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Thermal grease |
Thermal grease (also called thermal compound, heat paste, heat transfer compound, thermal paste, 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.
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Thermal greases use one or more different thermally conductive substances:
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
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.2 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 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.3
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 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:
The compound must also be smooth so that it is easy to apply in a very thin layer.
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, increasing the chances of overheating. It should also be noted that silver-based thermal grease can also be slightly electrically conductive. If excess were flow onto the circuits, it could cause a short circuit.
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.
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