Thermal grease

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Thermal grease (also called thermal compound, thermal paste, heat paste, heat sink paste, heat transfer compound, or heat sink compound) is a fluid substance, originally with properties akin to grease, which increases the thermal conductivity of a thermal interface by compensating for the irregular surfaces of the components. In electronics, it is often used to aid a component's thermal dissipation via a heat sink.

Contents 1 Conductor types 2 Purpose 3 Properties 4 Applying and removing 5 See also 6 References 7 External links

[edit] Conductor types

Thermal greases use one or more different thermally conductive substances:

• Ceramic-based thermal grease has generally good thermal conductivity and is usually composed of a ceramic powder suspended in a liquid or gelatinous silicone compound, which may be described as 'silicone paste' or 'silicone thermal compound'. The most commonly used ceramics and their thermal conductivities (in units of W/(m ·K)) are:^[1] beryllium oxide (218), aluminium nitride (170), aluminum oxide (39), zinc oxide (21), and silicon dioxide (1). Thermal grease is usually white in colour since these ceramics are all white in powder form.

• Metal-based thermal grease contain solid metal particles (usually silver or aluminum). It has a better thermal conductivity^{[citation needed]} (and is more expensive) than ceramic-based grease.

• Carbon based. There are products based on with carbon-based conductors, using diamond powder^[2][3][4], or short carbon fibers[2], they have the best thermal conductivity and are generally more expensive than metal-based thermal grease.

• 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.^{[citation needed]}

[edit] 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.^[5] 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.

As such, both the thermal conductivity and the "conformability" (i.e., the ability of the material to conform to irregular surfaces) are the important characteristics of thermal grease.

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.^[6]

[edit] Properties

The metal oxide and nitride particles suspended in silicone thermal compounds have thermal conductivities of up to 220 W/(m·K).^[1] (In comparison, the thermal conductivity of metals used particle additions, copper is 380 W/(m·K), silver 429 and aluminum 237.) The typical thermal conductivities of the silicone compounds are 0.7 to 3 W/(m·K). Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more.

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

The compound must also be smooth so that it is easy to apply in a very thin layer.

[edit] Applying and removing

Computer processor heatsinks utilize a variety of designs to promote better thermal transfer between components. Flat and smooth surfaces may use a small line method to apply material, and exposed heat-pipe surfaces will be best prepared with multiple lines.^[7]

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, increasing the chances of overheating. It should also be noted that silver-based thermal grease can also be slightly electrically conductive. If excess were to flow onto the circuits, it could cause a short circuit.

Over time, some thermal greases may set like glue and make it difficult to remove the heat sink. If too much preasure is applied the processor may be damaged. Heating the grease by turning the processor on for a short period often softens the glue.

The preferred way to remove typical silicone oil-based thermal grease from a component or heat sink is by using isopropyl alcohol (rubbing alcohol). If none is available, pure acetone is also a valid method of removal. There are also purpose made cleaners for removing and purifying the surfaces of the contacts.

[edit] See also

• Computer cooling
• Arctic Silver
• Phase Change Material
• Thermal pads
• Thermal adhesive
• List of thermal conductivities

[edit] References

[edit] External links

• 80-way Thermal Interface Material Performance Test by Olin Coles
• AI Technology Cool Silver Thermal Interface Material by Shane Higgins
• Latest Thermal Compound Faceoff by Joe Anderson
• How to apply thermal grease to CPUs by Gabriel Torres, Daniel Barros and Cássio Lima