles by virtue of increased convection air velocity through the chimney. However, the most effective means of increasing air flow is through the use of forced air. This paper discusses the effects of forced air flow provided by fans on the thermal impedance ratings of heat sinks.