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 thermally conductive (but usually electrically insulating)

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

, which is commonly used as an interface between

heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...

s and heat sources such as high-power

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

devices. The main role of thermal paste is to eliminate air gaps or spaces (which act as

thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with s ...

) from the interface area in order to maximize

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

and dissipation. Thermal paste is an example of a

thermal interface material A thermal interface material (shortened to TIM) is any material that is inserted between two components in order to enhance the thermal coupling between them. A common use is heat dissipation, in which the TIM is inserted between a heat-producing de ...

. As opposed to

thermal adhesive Thermal adhesive is a type of thermally conductive glue used for electronic components and heat sinks. It can be available as a paste (similar to thermal paste) or as a double-sided tape. It is commonly used to bond integrated circuits to heats ...

, thermal paste does not add mechanical strength to the bond between heat source and heat sink. It has to be coupled with a mechanical fixation mechanism such as screws to hold the heat sink in place and to apply pressure, spreading and thinning the thermal paste.

Composition

Thermal paste consists of a

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

izable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials are

epoxies The Epoxies were an American new wave band from Portland, Oregon, formed in 2000. Heavily influenced by new wave, the band jokingly described themselves as robot garage rock. Members included FM Static on synthesizers, guitarist Viz Spectrum, ...

silicone A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...

s (

silicone grease Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this ...

), urethanes, and

acrylates Acrylates (IUPAC: prop-2-enoates) are the salts, esters, and conjugate bases of acrylic acid. The acrylate ion is the anion C H2=CHC OO−. Often, acrylate refers to esters of acrylic acid, the most common member being methyl acrylate. These acry ...

; solvent-based systems, hot-melt adhesives, and pressure-sensitive adhesive tapes are also available. Aluminum oxide,

boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...

zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...

, and increasingly

aluminum nitride Aluminium nitride ( Al N) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potent ...

are used as fillers for these types of adhesives. The filler loading can be as high as 70–80% by mass, and raises the

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of the base matrix from 0.17–0.3 W/(m·K) (watts per meter-kelvin) up to about 4 W/(m·K), according to a 2008 paper. Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more, and consist of

micronized Micronization is the process of reducing the average diameter of a solid material's particles. Traditional techniques for micronization focus on mechanical means, such as milling and grinding. Modern techniques make use of the properties of superc ...

silver particles suspended in a silicone/ceramic medium. However, metal-based thermal paste can be electrically conductive and capacitive; if some flows onto the circuits, it can lead to malfunction and damage. The most effective (and most expensive) pastes consist almost entirely of

liquid metal A liquid metal is a metal or a metal alloy which is liquid at or near room temperature. The only stable liquid elemental metal at room temperature is Mercury (element), mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9&nbs ...

, usually a variation of the alloy

galinstan Galinstan (R) is a brand name for a alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. However, it is not a eutectic alloy but a near eutectic alloy. In scientific literature, galinstan is also used ...

, and have thermal conductivities in excess of 13 W/(m·K). These are difficult to apply evenly and have the greatest risk of causing malfunction due to spillage. These pastes contain

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...

, which is highly corrosive to

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

and cannot be used on aluminium heat sinks. The lifespan of thermal paste varies depending on the manufacturer and typically ranges from 3 to 5 years.

Uses

Thermal paste is used to improve the heat coupling between different components. A common application is to drain away waste heat generated by electrical resistance in semiconductor devices including power

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

s, CPUs,

GPU A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobi ...

s, and LED COBs. Cooling these devices is essential because excess heat rapidly degrades their performance and can cause a runaway to

catastrophic failure A catastrophic failure is a sudden and total failure from which recovery is impossible. Catastrophic failures often lead to cascading systems failure. The term is most commonly used for structural failures, but has often been extended to many oth ...

of the device due to the negative temperature coefficient property of semiconductors. Factory PCs and laptops (though seldom tablets or smartphones) typically incorporate thermal paste between the top of the CPU case and a heat sink for

cooling Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...

. Thermal paste is sometimes also used between the CPU

die Die, as a verb, refers to death, the cessation of life. Die may also refer to: Games * Die, singular of dice, small throwable objects used for producing random numbers Manufacturing * Die (integrated circuit), a rectangular piece of a semicondu ...

and its

integrated heat spreader A heat spreader transfers energy as heat from a hotter source to a colder heat sink or heat exchanger. There are two thermodynamic types, passive and active. The most common sort of passive heat spreader is a plate or block of material having hi ...

, though

solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...

is sometimes used instead. When a CPU heat spreader is coupled to the die via thermal paste, performance enthusiasts such as

overclockers In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly, operating voltage is also increased to maintain a component's operational stability at accelerated spe ...

are able to, in a process known as "delidding", pry the heat spreader, or CPU "lid", from the die. This allows them to replace the thermal paste, which is usually of low-quality, with a thermal paste having greater thermal conductivity. Generally, liquid metal thermal pastes are used in such instances.

Challenges

The consistency of thermal paste makes it susceptible to failure mechanisms distinct from some other thermal interface materials. A common one is pump-out, which is the loss of thermal paste from between the die and the heat sink due to their differing rates of thermal expansion and contraction. Over a large number of power cycles, thermal paste gets pumped out from between the die and the heat sink and eventually causes degradation of thermal performance. Another issue with some compounds is the separation of the polymer and filler matrix components occurs under high temperatures. The loss of polymeric material can result in poor

wettability Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with ...

, leading to increased thermal resistance.

References

External links

* {{DEFAULTSORT:Thermal Grease Adhesives Cooling technology Computer hardware cooling

Conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured f ...

Composition

Uses

Challenges

See also

References

External links