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I would like to waterproof an FR4 circuit board that has no external connectivity requirements other than permanently attached USB cable, but it does have a number of SMD LEDs that must show. Most significant other components are an TSSOP20 MSP430 microcontroller and an LM4970. Power is via a small rechargable 3.7 volt LiPo battery.

Also, the board is to be fitted in a shoe heel(!), so adding an additional waterproof enclosure is not feasible. The water exposure is essentially moisture build-up, not actual soaking (hopefully). The device needs to last just a few months to a year.

What are my options and and what problems would I face?

  • If I use potting epoxy, will there be heat build-up?
  • If I simply use a see-through latex balloon will that survive circuit board heat?

Is there any additional detail I could add to make this question clearer?

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Ha! So your musical light-up shoe has progressed! –  Anindo Ghosh Jan 28 '13 at 15:59
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What is the board's power dissipation under normal operating conditions? That sort of thing is kinda useful when answering questions like "will it get too hot?" –  John U Jan 28 '13 at 16:06
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Conformal coating. :) –  Toby Lawrence Jan 28 '13 at 16:10
    
@JohnU 14 mW average, 80 mW peak. –  Aashima A Jan 28 '13 at 16:50
    
At those powers, don't worry about thermal buildup. In any case, epoxy conducts head better than (still) air. You might consider marine epoxy for toughness. Any epoxy is somewhat vulnerable to UV but in less than a year of sunlight you should be OK. –  Brian Drummond Jan 28 '13 at 21:20
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2 Answers

Ask you FR4 supplier about conformal coating options. They can mask it off, on the windows of the LED's and such. Check the water resistance of the LED's and such exposed parts. Most electronics are, as they are washed. Conformal Coating is very common on military and automotive electronics. A long time ago, I used it on video equipment that went in the trunk of vehicles to protect from salt build up from DEW point condensation.

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+1. They have clear conformal coating as well. You could possibly top it off with clear two part silicone. –  MandoMando Jan 31 '13 at 15:21
    
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Potting epoxy is not a good water barrier. Silicone based products are usually superior. Conformal coatings may meet some of your needs with subsequent encapsulation in a product aimed at mechanical protection.

Coatings of silicone do not block water 100%. What is not widely understood is that NOTHING that is used as an add on coating blocks water 100%. [There is a special coating - Parylene and similar names - applied via vacuum deposition of vaporised coating which changes its molecular arrangement during the process, which is as good as you get, not perfect and very much not a do-at-home treatment]. BUT water permeates through and coating. A good coating works by having very low % water dissolved in it and by forming an aggressive voidless bond to the target. This means mainly only water vapor is present and lack of surface micro-voids which can form liquid water pockets means corrosion rates are far far far lower than otherwise. More on such below.

When applied as an after-market treatment ALL conformal coatings have the potential to degrade performance or do damage. This is usually at the "obvious enough" level - use your brain, look carefully and think ahead. Covering any screen with a coating will usually be a bad idea. Getting goo that sets on any electrically connecting surface will be a bad idea. If it is meant to move (hinge, keyboard flexure, ...) then gumming up the motion is liable to be a very bad idea. Sounders/beepers/speakers that make noise may make different noises if plastic coated. Thinks that get hot and are cooled by air movement usually get hotter and are less cooled when coated. Holes that allow airflow may vanish ... . Once such 'little things' have been addressed, a range of coatings MAY help.

Dow Corning make a material named Dow Corning 1-2577, which does a better than most job. It can be brushed sprayed or dipped and air sets to about a 0.1mm layer. It is not cheap, contains enough volatiles to be used in a fume hood or outdoors and you are unlikely to see it in retail sales. A "low VOC" version exists.

Dow Corning conformal coating products here

Dow advertised a PV6100 material a few years ago that sounded like it would be ideal (aimed at solar cell encapsulation) but they have "gone all quiet" re it recently. May yet be available.

The older Dow Corning "Sylgard 184" is much used by the DIY PV panel manufacturing community with good reported results. Also see slower setting Sylgard 182

Note: I have no involvement with Dow Corning, except as a satisfied user of their products.

"Silicone spray" will provide a degree of protection against water but is far from ideal.

Clear polyurethane plastic spray (sold as "lacquer" or "clear varnish") will do a passingly good job as an aftermarket conformal coating. You could spray in an excess and let it run to and from over semi inaccessible surfaces.

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