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16-02-2011, 04:33 PM
Heat sink.doc (Size: 195 KB / Downloads: 57)
A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator (also a heat exchanger) in a car. Heat sinks also help to cool electronic and optoelectronic devices, such as higher-power lasers and light emitting diodes (LEDs).
A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer CPU or graphics processors. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Thermal adhesive (also known as thermal grease) is added to the base of the heatsink to help its thermal performance. Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance.
Basic heat sink heat transfer principle
A heat sink is an object that transfers thermal energy from a higher temperature to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water or in the case of heat exchangers, refrigerants and oil. If the fluid medium is water, the 'heat sink' is frequently called a cold plate.
To understand the principle of a heat sink, consider Fourier's law of heat conduction. Joseph Fourier was a French mathematician who made important contributions to the analytical treatment of heat conduction. Fourier's law of heat conduction, simplified to a one-dimensional form in the x-direction, 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, qk, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred.
Design factors which influence the thermal performance of a heat sink
The most common heat sink material is aluminium. Chemically pure aluminium is not used in the manufacture of heat sinks, but rather aluminium alloys. Aluminium alloy 1050A has one of the higher thermal conductivity values at 229 W/m•K. However, it is not recommended for machining, since it is a relatively soft material. Aluminium alloys 6061 and 6063 are the more commonly used aluminium alloys, with thermal conductivity values of 166 and 201 W/m•K, respectively. The aforementioned values are dependent on the temper of the alloy.
Copper is also used since it has around twice the conductivity of aluminium, but is three times as heavy as aluminium. Copper is also around four to six times more expensive than aluminium, but this is market dependent. Copper and aluminium prices can be compared in figures 3 and 4, or on Internet websites, such as the London Metal Exchange. Aluminium has the added advantage that it is able to be extruded, while copper can not. Copper heat sinks are machined and skived. Another method of manufacture is to solder the fins into the heat sink base.
Another heat sink material that can be used is diamond. With a value of 2000 W/mK it exceeds that of copper by a factor of five. In contrast to metals, where heat is conducted by delocalized electrons, lattice vibrations are responsible for diamond's very high thermal conductivity. For thermal management applications, the outstanding thermal conductivity and diffusivity of diamond is an essential. Nowadays CVD diamond is used as submounts for high-power integrated circuits and laser diodes.
Composite materials can be used. Examples are a copper-tungsten pseudoalloy, AlSiC (silicon carbide in aluminium matrix), Dymalloy (diamond in copper-silver alloy matrix), and E-Material (beryllium oxide in beryllium matrix). Such materials are often used as substrates for chips, as their thermal expansion coefficient can be matched to ceramics and semiconductors.
• Fin efficiency is one of the parameters which 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.
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 better thermal conductivity
• Use a vapour chamber or heat pipe in the heat sink base.
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 by far 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 but at a coarser pitch than a pin fin type.
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. Kordyan  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 50 °C. 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 (see figure 17) 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 figure 5. 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. have published data on tests conducted on pin fin, straight fin and flared fin heat sinks. They found that for low approach air velocity, typically around 1 m/s, the thermal performance is at least 20% better than straight fin heat sinks. Lasance and Eggink  also found that for the bypass configurations that they tested, the flared heat sink performed better than the other heat sinks tested.
The heat transfer from the heatsink is mediated by two effects: conduction via the coolant, and thermal radiation. The surface of the heatsink influences its emissivity; shiny metal absorbs and radiates only a small amount of heat, while matte black radiates highly.
In coolant-mediated heat transfer, the contribution of radiation is generally small. A layer of coating on the heatsink can then be counterproductive, as its thermal resistance can impair heat flow from the fins to the coolant. Finned heatsinks with convective or forced flow will not benefit significantly from being colored.
In situations with significant contribution of radiative cooling, e.g. in case of a flat non-finned panel acting as a heatsink with low airflow, the heatsink surface finish can play an important role. Matte-black surfaces will radiate much more efficiently than shiny bare metal.
The importance of radiative vs coolant-mediated heat transfer increases in situations with low ambient air pressure (e.g. high-altitude operations) or in vacuum (e.g. satellites in space).
Heat dissipation is an unavoidable by-product of all but micropower 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 temperature 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:
1. The method by which the heat sink is mounted on a component or processor. This will be discussed under the section attachment methods.
2. 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 . This temperature change may be attributed to what is known as the thermal contact resistance . Thermal interface materials (TIM) decrease the thermal contact resistance.
Epoxy 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.
• 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 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 . 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
Firestopping and fireproofing
A heat sink is rarely a desired thing in passive fire protection. Rather, it is usually a problem that must be overcome to maintain fire-resistance ratings. The proven ability to overcome heat sinks in construction is subject to building code and fire code regulations.
• Problem – Metallic penetrants and sleeves, at a density of 7.9 kg/L are denser than common firestops or concrete. Consequently, during a fire, they will absorb more heat and conduct it to the unexposed side of a fire barrier (thus "cooling" the exposed side at the expense of the unexposed side), such as the cold side of a firewall. This is undesirable. Even if the fire is stopped by the barrier, one must keep the unexposed side cool to prevent autoignition of combustibles on the unexposed side of a fire barrier. The unexposed side may very well be an area of refuge, which must be safeguarded to comply with the building code. Greater penetrant and sleeve conductivity leads to lower T-ratings. Higher density firestops, such as firestop mortars act as heat sinks to absorb heat away from small penetrants, such as cables, thus increasing T-ratings.
• Benefit – a rare exception where heat sinks are beneficial in firestops is where intumescents must be activated, such as in a firestop containing a plastic pipe. Heat sinks such as wire mesh and extra metallic sleeving may be used to carry heat to intumescents to activate expansion which should choke off a melting plastic pipe or melting pipe covering, such as foamed plastic or fibreglass.
In fireproofing of structural steel as well as providing circuit integrity to cables, cable trays, junction boxes and electrical conduit, the metallic items that are protected by the fireproofing measures act as heat sinks. Fireproofing methods are used to defeat the heat sink properties of the items they protect. In the case of circuit integrity measures, electrical services will fuse and short circuit above 140°C.
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, 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 .
Methods to determine heat sink thermal performance
In general, a heat sink performance is a function of material thermal conductivity, dimensions, fin type, heat transfer coefficient, air flow rate, 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 in applications, numerical methods or 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
: Thermal resistance and heat transfer coefficient plotted against flow rate for the specific heat sink design used in . The data was generated using the equations provided in the article. The data shows that for an increasing air flow rate, the thermal resistance of the heat sink decreases.
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, Rb, and the resistance in the fins, Rf. The heat sink base thermal resistance, Rb, can be written as follows if the source is a uniformly applied the heat sink base. If it is not
where tb is, then the base resistance is primarily spreading resistance:
(4) the heat sink base thickness, k is the heat sink material thermal conductivity and Ab is the area of the heat sink base.
The thermal resistance from the base of the fins to the air, Rf, can be calculated by the following formulas.
Active In SP
Joined: Feb 2012
27-02-2012, 05:08 PM
This is bilal from mechanical 8th sem, i am searching synopsis of composition of alluminium alloy-6061 with tungston carbide.
plz send me the synopsis.
|Tagged Pages: design of heat sinks, heat sink doc, straight fin heat sink vs pin fin heatsink, dymalloy performance vs copper, make of pin fin heat sink, seminar topics for mechanical heat sink, heat sink doc mechanical,|
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