Heat sink

A heat sink increases the heat-emitting surface of a heat -producing component. Thus, a possible damage due to overheating can be prevented.

Function

The heat transfer from a heat source to the surrounding cooling medium ( usually air, but also water or other fluids ) is dependent primarily on the temperature difference, the effective surface area and the flow velocity of the cooling medium. A heat sink has the task of conduct away heat loss by conduction from the heat-generating component and then leave by radiation and convection to the environment. In order to keep the thermal resistance as low as possible, it must

Heatsinks are typically of a highly thermally conductive metal, usually aluminum or copper. In industrial mass production, parts of an aluminum or steel sheet metal housing is often used as a cooling body.

A more recent development is the heat sink made ​​of ceramic materials (aluminum oxide and aluminum nitride ), which are coming especially to dissipate heat in the power electronics and LED applications.

Depending on the requirements heatsinks are manufactured in various versions:

• Ribbed metal block, usually made of aluminum by extrusion
• Cooler made ​​of copper milled as solid metal plate with pressed or (rarely ) soldered fins made ​​of copper or aluminum, but also from solid material.
• Punched and bent sheets
• Attachable cooling Stars and cooling fins of aluminum, bronze or spring steel sheet

The component to be cooled is connected by screws, clamps, glue or staples with a minimum distance among themselves. A side effect is often held mechanical attachment, especially by screws.

In order to promote the transfer of heat, the contact surfaces are to each other just made ​​two-dimensional plane or by mechanical machining (milling, turning, grinding ).

In microelectronics, other materials are used, they are used here, however, mainly the heat distribution within components. In addition to a good heat conductivity and electrical insulation required, diamond is used having a coefficient of thermal conductivity approximately five times greater compared with silver. A recent development are carbon nanotubes with an almost four times better thermal conductivity coefficient over silver.

Applications

Heatsinks are used in power electronics and computing, especially for cooling of power semiconductors, such as in electric locomotives, power amplifiers in hi-fi amplifiers in power supplies, for Peltier elements in cooler bags, or even for processors. Also radioisotope generators are used to supply satellite or remote lighthouses with electrical energy, heat sinks are required. And LED lights powered require a heat sink to dissipate the heat which forms in particular efficient and to reduce the temperature of LEDs.

Dimensioning and calculation

Shape and size of the heat sink are mainly dependent on physical initial conditions.

The characteristic variable for the specification of a cooling body, the absolute thermal resistance is equivalent to the electrical resistance, and is calculated by:

Used parameters:

The thermal resistance is the basis for further calculations in the construction of the heat sink. Based on that additional factors and can now be included, for example:

• The nature and characteristics of the cooling medium ( gas or liquid)
• Free or forced convection
• Horizontal or vertical installation
• ( dark ) color of the heat sink surface
• Retroactive heating by possibly necessarily related increase in the ambient temperature
• Reflected heat radiation

In free convection in enclosed spaces the installation location in space, the mounting position and the shape of the heat sink must be taken into account. A vertical mounting position with vertically oriented fins corresponds to the optimum. The heat sink on the other hand horizontally installed, the arrangement of the cooling ribs play an important role. In which case, the cooling performance is enhanced when the cooling fins are arranged perpendicular and not parallel to the longitudinal axis of the heatsink.

But shape and size of the heat sink are also dependent on economic factors. There are observed under certain circumstances special features of the components to be cooled.

Passive heatsink

A passive heat sink works mainly by convection: The ambient air is heated, less dense and rises to it, so cooler air. At higher temperatures, the heat radiation is involved, and therefore the surfaces of heat sinks in the electronics field will often anodized ( see black body ). This increases in the relevant wavelength range ( about 10 microns) of the emissivity to near unity. The color of the anodizing plays against the widespread assumption that heatsink would have to be black, does not matter since it concerns only the visible wavelength range.

The most commonly used material for passive heat sink is aluminum. The reasons are:

• Low material price
• Light processing ( extrusion profiles)
• Low density
• High heat capacity
• Satisfactory thermal conductivity

Although copper has a higher thermal conductivity, but is heavier, more expensive and difficult to edit. It is therefore primarily for active cooler for the application.

Active Heatsink

An active heatsink has a mostly electrically powered fan to provide sufficient lead air mass along the body. Power requirement and noise can be reduced if the fan speed is controlled by temperature. The active coolers include liquid cooling.

Heatsink with forced cooling or forced air cooling to achieve at the same material costs to about six times the cooling capacity of a system based only on passive convection cooler and can therefore be made very compact. Disadvantages are the resulting noise, also the risk of overheating by dusting, dirt or fan failure. Sensors therefore frequently monitor the fan speed, air flow and temperature. An axial fan may be mounted on a heat sink on the cooling fins so that it blows in the direction of the cooling fins ( Conversely, the unfavorable) or buried in the heat sink (integrated) is. Little amount need arrangements in which a flat radial fan from the side blowing through the cooling fins.

Coolers can often have a much finer ribbing on as a passive cooler and are therefore not due to the high flow resistance for convection cooling suitable.

In high power diode lasers is meant by " active cooling " a so-called microchannel cooler ( chiller with very fine, lying close to the heat source, strongly perfused cooling channels ). Whereas, a in this industry under " passive cooling " a heat exchanger in which the heat initially propagates through thermal conduction.

Heat pipes ( heat pipes )

A heat pipe does not replace a heatsink, but serves only the heat transfer and improves heat distribution and the dynamic behavior. It is often used in tight spaces to conduct heat from the component to the actual heatsink. The heat pipe principle is used in notebooks and compact power electronic assemblies and, increasingly, for the cooling of components such as graphics cards, chipsets and processors of course in powerful computers.

Another area of ​​application satellites and space because there is no air for direct heat dissipation is present here. A heat pipe conducts the heat energy to the outside wall, where large, dark radiators provide radiation.

Assembly

The remaining when mounting unevenness of the surfaces lead to air pockets, the - lead to the so-called hot spots - because of the relatively low thermal conductivity of air. To compensate for these irregularities to produce a better heat transfer between the component to the heat sink, a thin layer of thermal compound is applied before assembly usually. Thermal pads used when an insulated mounting ( galvanic separation) or to an assembly clearance is required. There are sheets of mica, ceramics (Al2O3, BeO ), silicone rubber or special plastic polyimide ( "Kapton " ® ) are used. The latter may in turn be coated, so that in the first heating an intimate connection is produced. Excellent mechanical fastening and excellent thermal conductivity is achieved with Wärmeleitklebern. This usually two-component adhesives harden under heat from within a few hours and after a few minutes already by hand.