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I've got some some ready-made 12v LED units. I wanted to pot each and put it in a metal container for robustness and to try and dissipate some heat.

I'd like to pot the circuit but was hoping to avoid expensive potting compound. Is there something cheap I could pot the circuitboard in? It would need to be electrically insulating but have better thermal conductance than air. Not necessarily that much better, I assume these units are meant to be used in enclosed spaces like desk lamps.

I was thinking of silicone sealant or clear casting resin but I have no idea of their properties. Bonus points for optical transparency but it's not a requirement.

Any suggestions?

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    \$\begingroup\$ Without quantification it's hard to say. The only additive that gets you up to significant thermal conductivity levels without electrical conductivity is BN (boron nitride), which is expensive. However, it's rare to see something potted that dissipates significant heat; you usually put some kind of thin thermal interface to an outside heatsink on the high power components. \$\endgroup\$ – user36129 May 11 '14 at 9:52
  • \$\begingroup\$ Some resins thermally expand or contract while hardening, that can tear boards up so watch out. Think about a conformal coating instead. Potting is tricky and makes things non repairable. \$\endgroup\$ – EternityForest May 11 '14 at 11:05
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Just about anything will have better thermal conductivity than air. That's not the whole story though, since most of the cooling in air will typically be from convection.

Typical potting compounds are available from suppliers such as Hysol. If you need guaranteed electrical characteristics, that's a good way to go, but they're often not that easy to source, and it's expensive to buy a 4l or gallon size can. As Eternity said, the dimensional change during curing can rip parts off the board or place them under great stress so they fail very soon with thermal cycling.

Clear silicone (depending on the type) can have highly corrosive (and conductive) acetic acid in it (some types do not). It's not all that good thermally (2 or 3 times worse than epoxy), but it can seal against moisture and is available in translucent. Do not use the acetic acid type on electronics.

Your best bet may be to use a minimal thickness of conformal material such as silicone rubber thermal pad to transfer the heat to an aluminum heatsink.

enter image description here

There are a wide range of these available, with varying prices and performance (including some that are anisotropic- they are more thermally conductive in one direction than another).

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  • \$\begingroup\$ Thanks very much for your answer. I'm reasonably sure that the LED boards aren't meant to cool by convection as they're designed to be used inside lighting mounts. I'm afraid the PCB doesn't look very thermally conductive, hence wanting to conduct the heat away. I'll look at my options, and will avoid corrosive silicone. \$\endgroup\$ – Joe May 11 '14 at 11:54
  • \$\begingroup\$ @Joe A thermal pad on the back of the board could do wonders, if you have a heatsink to conduct the heat away. They're just sheets of rubbery material you can cut to size. \$\endgroup\$ – Spehro Pefhany May 11 '14 at 11:56
  • \$\begingroup\$ The back of the board contains the driver circuitry (although you can't see it from the photos). Two capacitors, two inductors, a rectifier, an IC and not much space for anything else. \$\endgroup\$ – Joe May 11 '14 at 11:57
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I just had the same query. Arctic silver adhesive is good for small uses, it's made for LED's and sinks and you can make a first coat to ensure electrical insulation.

HBN boron nitride conducts very well and you can mix it with tin/platinum cured silicone, and perhaps check your shops out for solventless silicone tubes as glue. HBN costs 20-40 a kilo on alibaba and will last you a lifetime.

If you stuff thick glass fiber into the potting compound it can treble the conduction of normal silicone at .35WmK, which is not much compared to 20WmK rating of HBN.

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The LED assembly you have has 27mm diameter and only dissipates 1.5W, so it should have no trouble getting rid of heat by convection alone if it is placed vertically, as hot air rises...

At this level of dissipation, which is low, the PCB will probably be FR4 for cost, not metal core, so the thermal conductivity through the board should suck, unless the manufacturer drilled lots of vias. Cooling it from the back would thus be kinda dubious if there are no thermal vias.

In fact, since I do not see any resistors or LED driver chip in the picture, I would guess they must be on the back. Obviously if there are components soldered on the back of the board, you won't be able to mount it on a heat sink, but for 1.5W as said above, it does not matter.

Therefore, some spray-on conformal coating should do the trick, but you must be sure it adheres everywhere, including the LED phosphor.

Since this LED assembly is certainly NOT CHEAP (£5 for 100 lumen? seriously?) if I were you I'd simply purchase 12V LED bulbs like these:

enter image description here

The bulb is glass, there is a glued-on plastic lens on the front, and the connections at the back look waterproof. If you manage to connect the 12V supply in a waterproof way, this should do the trick for you, and it is much simpler and cheaper than waterproofing your PCB.

Googling a bit, I also found submersible LED strips, so there should be many simpler solutions that will end up cheaper than purchasing potting resin...

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