Heat spreader
A device that transfers heat from a source to a sink.
A heat spreader moves thermal energy from a hotter object to a cooler heat sink or heat exchanger. It comes in two thermodynamic varieties: passive and active. A typical passive heat spreader is a plate or block made of a highly thermally conductive material like copper, aluminum, or diamond. An active heat spreader accelerates heat transfer by using energy supplied from an external source.
The mechanism can involve a sealed heat pipe containing fluid that circulates either passively through spontaneous convection (triggered by a sufficient temperature difference) or actively via an impeller powered externally. Without a sealed loop, energy can also be carried by moving fluid matter, such as externally supplied cold air driven by an external work source, though this is not strictly heat transfer as defined in physics.
A passive heat spreader disperses heat, increasing entropy per the second law of thermodynamics, so that heat exchangers can be used more fully. This can raise the total assembly's heat capacity, but added thermal junctions limit that capacity. The spreader's high conductivity makes it more effective as an air heat exchanger than the original, smaller source. The spreader's larger surface area compensates for air's low convective heat conduction, improving heat transfer.
Heat spreaders are used when a heat source has high heat-flux density (high heat flow per unit area) and the heat exchanger cannot conduct heat away effectively—for example, with air cooling, which has a lower heat transfer coefficient than liquid cooling. A sufficiently high heat exchanger transfer coefficient can eliminate the need for a spreader. Using a heat spreader is key to an economically optimal design for moving heat from high to low heat flux media. Examples include a copper-clad bottom on steel or stainless steel cookware, air-cooling microprocessors, and air-cooling photovoltaic cells in concentrated photovoltaics. Diamond, with its very high thermal conductivity, is used as synthetic submounts for high-power integrated circuits and laser diodes. Composite materials like copper-tungsten, AlSiC (silicon carbide in aluminum), Dymalloy (diamond in copper-silver alloy), and E-Material (beryllium oxide in beryllium) are used as chip substrates because their thermal expansion can match ceramics and semiconductors.
- Types
- passive and active
- Common passive materials
- copper, aluminum, diamond
- Example composite materials
- copper–tungsten, AlSiC, Dymalloy, E-Material
Lore & Background
A heat spreader is generally used when the heat source tends to have a high heat-flux density, and heat cannot be conducted away effectively by the 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 sufficient to avoid the need for a heat spreader. The use of a heat spreader is an important part of an economically optimal design for transferring heat from high to low heat flux media.
Examples include a copper-clad bottom on a steel or stainless steel stove-top cooking container, air-cooling integrated circuits such as a microprocessor, and air-cooling a photovoltaic cell in a concentrated photovoltaics system. Diamond has a very high thermal conductivity, and synthetic diamond is used as submounts for high-power integrated circuits and laser diodes. Composite materials such as copper–tungsten, AlSiC, Dymalloy, and E-Material are often used as substrates for chips, as their thermal expansion coefficient can be matched to ceramics and semiconductors.
A heat pipe uses fluids inside a sealed case. The fluids circulate either passively, by spontaneous convection, triggered when a threshold temperature difference occurs; or actively, because of an impeller driven by an external source of work. Without sealed circulation, energy can be carried by transfer of fluid matter, for example externally supplied colder air, driven by an external source of work, from a hotter body to another external body, though this is not exactly heat transfer as defined in physics.
Reader's Guide
Exemplifying increase of entropy according to the second law of thermodynamics, a passive heat spreader disperses or 'spreads out' heat, so that the heat exchanger(s) may be more fully utilized. This has the potential to increase the heat capacity of the total assembly, but the additional thermal junctions limit total thermal capacity. The high conduction properties of the spreader will make it more effective to function as an air heat exchanger, as opposed to the original (presumably smaller) source. The low heat conduction of air in convection is matched by the higher surface area of the spreader, and heat is transferred more effectively.
In May 2022, researchers at the University of Illinois at Urbana-Champaign and University of California, Berkeley devised a new solution that could cool modern electronics more efficiently than other existing strategies. Their proposed method is based on the use of heat spreaders consisting of an electrical insulating layer of poly (2-chloro-p-xylylene) (Parylene C) and a coating of copper. This solution would also require less expensive materials.
Did You Know?
- A 120 mm-diameter vapor chamber heat sink design thermal animation was created using high resolution computational fluid dynamics (CFD) analysis.
- Composite materials such as AlSiC (silicon carbide in aluminium matrix) and Dymalloy (diamond in copper-silver alloy matrix) are used as substrates for chips.
- In May 2022, researchers proposed a heat spreader using Parylene C and copper for more efficient cooling of electronics.
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