Components, Electronics & Scientific Guide

High performance cooling for critical assemblies

The increasing miniaturization and performance increase of electronic components and power electronics generates increasing demand for high-performance cooling despite increasing electrical efficiency. High performance with minimum space inevitably makes high thermal loads. Since usually, the heat-transferring surface limits the heat dissipation from the electronics, the targeted influencing, as well as the improvement of the heat transfer, is the key to spreading the limits of heat dissipation.
The Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM in Dresden has been working for more than 25 years on materials and manufacturing technologies for compact heat and mass transfer systems, which are also used in cooling applications. The developed metallic structures are characterized by a large volume-specific exchange surface and high porosity. The process of 3D screen printing, as well as metallic fiber structures, occupy a special position.
3D screen printing is based on the structured, layer-by-layer application of metal-containing screen printing pastes with a printing screen. After each pressure, drying takes place. This cycle is repeated until the target height of the component is reached. Another operation then freed the produced green particles from powder particles and organic from the organic components and sintered them into a compact, solid component.

The printed components are characterized above all by high structuring resolution and surface quality as well as low residual porosity. Due to the possibility of producing components with structural details of up to 60 μm and aspect ratios greater than 100, high specific surfaces can be realized.

Fiber structures are made up of many metallic fibers, which connect a sintering process to one another in a material-locking manner. For the production of fibers, processes such as the melt extraction process further developed by Fraunhofer IFAM or the commercial process of stripping are used. This makes it possible to use a wide variety of materials, including copper, aluminum, iron and their alloys.

The further processing of the fibers with final heat treatment (sintering) produces a highly porous, open and anisotropic structure. This stands out from known porous structures by directionally optimized properties. The resulting fiber structures have pore sizes of 50 to 500 microns and a porosity (air content) of 60 to 90 percent.

Temperature control of electronic components with micro heat exchangers

Highly integrated, power electronic components are increasingly increasing their efficiency and compactness. This results in the challenge of dissipating the resulting heat loss via steadily decreasing contact surfaces. Convective cooling applications with air require relatively large areas due to the limited heat transfer that researchers and developers create through heat pipes and fins that require a lot of space.

As an alternative, the liquid cooling is available, which allows flat designs. The limited design of the heat exchanger still requires a heat spreader directly at the heat source. However, this additional thermal resistance reduces the efficiency of the cooling application. The 3D screen printing process allows complex structures to be created in a small space. An optimized flow guidance, as is the case with a meander structure, can dissipate conventional heat outputs of up to 100 W over an area of approximately 2.6 cm 2. The use of a heat spreader is unnecessary. Only the water-carrying area is colored according to the water temperature. The 3D screen printed copper component is also heated.

The Fraunhofer IFAM tested the microwave transformer in the laboratory for various applications. The power loss of a Desktop CPU of 85 W (TDP) can thus be dissipated in a pressureless system already with a water volume flow of 0.017 L / min. With a corresponding increase in the system pressure, it is also possible to dissipate larger amounts of heat.

The geometry of the heat exchanger is variable over a wide range. Further alternative geometries for the internal structure of the heat exchanger. The flow is distributed in these cases and increases the heat transfer area by pin structures. Due to the larger flow cross-section, these variants produce lower pressure losses than meander structures.

Heat dissipation through structured surfaces for evaporation

In the evaporation of a liquid cooling medium, the achievable heat transfer coefficients and thus the dissipated heat per area are significantly higher than in a pure liquid or air cooling. Intelligent structuring of the evaporation surface also makes it possible to exceed this limit. The experimental comparison of the structured and smooth surface shows a significant increase in heat removal in the case of evaporation in nucleate boiling.

The 3D-screen-printed, detailed structures can use different effects. For example, the star-shaped copper structure conducts the heat far into the fluid. In addition, the filigree tips increase the heat transferring surface. Together, the heat dissipation improves by about 50 percent relative to the flat surface.

During evaporation, the system must supply a large amount of heat to the liquid in a small space. Especially with materials with high thermal conductivity is absolutely necessary to ensure sufficient fluid supply to prevent the formation of a thermally insulating vapor cushion. 3D screen printing escapes this problem by allowing optimized cooling structures to be printed on a base plate to ensure fluid delivery. This base plate directly contacts the assembly to be cooled.

Highly porous fiber structures transport the heat from the hot wall into the liquid via the enormously enlarged contact area (> 5000 m² / m³). The large void content allows a steady supply of liquid. Coupled with the good heat conduction in the solid structure, the evaporation can be maintained even under demanding conditions. Compared to a smooth surface, the heat flow increases by more than 75 percent. A variable range of materials, pore sizes and porosity allows the adaptation of liquid and heat transfer to a variety of applications to optimize heat and mass transfer.

The application of the structures shown by way of example lies above all in the high-power range, when high heat loss currents have to be dissipated from a limited surface of electronic components. They allow the heat transfer to be improved both in liquid cooling and in cooling applications using evaporation. An application in heat pipes (heat pipes) or on the heat pipe principle based surface coolers (Vapor Chambers) is also realistic possible.

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