TheInfoList

A heat sink (also commonly spelled heatsink) is a passive
heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (m ...

that transfers the heat generated by an electronic or a mechanical device to a
fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature. In computers, heat sinks to cool
CPUs A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controllin ...

,
GPUs A graphics processing unit (GPU) is a specialized designed to rapidly manipulate and alter to accelerate the creation of in a intended for output to a . GPUs are used in s, s, s, s, and s. Modern GPUs are very efficient at manipulating and ...
, and some chipsets and RAM modules. Heat sinks are used with high-power semiconductor devices such as power
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s and optoelectronics such as lasers and light-emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature. A heat sink is designed to maximize its surface area in contact with the cooling medium surrounding it, such as the air. Air velocity, choice of material, protrusion design and surface treatment are factors that affect the performance of a heat sink. Heat sink attachment methods and thermal interface materials also affect the
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 semiconduct ...
temperature of the integrated circuit.
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 heatsink ...
or
thermal paste File:Thermalgrease.jpg, Metal thermal paste applied to 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 Therma ...
improve the heat sink's performance by filling air gaps between the heat sink and the
heat spreader A heat spreader transfers energy as heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than Work (thermodynamics), thermodynamic work or Mass transfer, transfer of matter. The various ...
on the device. A heat sink is usually made out of aluminium or copper.

# Heat transfer principle

A heat sink transfers thermal energy from a higher-temperature device to a lower-temperature
fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
medium. The fluid medium is frequently air, but can also be water, refrigerants or oil. If the fluid medium is water, the heat sink is frequently called a cold plate. In
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...

a heat sink is a heat reservoir that can absorb an arbitrary amount of heat without significantly changing temperature. Practical heat sinks for electronic devices must have a temperature higher than the surroundings to transfer heat by convection, radiation, and conduction. The power supplies of electronics are not absolutely efficient, so extra heat is produced that may be detrimental to the function of the device. As such, a heat sink is included in the design to disperse heat. Fourier's law of heat conduction shows that when there is a temperature gradient in a body, heat will be transferred from the higher-temperature region to the lower-temperature region. The rate at which heat is transferred by conduction, $q_k$, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred. When it is simplified to a one-dimensional form in the ''x'' direction, it can be expressed as: : $q_k = -k A \frac.$ For a heat sink in a duct, where air flows through the duct, the heat-sink base will usually be hotter than the air flowing through the duct. Applying the conservation of energy, for steady-state conditions, and Newton’s law of cooling to the temperature nodes shown in the diagram gives the following set of equations: : $\dot = \dot c_\left(T_\text - T_\text\right),$ : $\dot = \frac,$ where : $T_\text = \frac.$ Using the mean air temperature is an assumption that is valid for relatively short heat sinks. When compact heat exchangers are calculated, the logarithmic mean air temperature is used. $\dot$ is the air mass flow rate in kg/s. The above equations show that: * When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat-sink base temperature. And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat-sink base temperature. ** The increase in heat-sink thermal resistance with decrease in flow rate will be shown later in this article. * The inlet air temperature relates strongly with the heat-sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat-sink base temperature. * If there is no air flow around the heat sink, energy cannot be transferred. * A heat sink is not a device with the "magical ability to absorb heat like a sponge and send it off to a parallel universe". Natural convection requires free flow of air over the heat sink. If fins are not aligned vertically, or if fins are too close together to allow sufficient air flow between them, the efficiency of the heat sink will decline.

# Design factors

## Thermal resistance

For semiconductor devices used in a variety of consumer and industrial electronics, the idea of ''
thermal resistance Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. *(Absolute) thermal resistance ''R'' in kelvin ...
'' simplifies the selection of heat sinks. The heat flow between the semiconductor die and ambient air is modeled as a series of resistances to heat flow; there is a resistance from the die to the device case, from the case to the heat sink, and from the heat sink to the ambient air. The sum of these resistances is the total thermal resistance from the die to the ambient air. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees Celsius per watt (°C/W). If the device dissipation in watts is known, and the total thermal resistance is calculated, the temperature rise of the die over the ambient air can be calculated. The idea of thermal resistance of a semiconductor heat sink is an approximation. It does not take into account non-uniform distribution of heat over a device or heat sink. It only models a system in thermal equilibrium and does not take into account the change in temperatures with time. Nor does it reflect the non-linearity of radiation and convection with respect to temperature rise. However, manufacturers tabulate typical values of thermal resistance for heat sinks and semiconductor devices, which allows selection of commercially manufactured heat sinks to be simplified.Nello Sevastopoulos et al., ''National Semiconductor Voltage Regulator Handbook'', National Semiconductor Corp., 1975, chapters 4, 5, 6. Commercial extruded aluminium heat sinks have a thermal resistance (heat sink to ambient air) ranging from for a large sink meant for
TO-3 In electronics, TO-3 is a designation for a standardized metal semiconductor package A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor device A semiconductor device is an elec ...

devices, up to as high as for a clip-on heat sink for a
TO-92 The TO-92 is a widely used style of semiconductor package A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor device A semiconductor device is an electronic component that reli ...
small plastic case. The popular
2N3055 The 2N3055 is a silicon NPN power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one ...

power transistor in a TO-3 case has an internal thermal resistance from junction to case of . The contact between the device case and heat sink may have a thermal resistance between , depending on the case size and use of grease or insulating mica washer.

## Material

The materials for heat sink applications should have high heat capacity and thermal conductivity in order to absorb more heat energy without shifting towards a very high temperature and transmit it to the environment for efficient cooling. The most common heat sink materials are
aluminium alloys Aluminium alloys (or aluminum alloys; see spelling differences) are alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly pr ...
.Anon, Unknown
"Heat sink selection"
, Mechanical engineering department, San Jose State University 7 January 2010
Aluminium alloy 1050 has one of the higher thermal conductivity values at 229 W/(m·K) and heat capacity of 922 J/(kg·K), but is mechanically soft. Aluminium alloys 6060 (low-stress), 6061, and 6063 are commonly used, with thermal conductivity values of 166 and 201 W/(m·K) respectively. The values depend on the temper of the alloy. One-piece aluminium heat sinks can be made by
extrusion allow bars to be joined with special connectors. Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile. A material is pushed through a Die (manufacturing), die of the desired cross-section. Th ...

,
casting Casting is a manufacturing Manufacturing is the creation or Production (economics), production of goods with the help of equipment, Work (human activity), labor, machines, tools, and chemical or biological processing or formulation. It is th ...

,
skiving Skive or skiving may refer to: * Skive, Denmark, a place in Denmark ** Skive Municipality ** Skive Airport ** Skive station ** Skive fH, a handball club ** Skive IK, a football club ** Skive RC, a rugby club * Skiving (leathercraft), the thinning o ...
or
milling Milling may refer to: * Milling (grinding), breaking solid materials into smaller pieces by grinding, crushing, or cutting in a mill * Milling (machining), a process of using rotary cutters to remove material from a workpiece * Milling (military tra ...
.
Copper Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and electrical conductivity. A fre ...

has excellent heat-sink properties in terms of its thermal conductivity, corrosion resistance, biofouling resistance, and antimicrobial resistance (see also
Copper in heat exchangers Heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids ...
). Copper has around twice the thermal conductivity of aluminium, around 400 W/(m·K) for pure copper. Its main applications are in industrial facilities, power plants,
solar thermal Solar thermal energy (STE) is a form of energy and a technology Technology ("science of craft", from Ancient Greek, Greek , ''techne'', "art, skill, cunning of hand"; and , ''wikt:-logia, -logia'') is the sum of Art techniques and materials, ...
water systems, HVAC systems, gas water heaters, forced air heating and cooling systems, geothermal heating and cooling, and electronic systems. Copper is three times as dense and more expensive than aluminium. One-piece copper heat sinks can be made by
skiving Skive or skiving may refer to: * Skive, Denmark, a place in Denmark ** Skive Municipality ** Skive Airport ** Skive station ** Skive fH, a handball club ** Skive IK, a football club ** Skive RC, a rugby club * Skiving (leathercraft), the thinning o ...
or
milling Milling may refer to: * Milling (grinding), breaking solid materials into smaller pieces by grinding, crushing, or cutting in a mill * Milling (machining), a process of using rotary cutters to remove material from a workpiece * Milling (military tra ...
. Sheet-metal fins can be soldered onto a rectangular copper body. Copper is less ductile than aluminium, so it cannot be extruded into heat sinks.

### Fin efficiency

Fin efficiency is one of the parameters that makes a higher-thermal-conductivity material important. A fin of a heat sink may be considered to be a flat plate with heat flowing in one end and being dissipated into the surrounding fluid as it travels to the other. As heat flows through the fin, the combination of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, therefore, the heat transfer to the fluid, will decrease from the base to the end of the fin. Fin efficiency is defined as the actual heat transferred by the fin, divided by the heat transfer were the fin to be isothermal (hypothetically the fin having infinite thermal conductivity). These equations are applicable for straight fins: : $\eta_\text = \frac,$ : $mL_c = \sqrt L_\text,$ where : ''h''f is the convection coefficient of the fin: :: 10 to 100 W/(m2·K) in air, :: 500 to 10,000 W/(m2·K) in water, : ''k'' is 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 fin material: :: 120 to 240 /( m·) for aluminium, : ''L''f is the fin height (m), : ''t''f is the fin thickness (m). Fin efficiency is increased by decreasing the fin
aspect ratio The aspect ratio of a geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic Arithmetic (from the Ancient Greek, Greek wikt:en:ἀριθμός#Ancient Greek, ἀριθμό ...

(making them thicker or shorter), or by using a more conductive material (copper instead of aluminium, for example).

Another parameter that concerns the thermal conductivity of the heat-sink material is spreading resistance. Spreading resistance occurs when thermal energy is transferred from a small area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat-sink base. The spreading resistance phenomenon is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform across the base of the heat sink. This nonuniformity increases the heat sink's effective thermal resistance. To decrease the spreading resistance in the base of a heat sink: * increase the base thickness, * choose a different material with higher thermal conductivity, * use a vapor chamber or
heat pipe A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa. ...

in the heat sink base.

## Fin arrangements

A pin-fin heat sink is a heat sink that has pins that extend from its base. The pins can be cylindrical, elliptical or square. A pin is one of the more common heat-sink types available on the market. A second type of heat-sink fin arrangement is the straight fin. These run the entire length of the heat sink. A variation on the straight-fin heat sink is a cross-cut heat sink. A straight-fin heat sink is cut at regular intervals. In general, the more surface area a heat sink has, the better it works. However, this is not always true. The concept of a pin-fin heat sink is to try to pack as much surface area into a given volume as possible. As well, it works well in any orientation. Kordyban has compared the performance of a pin-fin and a straight-fin heat sink of similar dimensions. Although the pin-fin has 194 cm2 surface area while the straight-fin has 58 cm2, the temperature difference between the heat-sink base and the ambient air for the pin-fin is , but for the straight-fin it was 44 °C, or 6 °C better than the pin-fin. Pin-fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along the pins rather than only tangentially across the pins. Another configuration is the flared-fin heat sink; its fins are not parallel to each other, as shown in . Flaring the fins decreases flow resistance and makes more air go through the heat-sink fin channel; otherwise, more air would bypass the fins. Slanting them keeps the overall dimensions the same, but offers longer fins. Forghan, et al.Forghan, F., Goldthwaite, D., Ulinski, M., Metghalchi, M., 2001, Experimental and Theoretical Investigation of Thermal Performance of Heat Sinks, ISME May. have published data on tests conducted on pin-fin, straight-fin and flared-fin heat sinks. They found that for low air approach velocity, typically around 1 m/s, the thermal performance is at least 20% better than straight-fin heat sinks. Lasance and EgginkLasance, C. J. M. and Eggink, H. J., 2001, A Method to Rank Heat Sinks in Practice: The Heat Sink Performance Tester, 21st IEEE SEMI-THERM Symposium. also found that for the bypass configurations that they tested, the flared heat sink performed better than the other heat sinks tested.

## Cavities (inverted fins)

Cavities (inverted fins) embedded in a heat source are the regions formed between adjacent fins that stand for the essential promoters of nucleate boiling or condensation. These cavities are usually utilized to extract heat from a variety of heat-generating bodies to a heat sink.

## Conductive thick plate between the heat source and the heat sink

Placing a conductive thick plate as a heat-transfer interface between a heat source and a cold flowing fluid (or any other heat sink) may improve the cooling performance. In such arrangement, the heat source is cooled under the thick plate instead of being cooled in direct contact with the cooling fluid. It is shown that the thick plate can significantly improve the heat transfer between the heat source and the cooling fluid by conducting the heat current in an optimal manner. The two most attractive advantages of this method are that no additional pumping power and no extra heat-transfer surface area, that is quite different from fins (extended surfaces).

## Surface color

The
heat transfer Heat transfer is a discipline of thermal engineering Thermal engineering is a specialized sub-discipline of mechanical engineering Mechanical engineering is an engineering Engineering is the use of scientific method, scientific pri ...

from the heat sink occurs by convection of the surrounding air, conduction through the air, and
radiation upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. In physics Physics (from grc ...
. Heat transfer by radiation is a function of both the heat-sink temperature and the temperature of the surroundings that the heat sink is optically coupled with. When both of these temperatures are on the order of 0 °C to 100 °C, the contribution of radiation compared to convection is generally small, and this factor is often neglected. In this case, finned heat sinks operating in either natural-convection or forced-flow will not be affected significantly by surface
emissivity work iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is ...
. In situations where convection is low, such as a flat non-finned panel with low airflow,
radiative cooling Radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation. Terrestrial radiative cooling Mechanism Infrared ...
can be a significant factor. Here the surface properties may be an important design factor. Matte-black surfaces radiate much more efficiently than shiny bare metal. A shiny metal surface has low emissivity. The emissivity of a material is tremendously frequency-dependent and is related to absorptivity (of which shiny metal surfaces have very little). For most materials, the emissivity in the visible spectrum is similar to the emissivity in the infrared spectrum; however, there are exceptions notably, certain metal oxides that are used as "
selective surface Image:Col·lectors solars de tub de buit.JPG, Solar thermal collector system based on evacuated glass tubes. Sunlight is absorbed by a special material at the center of each tube that has a selective surface. The surface absorbs sunlight nearly comp ...
s". In
vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Gree ...

or
outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting ...
, there is no convective heat transfer, thus in these environments, radiation is the only factor governing heat flow between the heat sink and the environment. For a satellite in space, a surface facing the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

will absorb a lot of radiant heat, because the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

's surface temperature is nearly 6000 K, whereas the same surface facing deep space will radiate a lot of heat, since deep space has an effective temperature of only several Kelvin.

# Engineering applications

## Microprocessor cooling

Heat dissipation is an unavoidable by-product of electronic devices and circuits. In general, the temperature of the device or component will depend on the thermal resistance from the component to the environment, and the heat dissipated by the component. To ensure that the component does not overheat, a thermal engineer seeks to find an efficient heat transfer path from the device to the environment. The heat transfer path may be from the component to a printed circuit board (PCB), to a heat sink, to air flow provided by a fan, but in all instances, eventually to the environment. Two additional design factors also influence the thermal/mechanical performance of the thermal design: # The method by which the heat sink is mounted on a component or processor. This will be discussed under the section ''attachment methods''. # For each interface between two objects in contact with each other, there will be a temperature drop across the interface. For such composite systems, the temperature drop across the interface may be appreciable.Incropera, F.P. and DeWitt, D.P., 1985, Introduction to heat transfer, John Wiley and sons, NY. This temperature change may be attributed to what is known as the thermal contact resistance. ''Thermal interface materials'' (TIM) decrease the thermal contact resistance.

### Attachment methods

As power dissipation of components increases and component package size decreases, thermal engineers must innovate to ensure components won't overheat. Devices that run cooler last longer. A heat sink design must fulfill both its thermal as well as its mechanical requirements. Concerning the latter, the component must remain in thermal contact with its heat sink with reasonable shock and vibration. The heat sink could be the copper foil of a circuit board, or a separate heat sink mounted onto the component or circuit board. Attachment methods include thermally conductive tape or epoxy, wire-form z clips, flat spring clips, standoff spacers, and push pins with ends that expand after installing. ; Thermally conductive tape Thermally conductive tape is one of the most cost-effective heat sink attachment materials.Azar, K, et al., 2008
"Thermally Conductive Tapes"
can-dotape.com, accessed on 3/21/2013
It is suitable for low-mass heat sinks and for components with low power dissipation. It consists of a thermally conductive carrier material with a pressure-sensitive adhesive on each side. This tape is applied to the base of the heat sink, which is then attached to the component. Following are factors that influence the performance of thermal tape: # Surfaces of both the component and heat sink must be clean, with no residue such as a film of
silicone grease Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil A silicone oil is any liquid polymerized siloxane with organic side chains. The most important member is polydimethylsiloxane. These pol ...
. # Preload pressure is essential to ensure good contact. Insufficient pressure results in areas of non-contact with trapped air, and results in higher-than-expected interface thermal resistance. # Thicker tapes tend to provide better "wettability" with uneven component surfaces. "Wettability" is the percentage area of contact of a tape on a component. Thicker tapes, however, have a higher thermal resistance than thinner tapes. From a design standpoint, it is best to strike a balance by selecting a tape thickness that provides maximum "wettability" with minimum thermal resistance. ; Epoxy
Epoxy Epoxy is the family of basic components or cured A cure is a completely effective treatment for a disease. Cure, or similar, may also refer to: Places * Cure (river), a river in France * Cures, Sabinum, an ancient Italian town * Cures, S ...

is more expensive than tape, but provides a greater mechanical bond between the heat sink and component, as well as improved thermal conductivity. The epoxy chosen must be formulated for this purpose. Most epoxies are two-part liquid formulations that must be thoroughly mixed before being applied to the heat sink, and before the heat sink is placed on the component. The epoxy is then cured for a specified time, which can vary from 2 hours to 48 hours. Faster cure time can be achieved at higher temperatures. The surfaces to which the epoxy is applied must be clean and free of any residue. The epoxy bond between the heat sink and component is semi-permanent/permanent. This makes re-work very difficult and at times impossible. The most typical damage caused by rework is the separation of the component die heat spreader from its package. ; Wire form Z-clips More expensive than tape and epoxy, wire form z-clips attach heat sinks mechanically. To use the z-clips, the printed circuit board must have anchors. Anchors can be either soldered onto the board, or pushed through. Either type requires holes to be designed into the board. The use of RoHS solder must be allowed for because such solder is mechanically weaker than traditional Pb/Sn solder. To assemble with a z-clip, attach one side of it to one of the anchors. Deflect the spring until the other side of the clip can be placed in the other anchor. The deflection develops a spring load on the component, which maintains very good contact. In addition to the mechanical attachment that the z-clip provides, it also permits using higher-performance thermal interface materials, such as phase change types. ; Clips Available for processors and
ball grid array A ball grid array (BGA) is a type of surface-mount File:SMDcompared.jpeg">A MOSFET transistor, placed upon a British postage stamp for size comparison. Surface-mount technology (SMT) is a method in which the electrical components are mount ...
(BGA) components, clips allow the attachment of a BGA heat sink directly to the component. The clips make use of the gap created by the ball grid array (BGA) between the component underside and PCB top surface. The clips therefore require no holes in the PCB. They also allow for easy rework of components. ; Push pins with compression springs For larger heat sinks and higher preloads, push pins with compression springs are very effective. The push pins, typically made of brass or plastic, have a flexible barb at the end that engages with a hole in the PCB; once installed, the barb retains the pin. The compression spring holds the assembly together and maintains contact between the heat sink and component. Care is needed in selection of push pin size. Too great an insertion force can result in the die cracking and consequent component failure. ; Threaded standoffs with compression springs For very large heat sinks, there is no substitute for the threaded standoff and compression spring attachment method. A threaded standoff is essentially a hollow metal tube with internal threads. One end is secured with a screw through a hole in the PCB. The other end accepts a screw which compresses the spring, completing the assembly. A typical heat sink assembly uses two to four standoffs, which tends to make this the most costly heat sink attachment design. Another disadvantage is the need for holes in the PCB.

### Thermal interface materials

Thermal contact resistance occurs due to the voids created by surface roughness effects, defects and misalignment of the interface. The voids present in the interface are filled with air. Heat transfer is therefore due to conduction across the actual contact area and to conduction (or natural convection) and radiation across the gaps. If the contact area is small, as it is for rough surfaces, the major contribution to the resistance is made by the gaps. To decrease the thermal contact resistance, the surface roughness can be decreased while the interface pressure is increased. However, these improving methods are not always practical or possible for electronic equipment. Thermal interface materials (TIM) are a common way to overcome these limitations. Properly applied thermal interface materials displace the air that is present in the gaps between the two objects with a material that has a much-higher thermal conductivity. Air has a thermal conductivity of 0.022 W/(m·K) while TIMs have conductivities of 0.3 W/(m·K) and higher. When selecting a TIM, care must be taken with the values supplied by the manufacturer. Most manufacturers give a value for the thermal conductivity of a material. However, the thermal conductivity does not take into account the interface resistances. Therefore, if a TIM has a high thermal conductivity, it does not necessarily mean that the interface resistance will be low. Selection of a TIM is based on three parameters: the interface gap which the TIM must fill, the contact pressure, and the electrical resistivity of the TIM. The contact pressure is the pressure applied to the interface between the two materials. The selection does not include the cost of the material. Electrical resistivity may be important depending upon electrical design details.

## Light-emitting diode lamps

Light-emitting diode A light-emitting diode (LED) is a semiconductor A semiconductor material has an value falling between that of a , such as metallic copper, and an , such as glass. Its falls as its temperature rises; metals behave in the opposite ...
(LED) performance and lifetime are strong functions of their temperature. Effective cooling is therefore essential. A case study of a LED based downlighter shows an example of the calculations done in order to calculate the required heat sink necessary for the effective cooling of lighting system. The article also shows that in order to get confidence in the results, multiple independent solutions are required that give similar results. Specifically, results of the experimental, numerical and theoretical methods should all be within 10% of each other to give high confidence in the results.

## In soldering

Temporary heat sinks are 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,
hemostat A hemostat (also called a hemostatic clamp, arterial forceps, or pean after Jules-Émile Péan) is a surgery, surgical tool used in many surgical procedures to control bleeding. For this reason, it is common in the initial phases of surgery for in ...

, 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 The reed switch is an electrical switch In , a switch is an that can disconnect or connect the conducting path in an , interrupting the or diverting it from one conductor to another. The most common type of switch is an electromechanica ...

es can malfunction if exposed to hotter soldering irons, so this practice is still very much in use.James Johnston
"Reed Switches"
''Electronics in Meccano'', Issue 6, January 2000.

# Methods to determine performance

In general, a heat sink performance is a function of material thermal conductivity, dimensions, fin type,
heat transfer coefficient The heat transfer coefficient or film coefficient, or film effectiveness, in thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and th ...
, air flow rate, and duct size. To determine the thermal performance of a heat sink, a theoretical model can be made. Alternatively, the thermal performance can be measured experimentally. Due to the complex nature of the highly 3D flow in present applications, numerical methods or
computational fluid dynamics#REDIRECT Computational fluid dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid dynamics, fluid flows. Computers are used ...
(CFD) can also be used. This section will discuss the aforementioned methods for the determination of the heat sink thermal performance.

## A heat transfer theoretical model

One of the methods to determine the performance of a heat sink is to use heat transfer and fluid dynamics theory. One such method has been published by Jeggels, et al., though this work is limited to ducted flow. Ducted flow is where the air is forced to flow through a channel which fits tightly over the heat sink. This makes sure that all the air goes through the channels formed by the fins of the heat sink. When the air flow is not ducted, a certain percentage of air flow will bypass the heat sink. Flow bypass was found to increase with increasing fin density and clearance, while remaining relatively insensitive to inlet duct velocity. The heat sink thermal resistance model consists of two resistances, namely the resistance in the heat sink base, $R_$, and the resistance in the fins, $R_$. The heat sink base thermal resistance, $R_$, can be written as follows if the source is a uniformly applied the heat sink base. If it is not, then the base resistance is primarily spreading resistance: :$R_b = \frac$ (4) where $t_b$ is the heat sink base thickness, $k$ is the heat sink material thermal conductivity and $A_b$ is the area of the heat sink base. The thermal resistance from the base of the fins to the air, $R_$, can be calculated by the following formulas: :$R_f = \frac$ (5) :$\eta_f = \frac$ (6) :$mL_c = \sqrtL_f$ (7) :$D_h = \frac$ (8) :$Re = \frac$ (9) :$f = \left(0.79 \ln Re - 1.64\right)^$Mills, A.F., 1999, Heat transfer, Second edition, Prentice Hall. (10) :$Nu = \frac$ (11) :$h_f = \frac$ (12) :$\rho = \frac$ (13) The flow rate can be determined by the intersection of the heat sink system curve and the fan curve. The heat sink system curve can be calculated by the flow resistance of the channels and inlet and outlet losses as done in standard fluid mechanics text books, such as Potter, et al. and White. Once the heat sink base and fin resistances are known, then the heat sink thermal resistance, $R_$ can be calculated as: :$R_=R_ + R_$ (14). Using the equations 5 to 13 and the dimensional data in, the thermal resistance for the fins was calculated for various air flow rates. The data for the thermal resistance and heat transfer coefficient are shown in the diagram, which shows that for an increasing air flow rate, the thermal resistance of the heat sink decreases.

## Experimental methods

Experimental tests are one of the more popular ways to determine the heat sink thermal performance. In order to determine the heat sink thermal resistance, the flow rate, input power, inlet air temperature and heat sink base temperature need to be known. Vendor-supplied data is commonly provided for ducted test results. However, the results are optimistic and can give misleading data when heat sinks are used in an unducted application. More details on heat sink testing methods and common oversights can be found in Azar, et al.

## Numerical methods

In industry, thermal analyses are often ignored in the design process or performed too late — when design changes are limited and become too costly. Of the three methods mentioned in this article, theoretical and numerical methods can be used to determine an estimate of the heat sink or component temperatures of products before a physical model has been made. A theoretical model is normally used as a first order estimate. Online heat sink calculators can provide a reasonable estimate of forced and natural convection heat sink performance based on a combination of theoretical and empirically derived correlations. Numerical methods or
computational fluid dynamics#REDIRECT Computational fluid dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid dynamics, fluid flows. Computers are used ...
(CFD) provide a qualitative (and sometimes even quantitative) prediction of fluid flows.Kuzmin, D., Unknown
"Course: Introduction to CFD"
Dortmund University of Technology.
What this means is that it will give a visual or post-processed result of a simulation, like the images in figures 16 and 17, and the CFD animations in figure 18 and 19, but the quantitative or absolute accuracy of the result is sensitive to the inclusion and accuracy of the appropriate parameters. CFD can give an insight into flow patterns that are difficult, expensive or impossible to study using experimental methods. Experiments can give a quantitative description of flow phenomena using measurements for one quantity at a time, at a limited number of points and time instances. If a full-scale model is not available or not practical, scale models or dummy models can be used. The experiments can have a limited range of problems and operating conditions. Simulations can give a prediction of flow phenomena using CFD software for all desired quantities, with high resolution in space and time and virtually any problem and realistic operating conditions. However, if critical, the results may need to be validated.

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