Thermal paste
Thermally conductive compound used to improve heat transfer between components.
Thermal paste—also called thermal compound, thermal grease, thermal interface material (TIM), thermal gel, heat paste, heat sink compound, heat sink paste, or CPU grease—is a chemical compound that conducts heat well but usually does not conduct electricity. It sits between a heat source, like a high-power semiconductor device, and a heat sink. Its job is to fill in the tiny air gaps that would otherwise trap heat and act as insulation, improving heat transfer away from the device. It is one type of thermal interface material.
Unlike thermal adhesive, thermal paste does not help hold the heat sink in place. It provides no mechanical strength to the bond. Instead, fasteners such as screws are needed to press the heat sink down, which spreads and thins the paste.
**Composition**
Thermal paste starts with a liquid matrix that can be polymerized, combined with a filler that acts as an electrical insulator. The filler makes up 70 to 80 percent of the total mass and is what actually conducts heat. Without the filler, the material’s thermal conductivity is around 0.17 to 0.3 W/(m·K); with it, conductivity can rise to about 4 W/(m·K). The liquid matrix can be an acrylate, epoxy, hot-melt adhesive, pressure-sensitive adhesive tape, silicone (silicone grease), a solvent-based system, or urethane. The filler can be made from aluminum nitride, aluminum oxide, boron nitride, diamond, or zinc oxide.
Silver-based thermal compounds can reach conductivities of 3 to 8 W/(m·K) or higher. They contain tiny silver particles suspended in a silicone or ceramic medium. But metal-based pastes can conduct electricity and also have capacitance; if any spills onto circuits, it can cause damage or malfunctions.
The most effective—and most expensive—pastes are almost entirely liquid metal, usually a variation of the alloy galinstan. Their thermal conductivity exceeds 13 W/(m·K). They are tricky to apply evenly and pose the highest risk of causing problems if spilled. Also, these pastes contain gallium, which corrodes aluminum badly, so they cannot be used with aluminum heat sinks.
**Uses**
Thermal paste improves heat coupling between components. A common use is drawing waste heat away from semiconductor devices such as power transistors, CPUs, GPUs, and LED COBs, where electrical resistance generates heat.
- Thermal conductivity range
- 0.17–0.3 W/(m·K) for unfilled matrix; up to about 4 W/(m·K) with filler
- Filler content
- 70 to 80% of total mass
- Silver compound conductivity
- 3 to 8 W/(m·K) or more
- Liquid metal conductivity
- in excess of 13 W/(m·K)
- Common fillers
- aluminum nitride, aluminum oxide, boron nitride, diamond, zinc oxide
- Liquid matrix types
- acrylate, epoxy, hot-melt adhesive, pressure-sensitive adhesive tapes, silicone (silicone grease), solvent-based systems, urethane
Lore & Background
Thermal paste is composed of a polymerizable liquid matrix combined with an electric insulator as filler. The filler, which takes up 70 to 80% of the total mass, conducts heat and may raise thermal conductivity from 0.17–0.3 W/(m·K) up to about 4 W/(m·K). The liquid matrix may be an acrylate, epoxy, hot-melt adhesive, pressure-sensitive adhesive tapes, silicone (silicone grease), solvent-based systems, or urethane. Fillers can be made from aluminum nitride, aluminum oxide, boron nitride, diamond, or zinc oxide. Silver thermal compounds consist of micronized silver particles suspended in a silicone/ceramic medium and may have a conductivity of 3 to 8 W/(m·K) or more; however, metal-based thermal paste can be electrically conductive and capacitive, and if it flows onto circuits it can lead to malfunction and damage. The most effective and most expensive pastes consist almost entirely of liquid metal, usually a variation of the alloy galinstan, with thermal conductivities in excess of 13 W/(m·K). These are difficult to apply evenly and carry the greatest risk of causing malfunction due to spillage; they contain gallium, which is highly corrosive to aluminium and cannot be used on aluminium heat sinks.
Reader's Guide
Thermal paste is used to improve heat coupling between different components, particularly to drain away waste heat generated by electrical resistance in semiconductor devices including power transistors, CPUs, GPUs, and LED COBs. Cooling these devices is essential because excess heat rapidly degrades their performance and can cause a runaway to catastrophic failure due to the negative temperature coefficient property of semiconductors. Factory PCs and laptops—although seldom tablets or smartphones—typically incorporate thermal paste between the top of the CPU case and a heat sink for cooling. Thermal paste is sometimes also used between the CPU die and its integrated heat spreader, though solder is sometimes used instead. Performance enthusiasts such as overclockers may perform a process known as 'delidding,' prying the heat spreader from the die to replace the usually low-quality thermal paste with one having greater thermal conductivity, often using liquid metal pastes. The consistency of thermal paste makes it susceptible to pump-out, the loss of paste from between the die and heat sink due to differing rates of thermal expansion and contraction over hot-cold cycles, which degrades thermal performance. Some compounds also suffer separation of polymer and filler matrix under high temperatures, leading to increased thermal resistance. Phase-change thermal pastes soften at intended working temperatures, allowing the paste to flow back and partially undo pump-out, improving lifespan in terms of resistance to hot-cold cycles; they require some time under heat to settle into place after applying.
Did You Know?
- Thermal paste typically consists of a polymerizable liquid matrix combined with an electric insulator as filler, which makes up 70 to 80% of the total mass.
- Liquid metal thermal pastes, usually based on the alloy galinstan, have thermal conductivities in excess of 13 W/(m·K) but are corrosive to aluminium.
- A common failure mechanism for thermal paste is pump-out, where paste extrudes from between the die and heat sink due to differing thermal expansion rates.
- Phase-change thermal pastes soften at working temperatures to flow back and partially reverse pump-out, improving resistance to hot-cold cycles.
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