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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 than shiny bare metal 17 18 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 citation needed however there are exceptions notably certain metal oxides that are used as selective surfaces in vacuum or outer space 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 100 c 373 k surface facing the sun will absorb a lot of radiant heat because the sun 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 edit microprocessor cooling edit cooling system of an asus gtx 650 graphics card three heat pipes are visible heat dissipation is an unavoidable by product of electronic devices and circuits 9 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 10 this temperature change may be attributed to what is known as the thermal contact resistance 10 thermal interface materials tim decrease the thermal contact resistance attachment methods edit 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 roll of thermally conductive tape thermally conductive tape is one of the most cost effective heat sink attachment materials 19 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 19 surfaces of both the component and heat sink must be clean with no residue such as a film of silicone grease 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 is more expensive than tape but provides a greater mechanical bond between the heat sink and component as well as improved thermal conductivity 19 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 19 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 a pin fin heat sink with a z clip retainer 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 mecha...
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