Coating a Motherboard In Thermal Resin?

Bat Country writes "I've had an idea in the back of my head for some time (and I'm surely not the only one) that it would be a worthwhile project to coat a motherboard in thermally conductive electrically insulating resin — complete with all of its various components — for the purpose of immersion, shock resistance, whatever. I'm curious to find out if anyone's undertaken a similar project or if it's known to be a shockingly bad idea (due to shrinkage during the curing process) already. Thoughts?" If you've done anything similar (even an experiment that failed), how did you go about it?

Comments

Conformal Coating

[DeathOverlord3]

yeah, it's called conformal coating

Re:Conformal Coating

[Anonymous Coward]

Conformal coat is generally a thin film applied over the board and components. I get the idea he is talking about something more like 'potting'.

Re:Conformal Coating

[Ethanol-fueled]

Conformal coating is protection against a lot of things, and submersion is not one of them.

Those of us who work in the electronics industry know that doing CC in-house is a bitch and inspecting an outsourced job is an even bigger bitch, especially when you're dealing with military parts.

Re:Conformal Coating

[Animats]

Right, it's conformal coating. People do that all the time. I've used Fine-L-Kote on boards.

Boards with connectors or jumpers have problems. If the CPU and RAM are soldered to the board (as they often are in industrial, consumer, mobile, and automotive devices), just mask off the connectors, jumpers, switches, battery contacts, etc. with masking tape and start spraying. Fine-L-Kote is transparent, but glows in UV, so you can use a UV lamp to check if you missed anything. There are much heavier coatings, ones that really encapsulate the board, and those are widely used for automotive, boat, and aircraft applications.

PC-type motherboards aren't a good choice for this, because of all those connectors. Those are a weak point for corrosion anyway, so protecting the soldered-in components may not be all that useful. But if, say, you're putting something like a single-board PC on your boat, it's quite reasonable.

Re:Conformal Coating

[effigiate]

We conformal coat our circuit boards at my current job and I can assure you that it is not what he's looking for.

Conformal coating is typically a thin layer of silicone/urethane/acrylic used to keep moisture from getting at the parts on the board. It can not sustain immersion in liquid.

He's looking for epoxy potting, which we also do occassionly. The trouble with epoxy potting is getting the heat out of the board. You need to leave the thermally conductive parts outside of the potting so that you can remove the heat. The epoxy itself isn't thermally conductive enough to get processor heat out, even on a processor with passive cooling.

You can do this yourself if you have enough time and epoxy, though I'm not sure how much success you'll have. A failed attempt probably means a board that is no longer useable.

The very best ones get cured in a vacuum so that all of the air bubbles are pulled out. There are many other types you can use that don't require a vacuum.

Re:Conformal Coating

[Zadaz]

I wonder where he could readily get a vacuum chamber big enough to pull the air out. Getting the air bubbles out is going to be pretty vital to not baking the mobo, especially the cavities under components.

The most readily available vacuum pumps are smaller than a full motherboard + components, used either for basic science, paint or latex molding. Maybe call up Tap Plastics or your local university and see if they have a big one you can borrow. Because you're going to need a real one. A vacuum cleaner on a Tupperware box or a concrete vibrator isn't going to get what you need.

But my main worry is about thermal expansion. When motherboards get hot they don't expand evenly. Locking everything in resin, not mater how thermally stable, will put a lot more physical stress on the components. And you won't be able to do a damn thing about it except chuck it in the bin and start over again. Though this would probably be lessened with a smaller form factor and lower energy components.

Re:Conformal Coating

[Cor-cor]

The trouble with epoxy potting is getting the heat out of the board. You need to leave the thermally conductive parts outside of the potting so that you can remove the heat. The epoxy itself isn't thermally conductive enough to get processor heat out, even on a processor with passive cooling.

Coming from a materials engineering background, this was my first thought about the question. A material which is very thermally conducting while also electrically insulating sounds like a pretty tall order to me. The reason is that one of the primary methods of heat transfer is through transferring excited electrons.

The two others are phonon transmission through the lattice and radiation. A lot of things which block electrons also block phonon movement (basically heat transfer through vibration) and I'm guessing radiant heat is not going to be sufficient. The one material I do know of which has high thermal conductivity and low electrical conductivity is diamond. It's kind of an oddball, though. I'd be interested to know if there are other, more common materials that would suit this application.

Then there's also the CTE mismatch issue mentioned later in the thread. Whether you design the specs for the high-heat state while it's in use or the cool state when it's off, the other state is going to put a lot of thermal strain on your circuit board components. I'd imagine you could solve some of these problems, but I doubt you're going to be able to take care of them all.

Re:Conformal Coating

[Artraze]

I'm surprised someone with a materials engineering background hasn't heard of it, but Beryllium Oxide would be the other big thermal conductor/electrical insulator. It's a little more popular than diamond since you can actually sinter it into large objects. However, like most beryllium based things it is rather pricey.

I think the OP's best bet is to coat the mobo with a CVD diamond film and hope that the liquid cooling keeps the temperature down low enough the film doesn't crack.

Technical Term

[aero6dof]

The technical term you're looking for is Potting.

Re:Not sure

[MozeeToby]

That's why you don't use water, you use something non-conductive. Mineral oil is a relatively cheap and widely available option (just go to your vet and ask for a few gallons of horse laxative) if you don't want to spend the money on commercial grade cooling fluid.

Look into Fluorinert

[grimsnaggle]

http://en.wikipedia.org/wiki/Fluorinert

It is electrically insulating and is commonly used for cooling electronics (think Cray supercomputers).

Part of the problem with conformal coat is that it makes it hard to service the electronics after it is cured. It also may or may not be uniformly distributed and thus may not pass muster in a tank of conductive liquid.

There are conductive epoxies like Stycast, but they're not particularly good conductors. The only reason to do immersion cooling is for good thermal contact to all components. A thick epoxy layer between your components and your liquid will quickly destroy that advantage.

Also, if you have connectors to the circuit board (like PCI connectors), then you cannot fill the pins. Last time I checked, most PCI connectors are just slots and have no bottom fill. Water will certainly get in under the coating through the slot.

No that's different

[pavon]

Potting is used to keep the components from moving (usually in high-G environments. Sometimes you use it to keep close conductors from shorting (like solder-cup connector), but again the risk there is mostly movement of the conductors, not the environment. Potting materials usually do not have good thermal dissipation properties, and aren't really the best thing for environmental protection (humidity, liquid immersion etc) either. Conformal coating is what you want for the latter.

Potting blocks air cooling, of course

[Animats]

Electronics has to be designed for potting, at least if it dissipates any significant power. You have to provide a heat path (usually a metal heat sink) out of the potted block. This is done routinely for DC-DC brick power supplies. But it's not going to work on a PC motherboard.

Re:Not sure

[Anti_Climax]

It is important to keep in mind that light mineral oil like that, while not as bad as other choices, will leech plasticizers out of insulators. The power supply wiring on my machine very quickly became stiff and brittle and it dissolved the soft rubber that was holding the fan assembly to the processor's heat sink. Not sure if it will have any long term effect on the plugs of the electrolytic caps on the board but I wouldn't be surprised.

If you can afford to split the difference between mineral oil and florinert (perfluorocarbon), you might consider a low viscosity silicon oil. That should bu much nicer to natural rubber compounds and plastic insulators.

Industrial Electronics

[Mike Rice]

In my former life I worked as an industrial electronics technician. My job was, in a nutshell, to modernize a manufacturing plant from its 1950s style, analog (pneumatic) technology, to digital electronic distributed control systems.

The environments these devices need to work in are quite harsh, with extreme temperatures and often corrosive atmospheres. The pneumatic control systems were quite robust in those environments... electronic devices need a lot of beefing up to survive these conditions.

One aspect of this was to treat all circuit boards with a conformal resin coating. The trick is to make sure the thermal coefficient of expansion of the resin, matches the expansion of the circuit board material. I am not a chemist, but I do know such coatings are available.

Another consideration which has been mentioned is how to treat connectors. The usual method is to apply a rubber like sealing compound after a connector is fitted and tested.

For less extreme environments, a much less expensive, but quite effective alternative, is to apply a cheap acrylic coating, using readily available sprays such as Krylon 1301. The procedure is...

Assemble the device (uncoated) and test thoroughly.
Disassemble the device.
Apply tape and / or petroleum jelly to connectors and contacts, to prevent damage from the spray.
Apply the spray to each component.
Assemble and re-test.

Hope this lights a bulb for you.

Re:Not sure

[Anti_Climax]

I thought about this a little more and I think I should put this into perspective a little bit.

The first machine I submerged was done in a plastic tub and covered with 5 gallons of Tractor Oil (no, really) that I bought at the wal-mart for $20. It smelled, it was fairly viscous and it definitely leeched plasticizers and crawled up the interior of cables. I ended up with a small puddle of oil under my optical mouse. Wireless worked okay through it though. I tried submerging hard drives after covering the breather holes - I figured if I was keeping them in a fairly narrow temp range they shouldn't need to equalize. Unfortunately none were sealed sufficiently and all eventually died, though one did make it a month before it flooded.

The next machine was a recreation of the Puget Systems submerged machine. I went with light mineral oil from the local feed and tack store but was seriously considering silicon oil in it's place. Light mineral oil was going for about $17 a gallon, was less viscous than the first oil I used, didn't stink and hasn't effected the components as quickly. I can't find it now but I recall silicon oil being somewhere around $100 a gallon. If I had the money to spare I would have gone that route. Looking online the best price I can find for appropriate Fluorinert is around $1,000 a gallon.

So for 5 gallons of submerged cooling you're looking at $20-$5,000. When you consider the effect it'll have on components, it's probably worth it to use the $500 fill of silicon oil over the $100 fill of mineral oil.