Thermal paste
Thermal paste is a paste that prevents heat transfer between two objects, e.g. B. a microchip and a heat sink improved.
composition
The composition of the respective thermal paste depends on the paste manufacturer, the thermal conductivity , the recommended application and the continuous operating temperature range of the paste. Classic thermal pastes mainly contain silicone oil and zinc oxide , high-priced versions are available with aluminum , copper , graphite and silver components. Silicone-free pastes are also offered. Relatively new on the market are pastes that are based on thermoplastics , similar to thermal pads . There are differences in the thermal conductivity of the various pastes : Commercially available thermal pastes have a thermal conductivity of 0.8 W / (mK) up to values of around 10 W / (mK). Exact values are given in the respective technical data sheets. For comparison: the metal copper has a thermal conductivity of approx. 380 W / (m · K), air approx. 0.024 W / (m · K).
Thermally conductive pastes made of liquid metal , these are metallic alloys with a comparatively very low melting point, which primarily consist of gallium and various additional metals such as indium , rhodium , silver , zinc and tin , conduct heat much better than conventional pastes. Usual values for thermal conductivity are in the range from 40 W / (m · K) to 80 W / (m · K). Since liquid metals are electrically conductive, they must be used with caution. It is not possible to use gallium-based liquid metal on aluminum heat sinks, as the formation of a local element with the aluminum removes the oxide layer that protects the base aluminum, which in conjunction with the constant air humidity leads to the formation of the corresponding hydroxide.
Mode of action
The mounting surfaces of heat sinks always contain more or less deep grooves, grooves, elevations and depressions due to mechanical processing. Even if these are small, the resulting cavities inhibit the transfer of heat by reducing the effective area. The same applies to the component to be cooled: processors often have lasered inscription that leads to indentations. Metal flanges of power semiconductors ( called heatspreaders ) that serve to dissipate heat are often uneven due to the manufacturing process ( punching ). Thermal compounds fill these unevenness and thus enable better heat transfer from the component / chip / processor to the heat sink. In contrast to some types of thermal pads, they are not intended or suitable for bridging large distances between the heat source and the heat sink.
Areas of application
Electrical engineering and electronics
Thermally conductive paste is often used in connection with small components to be cooled, for example processors and components of power electronics , in order to improve the heat transfer from the component to the heat sink. This is particularly important when the component that is to be cooled has a large thermal power loss (heat dissipation) per area.
Engine construction
Thermally conductive pastes are also used in engine construction: an example is the three-cylinder two-stroke radial engine from König. It is mainly used in light aircraft and has a combustion chamber that is closed towards the cylinder head. The cylinder head is therefore not used for sealing as usual, but only for cooling. It has only one cooling surface towards the cylinder. In order to better dissipate the heat of combustion generated at the cylinder base, it is absolutely necessary to use thermal conductive paste between the cylinder base and the cylinder (cooling) head with this unusual cylinder shape.
Heating and cooling devices
In Peltier cooling devices , thermal paste is used between the Peltier element and the heat exchangers. In the sanitary area and heating construction, thermal paste is used, for example, for better heat transfer to thermostats or sensors. Another use is freezing devices, which can freeze pipes via cooling lines at a certain point in order to e.g. B. to be able to carry out maintenance or installation work without emptying or downtime. Here, the thermal grease is applied to the connection point on the pipe to promote heat transfer.
Application of thermal paste
General
The correct, "best" application of thermal paste to a processor is controversial, but all approaches pursue the goal of keeping the layer thickness of the thermal paste as low as possible. The layer thickness should be such that the thermal grease completely fills the cavities between the two bodies, but does not further increase the distance between the two bodies. A common mistake when using it is to use too much paste.
Application to electronic components
Typically, a small amount of thermal grease is applied to the the or heatspreader applied, the further procedure is dependent on the manufacturer's recommendations and the personal preference.
One method is to put the cooler on immediately after placing a blob of paste in the center of the processor core or heat spreader. The paste is distributed by the contact pressure of the heat sink and forms a thin, circular layer. For processors without heatspreader or other semiconductors with a small contact area, this is the simplest and safest option. Thermally conductive pastes based on thermoplastics should always be applied in this way according to the manufacturer's instructions. With such pastes, the optimal distribution only takes place through heating during operation.
Another method is to smear the thermal paste before putting on the cooler. For this purpose, a small blob of thermal paste is also placed on the middle of the processor core or heat spreader, but this is then spread so that it covers the entire heat spreader or processor core. Old residual thermal paste should be removed. In the case of processors with heat spreader or power semiconductors in relatively large housings, a more even distribution of the paste can be achieved. Then the cooler or the semiconductor is mounted. An inclusion of air bubbles, which lead to a reduction in thermal conductivity, cannot be ruled out with this method.
Alternatives to thermal paste
Thermal pads
As an alternative to the various pastes, heat-conducting pads are often used, which are much easier to handle during initial assembly. If galvanic separation between the component and the heat sink is necessary, insulating heat-conducting pads or insulating washers can be used in conjunction with paste.
Thermally conductive adhesive
In addition, thermally conductive adhesives can also be used for better thermal coupling . These adhesives usually contain a proportion of particles or fillers that increase the thermal conductivity.
literature
- Chakravarti V. Madhusudana: Thermal Contact Conductance . 2nd Edition. Springer, 2013, ISBN 978-3-319-01276-6 .
Web links
Individual evidence
- ↑ Thermal Greases, Data Sheets. Aavid Thermalloy, LLC, accessed September 24, 2014 .
- ↑ Technical Datasheet KP 96, Keratherm Thermal Grease. (PDF) (No longer available online.) Keramische Folien GmbH, archived from the original on March 17, 2017 ; Retrieved September 25, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Safety data sheet "Coollaboratory Liquid Pro", liquid metal. (PDF) Coollaboratory, accessed September 25, 2014 .
- ^ Thermal Conductivity Of Liquid Metals. Electronics Cooling, accessed September 25, 2014 .
- ↑ Thermal compound made of liquid metal, safety data sheet. (PDF) Coollaboratory, accessed September 24, 2014 .
- ↑ ROFROST TURBO / - II (PDF; 3.33 MB) Rothenberger Group . Archived from the original on April 10, 2011.
