Conformal Coat

If it is east to peel off it will probably allow moisture intrusion. Some silicones will peel off.

What does it need to do?

MIL-I-46058 will list government approved coatings.

Conformal coating - SAE International

Go to your favorite supplier and tell them what you want.

Conformal Coatings for Automotive Electronics - HumiSeal

What it needs to do is be waterproof, primarily. The one I've seen that I like is in the transmission ECM of an auto, sealed in a housing but still somewhat exposed to the elements. While it is removable without a solvent, it's still very tacky and takes some work to remove. I'm used to hard coatings, arathane, conathane, etc., but I'm looking for something more prototype-friendly.

Thanks for your response.

I don't like the idea of peel-able conformal coatings. This sounds more like a way to trap contamination to the PCB than protect it.

What you might be looking for is a silicone based potting material not a conformal coating.

This stuff doesn't feel like a silicone potting. It feels more like a thick, tough, rubber cement. If you've ever used a glue trap for rats, it's like that, only not sticky on the surface. It takes some effort to penetrate, and doesn't come off as a sheet, - tending to stick to the board and come off in pieces, but it does come off with only mechanical means.

I suppose a potting compound is an option. I like the idea of dipping the board in something as opposed to spraying.

Thanks

You may be talking about a plastisol material, I don't know, from your description.

Custom Military Grade Plastisol, Liquid Vinyl, Latex ...

In my experience, military satellites and aircraft electronics, spray coating is only as effective as the applicator. When I instructed operators in the fine art of coating application, I stressed the importance of spraying at an angle that would be most likely to penetrate under leaded components, from all 4 sides. There is a bit of an art to effectively spray coating anything.

Of course satellite electronics only needs moisture protection while waiting on Earth to be deployed. Vibration and particle migration are more of a concern in orbit insertion.All of these things are important in a terrestrial application.

My experience is that the only effective way to assure complete coverage is to slowly dip and slowly remove the board from the compound. Connectors and edge connector fingers can be masked.

Talk to a supplier tech rep.

I remember from my military design days that we always had the boards conformal coated, and I always had no problem getting a good connection to the underside of an IC pin with a hook probe.

The auto stuff I saw looks like it's been poured on. A dam of black rubbery something is traced around the outside of the board and the thick clear stuff poured into it.

Thanks

My only experience was with HumiSeal product (our usage was avionics). It was acrylic based material and used toluene as its solvent. I never could make a product water proof relative to condensate testing with this (condensing humidity environment). Our operations sprayed this on. Which did not fully wet under surface mount components. Hand application was used where leakage currents were a problem. But in my opinion the only products that were able to be made water resistant were ones hand made by engineering.

Just before I retired 3 years ago, the same manufacture (of the acrylic product) had a water based product. This was a dismal field problem with it trapping metal ions and enabling dendrites to grow between pins. It required an extreme solvent (I don't recall but was a paint stripper). The intent of this product was to be "green" as toluene was seen as a VC pollutant.

Our product could not be dipped, as this would contaminate the board connectors. The spraying operation required masking of the board, so factory operations disliked the process as product had to be dispatched to a different area of the factory floor. And there was much hand labor and touchup/cleanup added to the manufacture.

Sorry I can't recommend anything, but don't use HumiSeal toluene based conformal coating. It did provide electrical insulation for accidental shorts of FOD.

Thanks - I remember the term humiseal, I must have used it many years ago.

CAGE Code 0J4Z5 -

MILLER STEPHENSON CHEMICAL CO

12261 FOOTHILL BLVD, SYLMAR, CA 91342

MS-460H Silicone Conformal Coating

A transparent, soft flexible coating which offers excellent moisture
resistance, good dielectric properties, and excellent light transmission
capabilities for solar applications. It has good weatherability, and
resistance to U.V. light and dirt pick-up. It is also solderable for
easy repair. Contains Dow Corning 1-2577 Silicone Resin. Silicon resin
meets the requirements of MIL-I-46058C, Type SR and UL Standard 746E.

It's what we use on some of our government contracts. Others use the typical Humiseal urethanes.

Good luck with your quest.

Thanks!

I use liquid rubber.

We have used parylene as a conformal coating on PCBs as an alternative to a solid enclosure which would have caused problems with heat dissipation. It has good dielectric properties and excellent moisture protection, to the extent that we were able to spray and even totally immerse the PCB without problems.

I thought parylene had to be vacuum deposited.

Not so?

Yes, parylene is normally vacuum deposited which also means that you get virtually perfect coverage with an absolutely uniform coating. And it costs more of course. Re-working parylene coated boards can be problematic.

Thanks.

I used it years ago and masking card edge fingers and connectors was a nightmare.

Very difficult to de-mask.

MIL-I-46058 finally accepted it as a coating in the late '80s.

BTW, it is not transparent to ultra-violet radiation.

We did a lot of conformal coating of automotive circuit boards at previous emloyment. Around 20L per day in two different formats.

Dymax was the supplier and had multiple otions.

We had a thin "sprayable" one that had red UV fluorescent pigment so that you could inspect for contamination on contact areas. This was a UV cure, but would also heat cure if you wanted.

The other was a thicker coating that we applied 2.5mm thick. We used compresed cast silicone moulds to hold the dispensed material while it was cured (UV also) but with a slower transit time to get pentration of the light for cure. This had a pale blue UV indicator in it, also for inspection purposes. Compression was between teflon coated steel sheets to allow celaning between process cycles. This thicker material would "climb" vertical walls before cure, so had some process development challenges.

Both could only be removed by immersion in methyl alcohol for more than 24 hours and then abrasion using "toothbrush" to remove.

Be careful with any conformal coating processes!! Heat dissipation from chips needs tobe re-assessed, thermal expansionissues can arise since it will get underneath components.

3M also had options marketed as RTV (RoomTemperature Vulcanising) materials, but most of theirs were solid colour and so eliminated any inspection capacity once coated.

This was 6 years ago!! Things will have changed.