The cards were packed right on top of each other, so the resulting stack was only about 3 inches high. With this sort of density there was no way any conventional air-cooled system would work; there was too little room for air to flow between the ICs. Instead the system would be immersed in a tank of a new inert liquid from 3M, Fluorinert.
However we were not dealing with such high powers as tens of watts. I think the problem you have is that with such high powers most low viscosity electrical inert fluids will boil.
I know a lot of overclockers have put their PC motherboards into pure mineral oil. It's non-conductive and carries heat better than air, to my knowledge. I doubt it has thermal conductive properties equal to proper heatsinking, though.To go this approach, you need to guarantee a few things.- All components are sealed. ICs are good. Caps must be airtight. Switches and potentiometers would cause problems if the oil were allowed to create a barrier between wiper/contact and element.- The case itself is sealed. Obviously.- The mineral oil is really pure, and stays that way. With liquid flowing around, you can't have metal shavings or dust hanging out in there.
Instead the system would be immersed in a tank of a new inert liquid from 3M, Fluorinert.
And lastly any dielectric fluid will increase stray capacitances around the board, this may affect some things.
Alternatively helium gas has a much higher thermal conductivity than air (due to low atomic mass).
I still think the first priority should be to see if the power consumption of the circuit can be reduced.
as an example, on the board are 8 transient suppressors dissipating up to ~400mW each, so we're at 3.2W.
Now include the 12V regulator, 5V regulator, 3.3V regulator...
...8 MOSFETs driving not insignificant currents...
...4 IGBTs that are self clamping...
...h-bridge IC, stepper motor IC
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