A heat spreader is a device that aids in the dissipation of heat from a thermal source into a heat exchange medium. This is a rather complex way of saying a heat exchanger helps keep appliances or equipment cool, or hot in some cases, by getting heat from where it is generated to where it needs to be. This is usually required where the heat exchange medium, or the place the heat needs to go, is incapable of absorbing the necessary amount of heat on its own. Good examples of this theory include the copper bases on stainless steel cookware or the finned heat sinks on high current electronic components. The copper base helps the stainless pot absorb and retain heat, and the heat sink helps the microprocessor shed heat.
Heat transfer, or rather adequate heat transfer, is not always as simple a concept as it may seem. Getting enough heat to or away from one medium or material into another depends on a number of technical factors which can, at times, confound the issue completely. The main problem in this regard is the difference in the heat flux density of different materials. Simply put, this means that some materials require a far larger area of exposure than others to absorb the same amount of heat. The heat sinks commonly mounted on electronic components or the fins on an oil heater or radiator are examples of how heat spreader theory works to get around this problem.
For example, the surface area of a high gain transistor generates a lot more heat than the air in contact with it can absorb over any given period of time. To get around this phenomenon, a heat spreader or heat sink is attached to the transistor. This is typically a heavy copper or aluminum base with a large number of fins projecting from its surface. This achieves a large increase in volume of air exposed to the heat source which negates the heat flux density differential between the transistor and the air. In this way, the head spreader becomes the primary heat exchanger mechanism which aids the secondary exchanger, the air, to effectively absorb the generated thermal energy.
Obviously heat spreader use is limited to applications where the secondary exchange medium is unable to overcome the heat flux density differences between it and the heat source material. Heat spreader materials should be good thermal conductors, and the surface profile needs to be fairly carefully calculated to offer the maximum exposure and circulation. The union between the heat source and the spreader also needs to be as efficient a thermal passage as possible. To this end, heat transfer pastes are often applied to the surfaces before the heat spreader is attached.