Heat Spreader

Heat spreader is most often having high thermal conductivity. Functionally, it is a primary heat exchanger that moves heat between a heat source and a secondary heat exchanger. The secondary heat exchanger is always larger in cross sectional area, surface area and volume. By definition, the heat is "spread out", such that the secondary heat spreader has a larger cross sectional area contacting the heat spreader than the heat source. The heat flow is the same in both heat exchangers, but the heat flux density is less in the secondary, so it can be made of a less expensive material such as aluminum, and is a better match to an air heat exchanger, since the low heat transfer coefficient for air convection is adequate for a low heat flux.

A heat spreader is generally used if and only if the heat source tends to have a high heat-flux density, (high heat flow per unit area), and for whatever reason, heat can not be conducted away effectively by the secondary heat exchanger. For instance, this may be because it is air-cooled, giving it a lower heat transfer coefficient than if it were liquid-cooled. A high enough heat exchanger transfer coefficient is often sufficient to avoid the need for a heat spreader.