Protecting a PCB from damp with varnish

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Ric

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I'm building a small electronic circuit for use on my boat from plans on the internet. I've read that it is a good idea to protect the circuit from dampness with a special varnish sold by the likes of RS electronics.

Is there really anything "special" about this electronic varnish? I already have pots of other "special" varnish sold by marine chandlers.
 
I believe what you are looking for is "conformal coating". One of the important properties of this is that it is non-conducting and I don't know whther that is true of other varnishes.

Rob.
 
How about
ACF-50 Motorcycle And Automotive All Metal Anti-Corrosion Spray
From amazon

whats left will be very useful om your boat

Ric said:
I'm building a small electronic circuit for use on my boat from plans on the internet. I've read that it is a good idea to protect the circuit from dampness with a special varnish sold by the likes of RS electronics.

Is there really anything "special" about this electronic varnish? I already have pots of other "special" varnish sold by marine chandlers.
Click to expand...
 
There are several issues to consider when choosing a material to coat a PCB, the big ones are:

1. The material must not have a component that will react with the copper, tinning or anything else. No point protecting your tracks from sea-water just to have them eaten by a solvent.
2. The material must adhere and flow well. It needs to wick into the gaps. Water is often better at this than the coating.
3. The material must have minimal shrink during curing. Otherwise it will pull tracks and components off the PCB.

I design electronic circuits and PCBs for a living, I have seen all of the problems listed above many times.

Some years ago I tested several coatings, only one provided protection for prolonged exposure to moisture and it couldn't be applied without a vacuum chamber. There are a surprising number of ways moisture gets into a PCB (including between the copper and insulation of a wire if the conditions are right).

Generally it is better to use suitably IP rated enclosures, glands and connectors. IE remove the PCB from the wet environment if at all possible.
 
In the past I have used vehicle aerosol ignition insulation sealants. Silicone based, I think, and went by such names as "Damp Start" or similar. I used them on the spark plugs, cables etc. of ancient Seagulls. Worked a treat for what seemed like ages. Withstanding serious dowsings at times in seawater. Perhaps not the solution for the op, but I've often thought about using it on any and all electrical tackle onboard.
Perhaps anyone with more understanding of these products might pass comment?
Good luck.
 
Huttoft said:
In the past I have used vehicle aerosol ignition insulation sealants. Silicone based, I think, and went by such names as "Damp Start" or similar. I used them on the spark plugs, cables etc. of ancient Seagulls. Worked a treat for what seemed like ages. Withstanding serious dowsings at times in seawater. Perhaps not the solution for the op, but I've often thought about using it on any and all electrical tackle onboard.
Perhaps anyone with more understanding of these products might pass comment?
Good luck.
Click to expand...

Works in a clean environment. In a dirty environment the coating traps particles. Once enough particles have collected the oil no-longer soaks them, then moisture is trapped and you're heading back to square one. Much depends on the exact environment. The wet environment you describe may well be washing the particles away:D
 
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Ric said:
I'm building a small electronic circuit for use on my boat from plans on the internet. I've read that it is a good idea to protect the circuit from dampness with a special varnish sold by the likes of RS electronics.

Is there really anything "special" about this electronic varnish? I already have pots of other "special" varnish sold by marine chandlers.
Click to expand...

The stuff we use for boards in traffic controllers is fungicidal, to suppress fungus growing in a nice warm roadside cabinet. It's a nuisance for repairs though.
 
Offshore, on ROV's which operate down to many thousands of metres, and do occasionally get wet, I use WD40 as the simple solution, primarily as a protective coating on connectors. It's main constituent is fish oil (seriously!)and won't cause any unwanted conductivity. If a repair to the PCB is needed the WD40 is easily removed by detergent, and it's a cheap and readily available 'stuff' that's completely benign.
 
I have used the RS acrylic coating on both work and boat pcb s . I think it is worth while. Particularly on small components. Having applied it there is no obvuois corrosion and I have no failures. Even after a drip has found it's way in. It is easy to remove if a repair or test is needed. Worth the price and simple to apply. My replacement fridge pcb that failed originally as the surface mount technology corroed has lasted twice the origional and shows no sign of a problem
 
Ric said:
I'm building a small electronic circuit for use on my boat from plans on the internet. I've read that it is a good idea to protect the circuit from dampness with a special varnish sold by the likes of RS electronics.

Is there really anything "special" about this electronic varnish? I already have pots of other "special" varnish sold by marine chandlers.
Click to expand...

It has to be waterproof, thermally conducting, electrically isolating and fast drying.
 
Not a lot of help, but I sprayed the circuit board on a new boat I did for a client about 18 years ago.

Made in France and claimed that you could solder through the coating without contamination.

The same boards are still in use and have had no problems at all.

Good luck and fair winds. :)
 
I used to work on industrial brine cell control electronics years ago. A more corrosive environment would be hard to find. The boards were coated with a varnish (ICI spec) and it didn't work if the outer housing failed, the other downside is that repairing and setting them up was made extremely difficult by the coating. The best coating I came across was made by Wacker which was a rubber two part compound. Coatings are only really effective if already protected in some kind of sealed housing as well.

These days, on yachts, I use "Duck Oil" spray which is a waxy spray similar to WD40 but with more substance.
 
Rediffusion television distribution boxes used to have modules (pcbs with wiring and ancillary components) encased in "potting compound", which I understand was epoxy resin. Connection to the modules was by screw terminals on top. They were virtually indestructable. The advantage of potting compound over a spray, is protection from vibration and shock, as well as the elements. The only downside is that they're impossible to repair in the event of an electrical malfunction, other than by replacement.
 
If potting compound cures too quickly then 2 things can happen;
1. The heat generated during curing damages components.
2. The shrinkage during curing pulls components and sometimes tracks off the PCB.

The "screw terminal" method of connection you describe can work. But wires soldered to the PCB and running through the compound rarely seal well because the compound shrinks during curing and leaves a small gap around the wire which allows water to creep or wick along to the PCB. This situation is actually worse than no potting because once the water is in it often can't get out. There are silicon potting compounds available but I have known them suffer from the same problems as epoxy. Unlike epoxy, they also insulate the components which can be a problem if the electronic design requires heat to be dissipated.

My experience matches that of savageseadog; conformal coating applied by brush or spray doesn't work very well. I also agree with a number of people here that a wax or oil based solution will do a good job. But I see that as a temporary fix since it will eventually fail as the solution evaporates or becomes contaminated. "Eventually" could well be long enough for your needs and you can clean it off and top it up.

If you have spent a lot of time, effort or money creating your electronics or it would be a problem if it failed, then maybe it deserves a water tight enclosure and connectors.
 
I would add that coatings can de-tune radio and other sensitive circuits.
I have used polyurethane one-pot varnish in the past.
 
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