Thermal paste
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
Although of the same function, thermal grease uses various compositions that can be broken down to the following broad classes:
- Zinc Oxide-based thermal grease has generally good thermal conductivity, is usually suspended in a silicone oil compound. If a product is described as a silicone paste or a generic silicone paste, usually, it is zinc oxide based. It is white in colour. A variant is a wax based adhesive that is factory applied to heat sinks. The oils used in many of these compounds may migrate out of the joint between surfaces and evaporate over extended time (5 to 10 years or more) causing loss of thermal conductivity over very long time periods [citation needed].
- Ceramic-based thermal grease contains ceramic particles. This type conducts heat better than the more common zinc oxide greases. [citation needed].
- 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.
- Beryllium-compound, sometimes found in very old (1960s or older) electronic equipment. Highly poisonous and carcinogenic hence discontinuation. It is a white greasy substance sometimes confused with zinc-oxide/silicone compound, a more modern and non-toxic alternative. [citation needed]
The first three classifications usually use liquid silicone as a medium, a heat conductor in itself, though some brands prefer use specific fractions of mineral oil to control their compound's behaviour over time at different heat levels. [citation needed]
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:
- 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
- How well it maintains its consistency over the required temperature range
- How well it resists drying out or flaking over time
- How well it insulates electrically
- 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
References
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External links
- The Heatsink Guide: Thermal Compound by Tillmann Steinbrecher
- Thermal Transfer Compound Comparison by Daniel Rutter
- Latest Thermal Compound Faceoff by Joe Anderson