Thermal paste

Silicone thermal compound

Metal (silver) thermal compound

Metal thermal grease applied to a chip

Surface imperfections

Thermal grease (also called thermal compound, heat paste, heat transfer compound, 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:

• 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: 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). It has a better thermal conductivity (and is more expensive) than ceramic-based grease. It is also more electrically conductive which can cause problems if it contacts the electrical connections of an integrated circuit.

• Carbon based. There are some experiments being done with carbon-based greases, using diamond powder[2] [3] or short carbon fibers[4].

• 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. 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 switpoint 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.

Properties

The thermal grease's most important property is its thermal conductivity, measured in watts per metre-kelvin (W/(m·K)). The metal oxide and nitride particles suspended in silicone thermal compounds have thermal conductivities of up to 220 W/(m·K). (In comparison, the thermal conductivity of metals used particle additions, copper is 401 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 2 to 3 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.

Applying and removing

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

The process is generally the same, regardless of brand. Simply put a thin "grain of rice" in the center of the CPU down the middle. Some people prefer to put a small cross in the center, though most high quality compound manufacturers will recommend the "grain of rice" method.

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 (rubbing alcohol). If none is available, distilled spirits such as gin will do the job.

See also

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

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
• How to apply thermal grease to CPUs by Gabriel Torres,Daniel Barros and Cássio Lima