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rs 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 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 19 two heat sink attachment methods namely the maxigrip left and talon clip right clips available for processors and ball grid array 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 a pair of push pins push pins with compression springs for larger heat sinks and higher preloads push pins with compression springs are very effective 19 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 19 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 summary of heat sink attachment methods 19 method pros cons cost thermal tape easy to attach inexpensive cannot provide mechanical attachment for heavier heat sinks or for high vibration environments surface must be cleaned for optimal adhesion moderate to low thermal conductivity very low epoxy strong mechanical adhesion relatively inexpensive makes board rework difficult since it can damage component surface must be cleaned for optimal adhesion very low wire form z clips strong mechanical attachment easy removal rework applies a preload to the thermal interface material improving thermal performance requires holes in the board or solder anchors more expensive than tape or epoxy custom designs low clip on applies a preload to the thermal interface material improving thermal performance requires no holes or anchors easy removal rework must have keep out zone around the bga for the clip extra assembly steps low push pin with compression springs strong mechanical attachment highest thermal interface material preload easy removal and installation requires holes in the board which increases complexity of traces in pcb moderate stand offs with compression springs strongest mechanical attachment highest preload for the thermal interface material ideal for large heat sinks requires holes in the board which increases complexity of trace layout complicated assembly high thermal interface materials edit main article thermal interface material thermal conductivity and the interface resistance form part of the thermal interface resistance of a thermal interface material 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 10 if the contact area is small as it is for rough surfaces the major contribution to the resistance is made by the gaps 10 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 20 while tims have conductivities of 0 3 w m k 21 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 selection based on interface gap 21 interface gap values products types available 0 05 mm 2 mil thermal grease epoxy phase change materials 0 05 0 1 mm 2 5 mil phase change materials polyimide graphite or aluminium tapes 0 1 0 5 mm 5 18 mil silicone coated fabrics 0 5 mm 18 mil gap fillers selection based on contact pressure 21 contact pressure scale typical pressure ranges product types available very low 70 kpa gap fillers low 140 kpa thermal grease epoxy polyimide graphite or aluminium tapes high 2 mpa silicone coated fabrics selection based on dielectric strength 21 electrical insulation dielectric strength typical values product types available not required n a n a n a thermal grease epoxy phase change materials graphite or aluminium tapes required low 10 kv mm 300 v mil silicone coated fabrics gap fillers required high 60 kv mm 1500 v mil polyimide tape tim application notes based on product type product type application notes thermal performance thermal paste messy labor intensive relatively long assembly time epoxy creates permanent interface bond phase change allows for pre attachment softens and conforms to interface defects at operational temperatures can be repositioned in field thermal tapes including graphite polyimide and aluminium tapes easy to apply some mechanical strength silicone coated fabrics provide cushioning and sealing while still allowing heat transfer gap filler can be used to thermally couple differing height components to a heat spreader or heat sink naturally tacky high power leds from philips lumileds lighting company mounted on 21 mm star shaped aluminium core pcbs light emitting diode lamps edit light emitting diode led performance and lifetime are strong functions of their temperature 22 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 23 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 edit 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 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 switches can malfunction if exposed to hotter soldering irons so this practice is still very much in use 24 methods to determine performance edit in general a heat sink performance is a function of material thermal conductivity dimensions fin type heat transfer coefficient 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 cfd can also be used this section will discuss the aforementioned methods for the determination of the heat sink thermal perfor...