Heat sink

Heat sink

Overview

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 exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s used in refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 and air conditioning systems and the radiator
Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...

 (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.
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Encyclopedia

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 exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s used in refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 and air conditioning systems and the radiator
Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...

 (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 central processing unit
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 (CPU) or graphics processors
Graphics processing unit
A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...

. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die
Die (integrated circuit)
A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated.Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon or other semiconductor through processes such as...

 temperature of the processor(s). Thermal adhesive
Thermal adhesive
Thermal adhesive is a type of thermally conductive glue used for electronic components and heatsinks. Thermal adhesive is similar to thermal paste in that it is used for transferring heat from one surface to another...

 (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
Joseph Fourier
Jean Baptiste Joseph Fourier was a French mathematician and physicist best known for initiating the investigation of Fourier series and their applications to problems of heat transfer and vibrations. The Fourier transform and Fourier's Law are also named in his honour...

 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, , is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred.

Consider a heat sink in a duct, where air flows through the duct, as shown in Figure 2. It is assumed that the heat sink base is higher in temperature than the air. Applying the conservation of energy, for steady-state conditions, and Newton’s law of cooling to the temperature nodes shown in Figure 2 gives the following set of equations.
(1)
(2)

where (3)

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. 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 in 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.
  • Therefore, if there is no air or fluid flow around the heat sink, the energy dissipated to the air can not be transferred to the ambient air. Therefore, the heat sink functions poorly.
  • Furthermore, 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".

Other examples of situations in which a heat sink has impaired efficiency:
  • Pin fins have a lot of surface area, but the pins are so close together that air has a hard time flowing through them.
  • Aligning a heat sink so that the fins are not in the direction of flow.
  • Aligning the fins horizontally for a natural convection heat sink. Whilst a heat sink is stationary and there are no centrifugal forces and artificial gravity, air that is warmer than the ambient temperature always flows upward, given essentially-still-air surroundings; this is convective cooling.

Material


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. 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 synthetic diamond
Synthetic diamond
Synthetic diamond is diamond produced in a technological process; as opposed to natural diamond, which is created in geological processes. Synthetic diamond is also widely known as HPHT diamond or CVD diamond, denoting the production method, High-Pressure High-Temperature synthesis and Chemical...

 is used as submounts for high-power integrated circuits and laser diodes.

Composite materials can be used. Examples are a copper-tungsten
Copper-tungsten
Copper-tungsten alloy is a pseudo-alloy of copper and tungsten. As copper and tungsten are not mutually soluble, the material is composed of distinct particles of one metal dispersed in a matrix of the other one...

 pseudoalloy, AlSiC
AlSiC
AlSiC, pronounced "alsick", is a metal matrix composite consisting of aluminium matrix with silicon carbide particles. It has high thermal conductivity , and its thermal expansion can be adjusted to match other materials, e.g. silicon and gallium arsenide chips and various ceramics...

 (silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

 in aluminium matrix), Dymalloy
Dymalloy
Dymalloy is a metal matrix composite consisting of 20% copper and 80% silver alloy matrix with type I diamond. It has very high thermal conductivity of 420 W/, and its thermal expansion can be adjusted to match other materials, e.g. silicon and gallium arsenide chips...

 (diamond in copper-silver alloy matrix), and E-Material
E-Material
E-Material, also called E Material, is a metal matrix composite consisting of beryllium matrix with beryllium oxide particles. It has high thermal conductivity , and its thermal expansion can be adjusted to match other materials, e.g. silicon and gallium arsenide chips and various ceramics...

 (beryllium oxide
Beryllium oxide
Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...

 in beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 matrix). Such materials are often used as substrates for chips, as their thermal expansion coefficient can be matched to ceramics and semiconductors.

Fin efficiency


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. 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. This factor is called the fin efficiency and 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). Equations 6 and 7 are applicable for straight fins. (6)
(7)

Where:
  • hf is the convection coefficient of the fin
    • Air: 10 to 100 W/(m2K)
    • Water: 500 to 10,000 W/(m2K)
  • k is the thermal conductivity
    Thermal conductivity
    In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

     of the fin material
    • Aluminum: 120 to 240 W
      Watt
      The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

      /(m
      Metre
      The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

      ·K
      Kelvin
      The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

      )
  • Lf is the fin height (m)
  • tf is the fin thickness (m)


To increase the fin efficiency of fins:
  • Decrease the fin aspect ratio
    Aspect ratio
    The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...

    , by:
    • Increasing the fin thickness, or
    • Decreasing the fin length
  • Increase the thermal conductivity of the fins, (e.g. by using copper instead of aluminum)

Spreading resistance



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 vapor chamber or heat pipe
    Heat pipe
    A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....

     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 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.

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 . 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.
Comparison of a pin fin and straight fin heat sink of similar dimensions. Adapted from data of
Heat sink fin type Width [cm] Length [cm] Height [cm] Surface area [cm²] Volume [cm³] Temperature difference, Tcase-Tair [°C]
Straight 2.5 2.5 3.2 58 20 44
Pin 3.8 3.8 1.7 194 24 51


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.

Surface color


The heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 from the heatsink occurs by convection of the surrounding air, conduction through the air, and radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal 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 effected significantly by surface emissivity
Emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...

.

In situations where convection is low, such as a flat non-finned panel with low airflow, radiative cooling
Radiative cooling
Radiative cooling is the process by which a body loses heat by thermal radiation.- Earth's energy budget :In the case of the earth-atmosphere system it refers to the process by which long-wave radiation is emitted to balance the absorption of short-wave energy from the sun.The exact process by...

 can be a significant factor. Here the surface properties may be an important design factor. Matte-black surfaces will radiate much more efficiently than shiny bare metal in the visible spectrum.http://ludens.cl/Electron/Thermal.html A shiny metal surface has low low effective emissivity due to its low surface area. While the emissivity of a material is tremendously energy (frequency) dependent, the noble metals demonstrate very low emissivity in the NIR spectrum. The emissivity in the visible spectrum is closely related to color. 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
Selective surface
In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency...

s".

In a vacuum or in outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

, 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 Kelvin) surface facing the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 will absorb a lot of radiant heat, since the sun's surface temperature is nearly 6000 Kelvin, whereas the same surface facing deep-space will radiate a lot of heat, since deep-space has an effective temperature of only a few Kelvin.

Processor/Microprocessor cooling


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.

Attachment methods for microprocessors and similar ICs


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 else 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. 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:
  1. Surfaces of both the component and heat sink must be clean, with no residue such as a film of silicone grease.
  2. 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.
  3. Thicker tapes tend to provide better "wettability" with uneven component surfaces. "Wettability" is a term used to describe 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 "wettablilty" with minimum thermal resistance.

Epoxy
Epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

 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
Ball grid array
A ball grid array is a type of surface-mount packaging used for integrated circuits.- Description :The BGA is descended from the pin grid array , which is a package with one face covered with pins in a grid pattern. These pins conduct electrical signals from the integrated circuit to the printed...

 (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. Examples of commercially available clips are the maxiGRIPTM and superGRIPTM range from Advanced Thermal Solutions (ATS) and the Talon ClipTM from Malico. The three aforementioned clipping methods use plastic frames for the clips, but the ATS designs uses metal spring clips to provide the compression force. The Malico design uses the plastic "arm" to provide a mechanical load on the component. Depending on the product requirement, the clipping methods will have to meet shock and vibration standards, such as Telecordia GR-63-CORE, ETSI 300 019 and MIL-STD-810.
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.
Summary of heat sink attachment methods
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. $
Epoxy Strong mechanical adhesion. Relatively inexpensive. Makes board rework difficult since it can damage component. Surface must be cleaned for optimal adhesion. $$
Wire form Z-clips Strong mechanical attachment. Easy removal/rework. Applies a preload to the thermal interface material, improving thermal performance. Requires holes in the board or solder anchors. More expensive than tape or epoxy. Custom designs. $$$
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. $$$
Push pin with compression springs Strong mechanical attachment. Highest thermal interface material preload. Easy removal and installation. Requires holes in the board which increases complexity of traces in PCB. $$$$
Stand-offs with compression springs Strongest mechanical attachment. Highest preload for the thermal interface material. Ideal for large heat sinks. Requires holes in the board which increases complexity of trace layout. Complicated assembly. $$$$$

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, or may not, be important, depending upon electrical design details.
Selection Based on Interface Gap
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
Polyimide
Polyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...

, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 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
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
Electrical insulation Dielectric strength Typical values Product types available
Not required N/A N/A N/A Thermal grease
Thermal grease
Thermal grease is a viscous fluid substance, originally with properties akin to grease, which increases the thermal conductivity of a thermal interface by filling...

, epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

, phase change materials, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 or aluminium tapes.
Required Low Silicone coated fabrics, gap fillers
Required High Polyimide
Polyimide
Polyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...

 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.
+



Light emitting diode lamps


Light emitting diode (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.

Firestopping and fireproofing



A heat sink is rarely a desired thing in passive fire protection
Passive fire protection
Passive fire protection is an integral component of the three components of structural fire protection and fire safety in a building. PFP attempts to contain fires or slow the spread, through use of fire-resistant walls, floors, and doors...

. Rather, it is usually a problem that must be overcome to maintain fire-resistance rating
Fire-resistance rating
A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for...

s. The proven ability to overcome heat sinks in construction is subject to building code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

 and fire code regulations.

Firestopping

  • Problem – Metallic penetrant
    Penetrant
    Penetrants, or penetrating items, are the mechanical, electrical or structural items that pass through an opening in a wall or floor, such as pipes, electrical conduits, ducting, cables and cable trays, or structural steel beams and columns...

    s and sleeves
    Sleeve (construction)
    In construction, a sleeve is used both by the electrical and mechanical trades to create a penetration.-Materials:Sleeves can be made of:* sections of steel pipe* plastic* sheet metal* proprietary devices that are listed firestop components...

    , at a density of 7.9 kg/L are denser than common firestop
    Firestop
    A firestop is a passive fire protection system of various components used to seal openings and joints in fire-resistance rated wall and/or floor assemblies, based on fire testing and certification listings....

    s or concrete
    Concrete
    Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

    . 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
    Firewall (construction)
    A firewall is a fireproof barrier used to prevent the spread of fire between or through buildings, structures, electrical substation transformers, or within an aircraft or vehicle.- Applications :...

    . 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
    Area of refuge
    An area of refuge is a location in a building designed to hold occupants during a fire or other emergency, when evacuation may not be safe or possible. Occupants can wait there until rescued by firefighters...

    , which must be safeguarded to comply with the building code. Greater penetrant and sleeve conductivity leads to lower T-ratings
    Fire-resistance rating
    A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for...

    . Higher density
    Density
    The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

     firestops, such as firestop mortars
    Mortar (firestop)
    Firestop mortars are mortars most typically used to firestop large openings in walls and floors required to have a fire-resistance rating. They are passive fire protection items...

     act as heat sinks to absorb heat away from small penetrants, such as cable
    Cable
    A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

    s, thus increasing T-ratings.

  • Benefit – a rare exception where heat sinks are beneficial in firestops is where intumescent
    Intumescent
    An intumescent is a substance which swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Intumescents are typically used in passive fire protection and, in America, require listing and approval use and compliance in their installed configurations in order to...

    s must be activated, such as in a firestop containing a plastic
    Plastic
    A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

     pipe
    Pipe (material)
    A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...

    . 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.

Fireproofing


In fireproofing
Fireproofing
Fireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...

 of structural steel
Structural steel
Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties...

 as well as providing circuit integrity
Circuit integrity
Circuit integrity refers to the operability of electrical circuits during a fire. It is a form of fire-resistance rating. Circuit integrity is achieved via passive fire protection means, which are subject to stringent listing and approval use and compliance.-Fireproofing:Providing fireproofing for...

 to cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

s, cable tray
Cable tray
In the electrical wiring of buildings, a cable tray system is used to support insulated electric cables used for power distribution and communication. Cable trays are used as an alternative to open wiring or electrical conduit systems, and are commonly used for cable management in commercial and...

s, junction box
Junction box
An electrical junction box is a container for electrical connections, usually intended to conceal them from sight and meter tampering. A small metal or plastic junction box may form part of an electrical conduit wiring system in a building, or may be buried in the plaster of a wall, concealed...

es and electrical conduit
Electrical conduit
An electrical conduit is an electrical piping system used for protection and routing of electrical wiring. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Flexible conduit is available for special purposes....

, 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
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

 above 140°C.

In soldering


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, 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 in applications, numerical methods or computational fluid dynamics
Computational fluid dynamics
Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...

 (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, , and the resistance in the fins, . The heat sink base thermal resistance, , 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: (4)

where is the heat sink base thickness, is the heat sink material thermal conductivity and is the area of the heat sink base.

The thermal resistance from the base of the fins to the air, , can be calculated by the following formulas. (5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(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, can be calculated as:
(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 Figure 14. It shows that 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, as shown in figure 15. However, figure 15 shows a test setup for a ducted flow heat sink application. 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. Another problem with the set up, be it unducted or ducted, is the losses into the board. These must be taken into account. 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. Numerical methods or computational fluid dynamics (CFD) provide a qualitative (and sometimes even quantitative) prediction of fluid flows. 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, but the quantitative or absolute accuracy of the result is not guaranteed.

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, the results still need to be validated. Another problem with CFD is that the inputs need to be correct. It is the classic case of "Garbage in, garbage out."

See also

  • Computer cooling
    Computer cooling
    Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits.Various cooling methods help to improve processor performance or reduce the noise of cooling fans....

  • Heat spreader
    Heat spreader
    A heat spreader is most often simply a copper plate, having high thermal conductivity. Functionally, it is a heat exchanger that moves heat between a heat source and a secondary heat exchanger whose surface area and geometry are more favorable. By definition, the heat is "spread out", such that...

  • Heat pipe
    Heat pipe
    A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....

  • Heat pump
    Heat pump
    A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...

  • Radiator
    Radiator
    Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...

  • Thermal management of electronic devices and systems
    Thermal management of electronic devices and systems
    Heat generated by electronic devices and circuitry must be dissipated to improve reliability and prevent premature failure. Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as liquid cooling....

  • Thermal resistance in electronics
    Thermal resistance in electronics
    Thermal resistance is a heat property - and a measure of a temperature difference, by which an object - or material resist a heat flow...