Thermal grease
From Wikipedia, the free encyclopedia
Thermal grease (also called thermal 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.
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[edit] Basic types
Today, thermal grease is produced by many companies. There are many types of thermal grease:
- Silicone-based thermal grease has moderate thermal conductivity, is usually white, and is often shipped with CPU heat sinks.
- Ceramic-based thermal grease contains ceramic particles suspended in other thermally conductive ingredients. Ceramic thermal grease conducts heat better than most silicone greases, but poorer than metal greases.
- Metal-based thermal grease contains metal particles (usually silver) and other conductive ingredients. Metal thermal grease has a better thermal conductivity (and is more expensive) than silicone-based grease. It is also more electrically conductive, however, which can cause problems if it contacts the pins of an IC.
[edit] Purpose
Thermal grease's primary purpose in electronics is to mediate small surface imperfections between an integrated circuit (also known as a chip) and a heat sink. When applied in appropriate quantities, it fills the tiny pits and grooves — particularly on today's CPUs — thereby increasing the amount of surface-to-surface contact with the CPU. Without thermal grease aiding the heat sink, CPU power dissipation overheating can occur and will generate logic errors as the heat raises electrical resistance on the multi-nanometer wide circuits.[1]
[edit] Properties
The thermal grease's most important parameter is the thermal conductivity, measured in watts per metre-kelvin (W/(m·K)). The typical silicon and zinc oxide thermal compound has thermal conductivity between 0.7 and 0.9 W/(m·K) (in comparison, copper is 401 W/(m·K) and aluminum is 237 W/(m·K)). The silver thermal compounds may have the conductivity of around 2 to 3 W/(m·K) or even more. The compound must also be smooth; easy enough to apply in a very thin layer.
In the suspended particle containing compounds, the properties of the fluid phase (component) may well be the most important. As seen by the thermal conductivity measures above, the conductivity is more close to the conductivity of their fluid components, as opposed to their ceramic or metal components. Their main job seems to be eliminating the air gaps between the surfaces as air is a terrible conductor of heat. Other important properties of the fluid components that play a major role might be:
- How well it fills the gaps and conforms to the component's uneven surfaces and the heat sink.
- How well it adheres to those surfaces.
- What consistency does it maintain or change over the required temperature range.
- Whether it resists drying out or flaking over time.
- How well it insulates electrically.
- Whether it degrades with oxidation or breaks down over time.
- And most important, how well it conducts heat.
[edit] Applying and removing
The grease is applied on both surfaces with a small plate or credit card. Even silver compounds do not conduct electricity well, so several minor drops on the chip pins usually do not damage the device.
Because heat sink compound's thermal conductivity is so poor in comparison to the metals they couple, it is important to use no more than is necessary to exclude any air gaps. Excess grease separating the metal surfaces further will only degrade conductivity.
The preferred way to remove typical thermal grease from a component or heat sink is by using isopropyl alcohol.
[edit] See also
[edit] References
[edit] External links
- The Heatsink Guide: Thermal Compound by Tillmann Steinbrecher
- Thermal Transfer Compound Comparison by Daniel Rutter
