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coolant where it is dissipated away from the device thereby allowing regulation of the device s temperature in computers heat sinks are used to cool cpus gpus and some chipsets and ram modules heat sinks are used with other high power semiconductor devices such as power transistors 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 temperature of the integrated circuit thermal adhesive or thermal paste improve the heat sink s performance by filling air gaps between the heat sink and the heat spreader on the device a heat sink is usually made out of a material with a high thermal conductivity such as aluminium or copper heat transfer principle edit main article heat pipe a heat sink transfers thermal energy from a higher temperature device to a lower temperature fluid medium the fluid medium is frequently air but can also be water refrigerants or even oil if the fluid medium is water the heat sink is frequently called a cold plate in thermodynamics 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 displaystyle 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 d t d x displaystyle q_ k ka frac dt dx sketch of a heat sink in a duct used to calculate the governing equations from conservation of energy and newton s law of cooling 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 q m c p in t air out t air in displaystyle dot q dot m c_ p text in t_ text air out t_ text air in q t hs t air av r hs displaystyle dot q frac t_ text hs t_ text air av r_ text hs where t air av t air in t air out 2 displaystyle t_ text air av frac t_ text air in t_ text air out 2 m displaystyle dot m is the air mass flow rate in kg s c p in displaystyle c_ p text in is the specific heat capacity of the incoming air in j kg c r hs displaystyle r_ text hs is the thermal resistance of the heatsink 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 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 1 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 edit power transistor heat sinks left for to 3 package right for to 220 package middle for two to 220 thermal resistance edit for semiconductor devices used in a variety of consumer and industrial electronics the idea of thermal resistance 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 2 commercial extruded aluminium heat sinks have a thermal resistance heat sink to ambient air ranging from 0 4 c w for a large sink meant for to 3 devices up to as high as 85 c w for a clip on heat sink for a to 92 small plastic case 2 the popular 2n3055 power transistor in a to 3 case has an internal thermal resistance from junction to case of 1 52 c w 3 the contact between the device case and heat sink may have a thermal resistance between 0 5 and 1 7 c w depending on the case size and use of grease or insulating mica washer 2 material edit 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 4 the most common heat sink materials are aluminium alloys 5 aluminium alloy 1050 has one of the higher thermal conductivity values at 229 w m k and heat capacity of 922 j kg k 6 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 casting skiving or milling copper has excellent heat sink properties in terms of its thermal conductivity corrosion resistance biofouling resistance and antimicrobial resistance see also copper in heat exchangers 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 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 5 and more expensive than aluminium and copper is less ductile than aluminum 5 one piece copper heat sinks can be made by skiving or milling sheet metal fins can be soldered onto a rectangular copper body 7 8 fin efficiency edit 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 9 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 10 η f tanh m l c m l c displaystyle eta _ text f frac tanh ml_ c ml_ c m l c 2 h f k t f l f displaystyle ml_ c sqrt frac 2h_ text f kt_ text f l_ text f where h f is the convection coefficient of the fin 10 to 100 w m 2 k in air 500 to 10 000 w m 2 k in water k is the thermal conductivity of the fin material 120 to 240 w m k 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 making them thicker or shorter or by using a more conductive material copper instead of aluminium for example spreading resistance edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed december 2010 learn how and when to remove this message 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 11 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 12 choose a different material with higher thermal conductivity use a vapor chamber or heat pipe in the heat sink base fin arrangements edit main article fin extended surface heat sink types pin straight and flared fin 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 second type of heat sink fin arrangement is the straight fin a variation on the straight fin heat sink is a cross cut heat sink a third type of heat sink is the flared fin heat sink where the fins are not parallel to one another 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 examples of the three types are shown in the image on the right forghan et al 13 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 eggink 14 also found that for the bypass configurations that they tested the flared heat sink performed better than the other heat sinks tested free convection flow around a pin fin heat sink generally the more surface area a heat sink has the better its performance 1 real world performance depends on the design and application the concept of a pin fin heat sink is to pack as much surface area into a given volume as possible while working in any orientation of fluid flow 1 kordyban 1 has compared the performance of a pin fin and a straight fin heat sink of similar dimensions although the pin fin has 194 cm 2 surface area while the straight fin has 58 cm 2 the temperature difference between the heat sink base and the ambient air for the pin fin is 50 c 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 optimal application where the fluid flows axially along the pins rather than only tangentially across the pins comparison of a pin fin and straight fin heat sink of similar dimensions 1 heat sink fin type width cm length cm height cm surface area cm 2 volume cm 3 temperature difference t case t air c straight 2 5 2 5 3 2 58 20 44 pin 3 8 3 8 1 7 194 24 51 cavities inverted fins edit 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 15 16 conductive thick plate between the heat source and the heat sink edit 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 citation needed 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 edit a server grade flash memory card with a black heat sink the heat transfer from the heat sink occurs by convection of the surrounding air conduction through the air and radiation 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 in situations where convection is low such as a flat non finned panel with low airflow radiative cooling can be a significant factor here the surface properties may be an important design factor matte black surfaces radiate much more efficiently ...
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