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Coating a Motherboard In Thermal Resin?
Posted by
timothy
on Wed Sep 03, 2008 04:58 PM
from the oh-did-you-want-to-use-it-afterwards-too? dept.
from the oh-did-you-want-to-use-it-afterwards-too? dept.
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?
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Conformal Coating (Score:5, Informative)
Re:Conformal Coating (Score:5, Informative)
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'.
Parent
Potting blocks air cooling, of course (Score:5, Informative)
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 [coselusa.com] power supplies. But it's not going to work on a PC motherboard.
Parent
Re:Conformal Coating (Score:5, Informative)
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.
Parent
Re:Conformal Coating (Score:5, Interesting)
Parent
Re:Conformal Coating (Score:5, Informative)
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.
Parent
Re:Conformal Coating (Score:5, Insightful)
For immersion, the only sensible means is to use a non-conductive coolant and not worry about having to protect your hardware from the coolant it will be immersed in. It might be more expensive than just cooling with water, but it will likely be cheaper than having to replace ALL of your hardware that you just fried.
Parent
Re:Conformal Coating (Score:5, Informative)
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.
Parent
Cray blood (Score:5, Insightful)
Probably a lot easier to source yourself a few liters of Cray blood (or some similar non-conductive coolant) to submerge the board in instead.
Cheers,
Not sure (Score:5, Insightful)
Re:Not sure (Score:5, Informative)
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.
Parent
Re:Not sure (Score:5, Funny)
Parent
Re:Not sure (Score:5, Funny)
You: Hello Doctor! 5 gallons of your finest horse laxative please!
Vet: Oh, got a sick horse then? Are you it needs a laxative?
You: No, it's for me, eh I mean, it's for my computer!
Vet: Ah, the computer! I see. Say no more. *wink wink*
Parent
Re:Not sure (Score:5, Informative)
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.
Parent
A Bad Thing (tm) (Score:5, Insightful)
Wouldn't that conduct the heat from the CPU over to the other components?
Technical Term (Score:5, Informative)
The technical term you're looking for is Potting [wikipedia.org].
No that's different (Score:5, Informative)
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.
Parent
SUN used to do it. (Score:5, Interesting)
When they offered the SUN Crypto Accelerator cards for offloading SSL computations, almost the entire PCI card was coated in resin to prevent tampering. I don't think they're still available for purchase from SUN but I'm sure we've still got a few in storage at work somewhere.
Re:SUN used to do it. (Score:5, Interesting)
I saw a documentary on card cheating devices, and one of the early card-counting computers was dipped in something to prevent people from backwards engineering it. It included a failsafe, as well, a thin filament wire designed to be pulled off if the stuff protecting the computer was scraped away, and without that wire in place it would malfunction.
Parent
You should see the IBM version (Score:5, Interesting)
The IBM crypto processors had the module containing the key wrapped in wires (which, if broken, or changed in length, would erase the key) and internal to the module were thermal and x-ray sensors to prevent sniffing the contents of the module that way.
SirWired
Parent
Re:SUN used to do it. (Score:5, Interesting)
Parent
I've done it. (Score:5, Funny)
Look into Fluorinert (Score:5, Informative)
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.
Try Mineral Oil (Score:5, Funny)
People have been running PC electronics submerged in mineral oil for decades.
Advantages:
1. Not too hard to do
2. If push comes to shove, you can can probably burn the PC in your fireplace or other suitable container to keep warm. Or just because you are pissed at it.
Problems:
1. It's messy.
2. The oil tends to creep up any wires to the outside world (capillary action?) and eventually show up at the other end.
3. I'm not sure if non-gas tight connectors are used in modern PCs, but if they are, they may be a problem.
4. It's messy.
Did I mention that it is messy?
cockroaches (Score:5, Interesting)
Years ago I worked at a company which had had problems with some telecom equipment in the field and no one could ever find any smoking gun. Random problems pointed to several different places on one particular board. One technician must have been working late, because apparently the CO filled with cockroaches once the sun went down. One of the theories was that bugs crawling across the board caused random short circuits. The customer was getting pissed, so management opted for a shotgun approach. Half a dozen shot-in-the-dark fixes were made, including adding an insulating coating. No one knows which one (or combination) of the fixes did the trick, but the random outages went away. That was engineering at its finest.