View source for Thermal management of electronic devices and systems

[[Heat]] generated by [[electronic device]]s and [[Electronic circuit|circuit]]ry must be dissipated to improve [[Reliability engineering|reliability]] and prevent premature [[failure]].<ref>[http://www.aavidthermalloy.com/technical/papers/cooling.shtml The Basics of Package/Device Cooling]</ref> Techniques for heat dissipation can include [[Thermal management of electronic devices and systems#Heat sinks|heatsinks]] and [[fan (mechanical)|fan]]s for air [[cooling]], and other forms of [[computer cooling]] such as [[Water cooling|liquid cooling]].<ref>[http://www.computerpoweruser.com/editorial/article.asp?article=articles/archive/u0413/02u13/02u13.asp&guid= Your Cooling Options - From Air To Water To Refrigeration: Cooling Goes High Tech]</ref>

In cases of extreme low environmental temperatures, it may actually be necessary to heat the electronic components to achieve satisfactory operation.<ref>[http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_4.html OSHA Technical Manual]</ref>

== Overview ==
=== Thermal resistance of devices ===
This is usually quoted as the [[Thermal resistance in electronics|thermal resistance]] from [[p–n junction|junction]] to case of the [[semiconductor]] device. The units are °C/W. For example, a heatsink rated at 10 °C/W will get 10°C hotter than the surrounding air when it dissipates 1 Watt of heat. Thus, a heatsink with a low °C/W value is more efficient than a heatsink with a high °C/W value.<ref>[http://www.digikey.com/Web%20Export/Supplier%20Content/Crydom_172/PDF/Crydom_WP_Forced_Air.pdf The Effect of Forced Air Cooling on Heat Sink Thermal Ratings]</ref>

=== Thermal time constants ===
A heatsink's thermal mass can be considered as a capacitor (storing heat instead of charge) and the thermal resistance as an electrical resistance (giving a measure of how fast stored heat can be dissipated). Together, these two components form a thermal [[RC circuit]] with an associated time constant given by the product of R and C. This quantity can be used to calculate the dynamic heat dissipation capability of a device, in an analogous way to the electrical case.<ref>[http://www.nap.edu/openbook.php?record_id=1624&page=59 Materials for High-Density Electronic Packaging and Interconnection (1990)]</ref>
A specific type of [[thermal]] [[Interface (chemistry)|interface]] material is put between the heat sink and the heat source to increase thermal throughput, such as a [[microprocessor]] chip or other [[Electric power|power]] handling [[semiconductor]] to stabilise its [[temperature]] through increased [[thermal mass]] and heat dissipation (primarily by conduction and convection and to a lesser extent by radiation).{{cn|date=February 2011}}

=== Thermal interface material ===
{{main|Thermal interface material}}
A '''Thermal Interface Material or Mastic''' (aka '''TIM''') is used to fill the gaps between [[thermal transfer]] surfaces, such as between [[microprocessor]]s and [[heatsink]]s, in order to increase thermal transfer [[thermodynamic efficiency|efficiency]]. These gaps are normally filled with air which is a very poor [[Thermal conductivity|conductor]].{{cn|date=February 2011}}

==Applications==
===Personal Computers===
Due to recent technological developments and public interest, the retail heat sink market has reached an all time high. In the early 2000s, [[CPU]]s were produced that emitted more and more heat than earlier, escalating requirements for quality cooling systems. 

[[Overclocking]] has always meant greater cooling needs, and the inherently hotter chips meant more concerns for the enthusiast. Efficient heat sinks are vital to [[overclocking|overclocked]] computer systems because the higher a microprocessor's cooling rate, the faster the computer can operate without instability; generally, faster operation leads to higher performance. Many companies now compete to offer the best heat sink for [[Personal computer|PC]] overclocking enthusiasts. Prominent aftermarket heat sink manufacturers include:   [[Aero Cool]], [[Foxconn]], [[Thermalright]], [[Thermaltake]], [[Swiftech]], and [[Zalman]].{{cn|date=February 2011}}

===Soldering===
Temporary heat sinks were sometimes used while [[soldering]] circuit boards, preventing excessive heat from damaging sensitive nearby electronics. In the simplest case, this means partially gripping a component using a heavy metal [[crocodile clip]] or similar clamp. Modern semiconductor devices, which are designed to be assembled by [[reflow soldering]], can usually tolerate soldering temperatures without damage. On the other hand, electrical components such as magnetic [[reed switch]]es can malfunction if exposed to higher powered soldering irons, so this practice is still very much in use.<ref>[http://www.eleinmec.com/article.asp?23 Reed Switches]</ref>

===Batteries===
In the battery used for electric vehicles, Nominal battery performance is usually specified for working temperatures somewhere in the + 20°C to +30°C range however the actual performance can deviate substantially from this if the battery is operated at higher or in particular lower temperatures, so some electric cars have heating and cooling for their batteries<ref>http://www.mpoweruk.com/thermal.htm</ref>

== Methodologies ==
=== Heat sinks ===
{{main|Heat sink}}
Heat sinks are widely used in [[electronics]], and have become almost essential to modern [[central processing units]]. In common use, it is a [[metal]] object brought into contact with an [[electronics|electronic]] component's hot surface &mdash; though in most cases, a thin [[thermal interface material]] mediates between the two surfaces. [[Microprocessor]]s and power handling [[semiconductor]]s are examples of electronics that need a heat sink to reduce their temperature through increased [[thermal mass]] and heat dissipation (primarily by [[heat conduction|conduction]] and [[convection]] and to a lesser extent by [[thermal radiation|radiation]]). Heat sinks have become almost essential to modern [[integrated circuits]] like [[microprocessors]], [[digital signal processor|DSP]]s, [[Graphics processing unit|GPU]]s, and more.

A heat sink usually consists of a metal structure with one or more flat surfaces to ensure good thermal contact with the components to be cooled, and an array of comb or fin like protrusions to increase the surface contact with the air, and thus the rate of heat dissipation. 

A heat sink is sometimes used in conjunction with a fan to increase the rate of airflow over the heat sink. This maintains a larger temperature gradient by replacing warmed air faster than convection would. This is known as a forced air system.

[[Image:AMD heatsink and fan.jpg|thumb|CPU heat sink with fan attached]]
A '''heat sink''' (or heatsink) is an environment or object that absorbs and dissipates heat from another object using [[thermal contact]] (either direct or radiant). Heat sinks are used in a wide range of applications wherever efficient heat dissipation is required; major examples include [[refrigeration]], [[heat engines]] and cooling electronic devices.{{cn|date=February 2011}}

====Principle====
<div style="float:right; clear:right;">
[[Image:Flow-vector-heat-sink-fluid-WBG.jpg|thumb|none|Radial Heat Sink with Thermal Profile and Swirling Forced Convection Flow Trajectories (using CFD analysis)]]
<!-- Deleted image removed: [[Image:Liquid-heat-sink-fluid-WBG.jpg|thumb|none|Liquid Cooled Heat Sink with Forced Convection Flow Trajectories (using CFD analysis) {{deletable image-caption}}]] -->
[[Image:Natural-convection-heat-sink-fluid-WBG.jpg|thumb|none|Pin Fin Heat Sink with Thermal Profile and Dione Convection Flow Trajectories (using CFD analysis)]]
[[Image:Copper heat sink with pipes.jpg|thumb|none|Heat sink in a workstation computer]]
[[Image:Heatsink povray.png|thumb|none|A [[motherboard]] heat sink]]
[[Image:Nelson tulsa test.jpg|thumb|none|[[Fire test]] where the [[steel]] [[pipe (material)|pipe]] [[penetrant]]s clearly act to absorb and conduct heat from the [[furnace]], through to the unexposed side.]]
</div>

Heat sinks function by efficiently transferring [[thermal energy]] ("heat") from an object at high temperature to a second object at a lower temperature with a much greater [[heat capacity]]. This rapid transfer of thermal energy quickly brings the first object into [[thermal equilibrium]] with the second, lowering the temperature of the first object, fulfilling the heat sink's role as a cooling device. Efficient function of a heat sink relies on rapid transfer of thermal energy from the first object to the heat sink, and the heat sink to the second object.

The most common design of a heat sink is a metal device with many fins. The high [[thermal conductivity]] of the metal combined with its large surface area result in the rapid transfer of thermal energy to the surrounding, cooler, air. This cools the heat sink and whatever it is in direct thermal contact with. Use of [[fluid]]s (for example coolants in refrigeration) and [[thermal interface material]] (in cooling electronic devices) ensures good transfer of thermal energy to the heat sink. Similarly, a fan may improve the transfer of thermal energy from the heat sink to the air.{{cn|date=February 2011}}

====Construction and materials====
A heat sink usually consists of a base with one or more flat surfaces and an array of comb or fin-like protrusions to increase the heat sink's surface area contacting the air, and thus increasing the heat dissipation rate. While a heat sink is a static object, a [[fan (mechanical)|fan]] often aids a heat sink by providing increased airflow over the heat sink &mdash; thus maintaining a larger temperature [[gradient]] by replacing the warmed air more quickly than passive convection achieves alone &mdash; this is known as a [[Forced-air|forced-air system]].{{cn|date=February 2011}}

Ideally, heat sinks are made from a good thermal conductor such as [[silver]], [[gold]], [[copper]], or [[aluminum]] alloy. Copper and aluminum are among the most-frequently used materials for this purpose within electronic devices.  Copper (401&nbsp;W/(m·K) at 300&nbsp;K) is significantly more expensive than aluminum (237&nbsp;W/(m·K) at 300&nbsp;K) but is also roughly twice as efficient as a [[thermal conductor]]. Aluminum has the significant advantage that it can be easily formed by [[extrusion]], thus making complex cross-sections possible. Aluminum is also much lighter than copper, offering less mechanical stress on delicate electronic components. Some heat sinks made from aluminum have a copper core as a trade off. The heat sink's contact surface (the base) must be flat and smooth to ensure the best thermal contact with the object needing cooling. Frequently a [[thermal grease|thermally conductive grease]] is used to ensure optimal thermal contact; such compounds often contain [[colloidal silver]]. Further, a clamping mechanism, screws, or thermal adhesive hold the heat sink tightly onto the component, but specifically without pressure that would crush the component.{{cn|date=February 2011}}

====Performance====
Heat sink performance (including free convection, forced convection, liquid cooled, and any combination thereof) is a function of material, geometry, and overall surface heat transfer coefficient.  Generally, forced convection heat sink thermal performance is improved by increasing the thermal conductivity of the heat sink materials, increasing the surface area (usually by adding extended surfaces, such as fins or foam metal) and by increasing the overall area heat transfer coefficient (usually by increase fluid velocity, such as adding fans, pumps, etc.).

Online heat sink calculators from companies such as Novel Concepts, Inc., can accurately estimate forced convection heat sink performance.  For more complex heat sink geometries, and/or heat sinks with multiple materials, and/or heat sinks with multiple fluids, computation fluid dynamics (CFD) analysis is recommended (see graphics on this page).{{cn|date=February 2011}}

=== Convective air cooling ===
This term describes device cooling by the [[convective heat transfer|convection]] currents of the warm air being allowed to escape the confines of the component to be replaced by cooler air. Since warm air normally rises, this method usually requires venting at the top or sides of the casing to be effective.{{cn|date=February 2011}}

=== Forced air cooling ===

If there is more air being forced into a system than being pumped out (due to an imbalance in the number of fans), this is referred to as a 'positive' airflow, as the pressure inside the unit is higher than outside. 

A balanced or neutral airflow is the most efficient, although a slightly positive airflow can result in less dust build up if filtered properly.{{cn|date=February 2011}}

=== Heat pipes ===

[[Image:Heat pipe.jpg|250px|thumb|A [[heat sink]] (aluminium) incorporating a heat pipe (copper)]]
A [[heat pipe]] is a heat transfer mechanism that can transport large quantities of heat with a very small difference in [[temperature]] between the hot and cold interfaces. A typical heat pipe consists of sealed hollow tube made of a thermoconductive metal such as [[copper]] or [[aluminium]]. The pipe contains a relatively small quantity of a "working fluid" or [[coolant]] (such as [[water]], [[ethanol]] or [[Mercury (element)|mercury]])  with the remainder of the pipe being filled with the vapour phase of the working fluid, all other gases being excluded.
The advantage of heat pipes is their great efficiency in transferring heat.  They are actually more "conductive" than a copper bar of equivalent cross-section.{{cn|date=February 2011}}

=== Peltier cooling plates ===

'''Peltier cooling plates''' ({{IPAc-en|icon|ˈ|p|ɛ|l|t|i|.|eɪ}}) take advantage of what is known as the [[Peltier effect#Peltier effect|Peltier effect]] to create a heat flux between the junction of two different types of materials. This effect is commonly used for cooling electronic components and small instruments. 

There are no moving parts and such a device is maintenance free. Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature outweighs the poor efficiency. Thermoelectric junctions are generally only around 10% as efficient as the ideal refrigerator ([[Carnot cycle]]), compared with 40% achieved by conventional compression cycle systems.{{cn|date=February 2011}}

=== Synthetic Jet Air Cooling===

A [[Synthetic Jet]] is produced by a continual flow of vortices that are formed by alternating brief ejection and suction of air across an opening such that the net mass flux is zero. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed can produce a net momentum to the flow of a system without net mass injection to the system. 

Synthetic jet air movers have no moving parts and are thus maintenance free.  Due to the high heat transfer coefficients, high reliability but lower overall flow rates, Synthetic jet air movers are usually used at the chip level and not at the system level for cooling.  However depending on the size and complexity of the systems they can be used for both at times.{{cn|date=February 2011}}

===Electrostatic Fluid Acceleration===
An [[electrostatic fluid accelerator]] (EFA) is a device which pumps a fluid such as air without any moving parts. Instead of using rotating blades, as in a conventional fan, an EFA uses an electric field to propel electrically charged air molecules. Because air molecules are normally neutrally charged, the EFA has to create some charged molecules, or ions, first. Thus there are three basic steps in the fluid acceleration process: ionize air molecules, use those ions to push many more neutral molecules in a desired direction, and then recapture and neutralize the ions to eliminate any net charge.

The basic principle has been understood for some time but only in recent years have seen developments in the design and manufacture of EFA devices that may allow them to find practical and economical applications, such as in micro-cooling of electronics components.

===Recent developments===
More recently, [[synthetic diamond]] cooling sinks are being researched to provide better cooling. Also, some heat sinks are constructed of multiple materials with desirable characteristics, such as [[phase change material]]s, which can store a great deal of energy due to their [[heat of fusion]].{{cn|date=February 2011}}

==See also==
*[[Thermal resistance in electronics]]
*[[Heat pipe]]
*[[Computer cooling]]
*[[Radiator]]
*[[Active cooling]]

==References==
{{Reflist}}



{{DEFAULTSORT:Thermal Management Of Electronic Devices And Systems}}
[[Category:Computer hardware cooling]]
[[Category:Passive fire protection]]
[[Category:Electronic design]]
[[de:Kühlkörper]]
[[es:Disipador]]
[[fr:Radiateur#.C3.89changeur solide.2Fair]]
[[it:Dissipatore (elettronica)]]
[[he:צלעות קירור]]
[[lt:Radiatorius (elektronikoje)]]
[[nl:Koelvin]]
[[ja:ヒートシンク]]
[[pl:Radiator]]
[[pt:Dissipador]]
[[ru:Кулер]]
[[sk:Chladič (elektronika)]]
[[zh:散热片]]
[[Category:2Fix]]