What makes efficient heat sinks?
3/16/2023 Technology & Processes Experts Know-how

What makes efficient heat sinks?

Efficient heat sinks are characterised by their properties. These include thermal conductivity, mechanical resilience and corrosion resistance. At the same time, the melt must remain castable in the production process and there must be no leakages at the end that could endanger the power electronics. The most efficient heat sinks are created in rheocasting. A guest article by Fabian Niklas.

Heat sink
To clarify what constitutes efficient heat sinks, we must look at the properties that are necessary for them. First, the thermal conductivity is crucial to be able to transport the heat away from the source. The mechanical properties must be sufficient to withstand the loads acting on the component without damage. The material must not be susceptible to corrosion, because heat sinks can be needed in the most unreal places. These are often difficult to reach in order to repair defects. Furthermore, there must be no leaks, because nothing is worse for power electronics than water and ice on the circuits. If the electronics fail, the entire heat sink loses its necessity.

Heat sink made of pure aluminium

According to this description, one possibility would be to take a large block of pure aluminium and mill the heat sink out of it. Pure aluminium has excellent conductivity and good corrosion resistance. However, this solution has two catches: this milling work is horrendously expensive and pure aluminium has very low strength.
Therefore, casting a suitable alloy is the logical consequence. Fine geometries and cooling fins can be produced by means of die casting. However, the alloys typically used in die casting have the disadvantage of having a rather high silicon content of seven to twelve percent. This is a significant disadvantage for thermal conductivity.

The thermal conductivity of pure aluminium is around 220 watts per metre per Kelvin. In metals, the Wiedemann-Franz law describes the correlation between electrical conductivity and thermal conductivity. Simplified, it can be said that a good electrical conductor is also a good heat conductor. This means that what disturbs the flow of electrons also disturbs heat conduction.

Every atom counts

At the atomic level, every foreign atom already disturbs the flow of electrons in the conduction band. At the structural level, there are several interference points. Each grain boundary, meaning the transition area between two grains of different orientation, reduces the conductivity. Different phases also have different conductivities. 
Even a more conductive phase interferes with the flow through the material because there is again a grain boundary. Inclusions and pores further reduce the conductivity. But high application temperatures also have a negative effect on conductivity. Increasing Brownian motion makes conduction more difficult.

Taking all factors into account, the upper limit for die-cast components is a thermal conductivity of 150 to 160 watts per metre per Kelvin. In contrast, typical structural casting alloys have thermal conductivities of only 100 to 140 watts per metre per Kelvin. Many heat sinks require thermal conductivities above 170 watts per metre per Kelvin. Otherwise, a fan is needed.

Achieve ideal conductivity and castability

With large 5G antenna heat sinks, the warm airflow attracts wildlife, which nests in the airflow. This may allow the birds' eggs to hatch faster, but the heat dissipation to the environment is massively disturbed. This leads to a significantly shorter operating time, as the power electronics overheat and then fail. Replacing the antennas in the wild is very time-consuming. They are usually suspended from high masts or inaccessible points. This quickly leads to costs in the five- to six-figure range.

So the solution is to reduce the elements in the cast alloy as much as possible to increase conductivity. However, if the silicon content falls below 7 per cent, the castability in die casting is massively reduced. 

If, on the other hand, rheocasting is used, an alloy with 1.7 per cent silicon, for example, is perfectly castable. Here, peak values of over 190 watts per metre per kelvin can be achieved, while the average is 170 to 180 watts per metre per kelvin. 

What is rheocasting?

Let me explain what rheocasting is: In die casting, liquid metal is transferred from a holding furnace to the casting machine. In rheocasting, a preparation unit is inserted between the holding furnace and the casting machine. There, the molten metal is cooled down into the two-phase region and a liquid-solid mixture, the so-called semi-solid slurry, is filled into the casting machine.

There are various methods of producing this slurry. Most of the processes are temperature-controlled. This means that the temperature interval has to be readjusted manually in the event of production-related fluctuations in the chemical composition. A better process here is one that works with a freshly produced melting mass. This has the same chemical composition and is therefore not affected by production-related fluctuations. 

Better thermal conductivity thanks to rheocasting

Due to the thixotropic properties of the semi-solid slurry, it achieves the same or usually even significantly better flow properties than a liquid melt. It is precisely the stirring of the melting mass that creates globulitic solid particles in the slurry - which also improve thermal conductivity. These provide ideal conditions for good make-up to ensure a pore- and leak-free casting.

Especially in comparison to the fir-tree-like dendrites in liquid die casting, the advantages become apparent. For example, this good feeding behaviour of rheocasting allows a base several millimetres thick and high as well as thin ribs (less than 0.5 millimetres at the tip) to be cast simultaneously in good condition. This also allows maximising the surface area available for heat exchange. Meaning more heat can be dissipated from the component.

The alloy, with its low silicon content, offers the highest thermal conductivities. This means that heat can be dissipated quickly via the slim fins. Due to the excellent feeding capacity, no leakages can be found. The corrosion resistance is given and the mechanical characteristic values are completely adequate. 

It is thus clear that the most efficient heat sinks can be produced by rheocasting.

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Fabian Niklas

Fabian Niklas

Managing Director

Casting-Campus GmbH