Information about Thermal Grease

Thermal grease (also called thermal compound, heat paste, thermal paste, or heat sink compound) is a substance that increases thermal conductivity between the surfaces of two or more objects. In electronics, it is often used to aid a component's thermal dissipation via a heat sink.

Basic types

Thermal greases use one or more different thermally conductive substances:

• Zinc oxide based thermal grease has generally good thermal conductivity and is usually suspended in a liquid or gelatinous silicone compound (hence the term 'grease'). If a product is described as a 'silicone paste' or a generic silicone thermal compound, usually it is zinc oxide based. It is white in colour.
• Ceramic based thermal grease contains ceramic particles. This type conducts heat better than the more common zinc oxide greases. .
• Metal based thermal grease contain solid metal particles (usually silver). It has a better thermal conductivity (and is more expensive) than zinc oxide based grease. It is also more electrically conductive which can cause problems if it contacts the electrical connections of an integrated circuit.
• Liquid metal based. Some thermal pastes are made of liquid metal alloys of gallium. Rare and expensive.

All but the last classification of compound usually use silicone grease as a medium, a heat conductor in itself, though some manufacturers prefer use of fractions of mineral oil.

Purpose

Thermal grease is primarily used in the electronics and computer industries to assist a heatsink to draw heat away from a semiconductor component such as an integrated circuit or transistor .

Thermally conductive paste improves the efficiency of a heatsink by filling air gaps that occur when the irregular surface of a heat generating component is pressed against the irregular surface of a heatsink, air being approximately 8000 times less efficient at conducting heat (see Thermal Conductivity) than, for example, aluminium, a common heatsink material.^[1] Surface imperfections inherently arise from limitations in manufacturing technology and range in size from visible and tactile flaws such as machining marks or casting irregularities to sub-microscopic ones not visible to the naked eye.

Both high power handling transistors, like those in a conventional audio amplifier, and high speed integrated circuits, such as the central processing unit (CPU) of a personal computer, generate sufficient heat to require the use of thermal grease in addition to the heatsink. High temperatures cause semiconductors to change their switching properties to the point of failure while CPU power dissipation overheating causes logic errors as heat raises electrical resistance on the multi-nanometer wide circuits of the CPU core.^[2]

Properties

The thermal grease's most important property is its thermal conductivity, measured in watts per metre-kelvin (W/(m·K)). Typical thermal conductivity for silicone and zinc oxide thermal compounds is 0.7 to 0.9 W/(m·K). (In comparison, the thermal conductivity of copper is 401 W/(m·K) and of aluminum 237.) Silver thermal compounds may have the conductivity of 2 to 3 W/(m·K) or more. The compound must also be smooth so that it is easy to apply in a very thin layer.

In compounds containing suspended particles, the properties of the fluid may well be the most important. As seen by the thermal conductivity measures above, the conductivity is closer to that of the fluid components rather than the ceramic or metal components. Other properties of fluid components that are important for thermal grease might be:

1. How well it fills the gaps and conforms to the component's uneven surfaces and the heat sink
2. How well it adheres to those surfaces
3. How well it maintains its consistency over the required temperature range
4. How well it resists drying out or flaking over time
5. How well it insulates electrically
6. Whether it degrades with oxidation or breaks down over time

Applying and removing

For computer CPU applications the grease is often applied on both surfaces with a small plastic spatula or credit card.

Because thermal grease's thermal conductivity is poorer than the metals they couple, it is important to use no more than is necessary to exclude air gaps. Excess grease separating the metal surfaces further will only degrade conductivity.

The preferred way to remove typical silicone oil based thermal grease from a component or heat sink is by using isopropyl alcohol.

See also

• Computer cooling
• Arctic Silver
• Phase Change Material
• Thermal pads
• Thermal adhesive

References

<http://www.tigernet.com/forums/message.jspa?messageID=4900133#4900133/>

External links

• AI Technology Cool Silver Thermal Interface Material by Shane Higgins
• The Heatsink Guide: Thermal Compound by Tillmann Steinbrecher
• Thermal Transfer Compound Comparison by Daniel Rutter
• Latest Thermal Compound Faceoff by Joe Anderson

References