Editing Thermal paste

{{Short description|Fluid used to maximize thermal contact}}
[[File:Thermal greases.JPG|thumb|Several containers of thermal paste of different brands. From left to right: [[Arctic Cooling]] MX-2 and MX-3, Tuniq TX-3, Cool Laboratory Liquid Metal Pro, Shin-Etsu MicroSi G751, [[Arctic Silver#Arctic Silver 5 (AS5)|Arctic Silver 5]], Powdered Diamond. In background: [[Arctic Silver#ArctiClean|Arctic Silver thermal paste remover]].]]
[[File:Wärmeleitpaste Thermal Compound.jpg|thumb|Silicone thermal compound]]
[[File:Thermal grease.jpg|thumb|Metal (silver) thermal compound]]
[[File:Thermalgrease.jpg|thumb|Metal thermal paste applied to a chip]]
[[File:Cpuimperfections.jpg|thumb|Thermal paste is designed to fill imperfections on the surface of a chip.]]

'''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 [[Thermal conductivity|thermally conductive]] (but usually not [[Insulator (electricity)|electrically conductive]]) [[chemical compound]], which is commonly used as an interface between [[heat sink]]s and [[Heat generation in integrated circuits|heat source]]s such as high-power [[semiconductor]] devices. The main role of thermal paste is to eliminate air gaps or spaces (which act as [[thermal insulation]]) from the interface area in order to maximize [[heat transfer]] and dissipation. Thermal paste is an example of a [[thermal interface material]].

As opposed to [[thermal adhesive]], thermal paste does not add mechanical strength to the bond between heat source and heat sink. It has to be coupled with a fastener 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]]izable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials are [[epoxy|epoxies]], [[silicone]]s ([[silicone grease]]), [[polyurethane|urethanes]], and [[Acrylate polymer|acrylates]]; solvent-based systems, [[hot-melt adhesives]], and [[pressure-sensitive adhesive tape]]s are also available. [[Aluminum oxide]], [[boron nitride]], [[zinc oxide]], diamond and increasingly [[aluminum nitride]] 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]] of the base matrix from 0.17–0.3&nbsp;W/(m·K) (watts per meter-kelvin)<ref>{{citation | author=Werner Haller | contribution=Adhesives | title=[[Ullmann's Encyclopedia of Industrial Chemistry]] | edition=7th | publisher=Wiley | year=2007 | pages=58–59 |display-authors=etal}}.</ref> up to about 4&nbsp;W/(m·K), according to a 2008 paper.<ref>{{cite conference | url = http://www.nrel.gov/docs/fy08osti/42972.pdf | title = Thermal interface materials for power electronics applications | last1 = Narumanchi | first1 = Sreekant | last2 = Mihalic | first2 = Mark | last3 = Kelly | first3 = Kenneth | last4 = Eesley | first4 = Gary | date = 2008 | publisher = IEEE | book-title = 11th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, 2008: ITHERM 2008: 28–31 May 2008 | at = Table 2 | doi = 10.1109/ITHERM.2008.4544297}}.</ref>

Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more, and consist of [[micronized]] 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]], usually a variation of the alloy [[galinstan]], 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. Furthermore, these pastes contain [[gallium]] which is highly corrosive to [[aluminium]] and thus cannot be used on aluminium heat sinks.

== 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]]s, [[Central processing unit|CPU]]s, [[Graphics Processing Unit|GPU]]s, and LED [[Chip on board|COB]]s. Cooling these devices is essential because excess heat rapidly degrades their performance and can cause a runaway to [[catastrophic failure]] of the device due to the negative temperature coefficient property of semiconductors.

Factory [[Personal computer|PC]]s and laptops{{Mdash}}although seldom tablets or smartphones{{Mdash}}typically incorporate thermal paste between the top of the CPU case and a heat sink for [[Computer cooling#Thermally conductive compounds|cooling]]. Thermal paste is sometimes also used between the CPU [[Die (integrated circuit)|die]] and its [[Heat spreader|integrated heat spreader]], though [[solder]] is sometimes used instead.

When a CPU heat spreader is coupled to the die via thermal paste, performance enthusiasts such as [[Overclocking|overclockers]] are able to, in a process known as "delidding",<ref>{{Cite news|url=https://www.ekwb.com/blog/what-is-delidding/|title=What is delidding? - ekwb.com|date=2016-08-25|work=ekwb.com|access-date=2018-10-18|language=en-US}}</ref> 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 cycling|power cycles]], thermal paste extrudes from between the die and heat sink, which eventually causes degradation of thermal performance inasmuch as there is less paste in place.<ref name="intel-2000">{{cite journal |last1=Viswanath |first1=Ram |last2=Wakharkar |first2=Vijay |last3=Watwe |first3=Abhay |last4=Lebonheur |first4=Vassou |title=Thermal Performance Challenges from Silicon to Systems |journal=Intel Technology Journal |date=2000 |url=http://mprc.pku.edu.cn/courses/architecture/autumn2005/thermal_perf.pdf|archive-url=https://web.archive.org/web/20170808033212/http://mprc.pku.edu.cn/courses/architecture/autumn2005/thermal_perf.pdf |archive-date=8 August 2017 |access-date=8 March 2020}}</ref>

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 [[Wetting|wettability]], leading to increased thermal resistance.<ref name="intel-2000"/>

==Health hazards==
Zinc oxide is highly toxic to aquatic organisms and may cause long-term negative effects to aquatic environments.<ref>{{cite web |title=ICSC 0208 - ZINC OXIDE |url=https://www.ilo.org/dyn/icsc/showcard.display?p_version=2&p_card_id=0208&p_lang=en#:~:text=Effects%20of%20long%2Dterm%20or%20repeated%20exposure&text=The%20substance%20is%20very%20toxic,chemical%20enter%20into%20the%20environment. |website=www.ilo.org |publisher=ILO}}</ref>

==See also==
* [[Computer cooling]]
* [[Hot-melt adhesive]]
* [[Phase-change material]]
* [[Thermally conductive pad]]
* [[List of thermal conductivities]]

==References==
{{Reflist}}

== External links ==
* {{Commons category-inline}}

{{DEFAULTSORT:Thermal Grease}}
[[Category:Adhesives]]
[[Category:Cooling technology]]
[[Category:Computer hardware cooling]]
[[Category:Heat conduction| ]]
[[Category:Heat transfer|Conduction]]