Heat sink

A heat sink is a body that increases the heat-emitting surface of a heat-producing component. This can prevent possible damage from overheating.

Various aluminum heat sinks

function

The heat transfer from a heat source to the surrounding cooling medium (mostly air , but also water or other liquids) is primarily dependent on the temperature difference, the effective surface and the flow rate of the cooling medium. A heat sink has the task of heat loss by conduction divert from the heat generating component and these then by thermal radiation and convection to give to the environment. To keep the thermal resistance as low as possible,

1. the heat sink is made of a material that conducts heat well
2. have a dark and as large as possible surface
3. can be installed vertically in order to support air circulation through the chimney effect .

Executions

Various aluminum profiles for heat sinks

Heat sinks usually consist of a metal with good thermal conductivity , usually aluminum or copper . In industrial mass production, parts of an aluminum or sheet steel housing are often used as heat sinks.

A more recent development are heat sinks made from ceramic materials ( aluminum oxide and aluminum nitride ), which are to be used in particular to dissipate heat in power electronics and LED applications.

Depending on the requirements, heat sinks are manufactured in a wide variety of designs:

• ribbed metal block, usually made of aluminum by extrusion
• For coolers made of copper as a solid metal plate with pressed or (rarely) soldered in lamellas made of copper or aluminum, but also milled from solid material .
• punched and bent sheets
• Attachable cooling stars and cooling flags made of aluminum, spring bronze or sheet steel

The component to be cooled is connected to one another by screws , clamps, gluing or clamps with the smallest possible spacing. A side effect is the mechanical fastening that often takes place, especially with screws.

In order to promote the transfer of heat, the contact surfaces are made flat or two-dimensionally flat by mechanical processing (milling, turning , grinding ).

Other materials are used in microelectronics, but here they primarily serve to distribute heat within components. Is in addition to good thermal conductivity and electrical insulation necessary is diamond with a roughly five times better thermal conductivity coefficient used to silver. A more recent development are carbon nanotubes with a thermal conductivity coefficient that is almost fourteen times better than that of silver .

Applications

Heat sinks are used in power electronics and in computers , especially for cooling power semiconductors , e.g. B. in electric locomotives , in output stages of HiFi amplifiers, in power supplies , for Peltier elements in cool bags, or for processors . Heat sinks are also required for radioisotope generators that are used to supply satellites or remote lighthouses with electrical energy. LED-operated luminaires also require heat sinks to dissipate the heat, which is particularly common with more powerful LEDs, and to lower the temperature.

Dimensioning and calculation

Transistor in the JEDEC TO-5 housing with slid-on aluminum cooling star

External heat sink for a water-cooled processor

The shape and size of the heat sink mainly depend on the initial physical conditions.

The characteristic quantity for the specification of a heat sink, the absolute thermal resistance , is equivalent to the electrical resistance and is calculated with:

${\displaystyle R_{\mathrm {thK} }=a\cdot {\frac {T_{\mathrm {i} }-T_{\mathrm {U} }}{P_{\mathrm {Q} }}}-b\cdot (R_{\mathrm {thG} }+R_{\mathrm {thM} })}$

used parameters:

${\displaystyle R_{\mathrm {th} }}$ = General thermal resistance

${\displaystyle R_{\mathrm {thK} }}$ = Thermal resistance of the heat sink

${\displaystyle R_{\mathrm {thG} }}$ = Thermal resistance of the component housing to be cooled

${\displaystyle R_{\mathrm {thM} }}$= Thermal resistance of the connection surface (thermal paste or thermal adhesive ) between component and heat sink

${\displaystyle T_{\mathrm {i} }}$= Temperature of the heat source, is, for example, the maximum temperature of the barrier layer in the case of semiconductor components

${\displaystyle T_{\mathrm {U} }}$ = Temperature of the cooling medium

${\displaystyle P_{\mathrm {Q} }}$ = heat output to be dissipated from the component to be cooled

The thermal resistance forms the basis for further calculations for the design of the heat sink. On this basis, can now additional factors and be involved, for example: ${\displaystyle a}$ ${\displaystyle b}$

• Type and properties of the cooling medium (gas or liquid)
• free or forced convection
• horizontal or vertical installation position
• (dark) color of the heat sink surface
• retroactive warming by possibly
  • an inevitable increase in the ambient temperature
  • reflected thermal radiation

With free convection in closed rooms, the installation location in the room, the installation position and the design of the heat sink must also be taken into account. A vertical installation position with vertically aligned cooling fins corresponds to the optimum. If, on the other hand, the heat sink is installed horizontally, the arrangement of the cooling fins plays an important role. In this case, the cooling performance is favored if the cooling fins are arranged perpendicular and not parallel to the longitudinal axis of the heat sink.

The shape and size of the heat sink are also dependent on economic factors. There may be special features of the components to be cooled that must be taken into account.

Passive heat sinks

Cooled transmission tube

A passive heat sink works primarily through convection : The ambient air is heated, becomes specifically lighter and thus rises, causing cooler air to flow in. At higher temperatures, thermal radiation also plays a role, which is why the surfaces of heat sinks in electronics are often anodized (see black bodies ). This increases the emissivity to close to one in the relevant wavelength range (by 10 µm) . The color of the anodization - contrary to the widespread assumption that heat sinks must be black - has no meaning; the color only affects the visible wavelength range, which does not play a role in the temperature range commonly used in electronics (<150 ° C).

The most common material used for passive heat sinks is aluminum. The reasons are:

• relatively low material price
• easy processing ( extruded profiles )
• low density
• high heat capacity
• satisfactory thermal conductivity

Although copper has a higher thermal conductivity, it is heavier, more expensive and more difficult to process. It is therefore mainly used for active coolers.

Active heat sinks

An active heat sink usually has an electrically driven fan wheel to guide sufficient air mass along the body. Power requirements and noise can be reduced if the fan speed is controlled depending on the temperature . Active coolers also include liquid cooling.

Heat sinks with forced cooling or forced air cooling achieve up to six times the cooling capacity of a passive cooler based only on convection with the same material expenditure and can therefore be built very compactly. Disadvantages are the resulting noise , as well as the risk of overheating from dust, dirt or fan failure. Sensors therefore often monitor the fan speed, the air flow or the temperature. An axial fan can be mounted on a heat sink above the cooling fins in such a way that it blows in the direction of the cooling fins (vice versa, this is unfavorable) or is sunk (integrated) in the heat sink. Arrangements in which a flat radial fan blows from the side through the cooling fins require little height.

Active coolers often have much finer ribs than passive coolers and are therefore not suitable for pure convection cooling due to the high flow resistance.

In the case of high-performance diode lasers , “active cooling” means a so-called microchannel cooler ( liquid cooler with very fine cooling channels, close to the heat source, with strong flow). In contrast, in this branch of industry, “passive cooling” is a heat exchanger in which the heat initially spreads through heat conduction.

Heat pipes

Processor cooler with copper heat pipes

A heat pipe does not replace a heat sink, but only serves to transport heat or improve heat distribution and dynamic behavior. It is often used in tight spaces to conduct heat from the component to the actual heat sink. The heat pipe principle is used in notebooks and very compact power electronics assemblies, and increasingly also for cooling assemblies such as graphics cards, chipsets and of course processors in high-performance computers.

Another area of ​​application is satellites and space travel, as there is no air for direct heat dissipation. A heat pipe conducts the thermal energy to the outer wall, where large, dark radiators provide radiation.

Assembly

Some insulating washers, three mica washers on the left, a silicone rubber pad on the right

The unevenness of the surfaces remaining during assembly leads to air pockets which - due to the comparatively low thermal conductivity of air - lead to so-called hot spots . To compensate for these unevenness in order to create a better heat transfer from the component to the heat sink, a thin layer of thermal paste is usually applied before assembly . Thermal pads are used when isolated mounting ( galvanic separation ) or mounting at a distance is required. Discs made of mica , ceramic (Al ₂ O ₃ , BeO), silicone rubber or a special plastic polyimide ("Kapton" ®) are used. The latter can in turn be coated, so that an intimate connection is created when they are heated for the first time. Excellent mechanical fastening and very good thermal conductivity can be achieved with thermally conductive adhesives. These mostly two-component adhesives harden under the influence of heat within a few hours and are already hand-tight after a few minutes.

See also

• Cooling fin , cooling

Web links

Commons : heat sink  - collection of images, videos and audio files

• Online design of axial fans: calculation application for dimensioning axial fans for cooling
• Special features when calculating heat sinks for high-performance LEDs ( Memento from December 9, 2011 in the Internet Archive )

Individual evidence

1. ↑ Materials in Technical Ceramics, Chapter Nitrides . Association of the Ceramic Industry, accessed October 25, 2009 . 
2. ↑ K.-D. Linsenmeier: Technical ceramics - material for the highest demands . In: The library of technology . tape 208 , 2010, ISBN 978-3-937889-97-9 , pp. 62 .
3. ↑ Heat sink as circuit carrier. (No longer available online.) Weka Fachmedien, archived from the original on October 11, 2009 ; Retrieved April 22, 2009 .