PCA Waterproof encapsulation

[DonkeyPhysics]

Howdy,

I'm working on an avionics product that has waterproofness requirement that we can't seem to pass with current processes. Currently, our PCA's are coated with a water-based humiseal. But, while that provides electrical isolation and prevents minor corrosion due to humidity, it's completely inadequate for waterproofness, even if we thicken it up to ridiculous thicknesses (at unreasonable time and cost).

The box our boards is in is ventilated, and cannot be dessicated, so I'm looking for some sort of waterproof encapsulation. And while I'm at it, since this is an aviation application, I need to make sure it's also inflammible and fungus inert.

Oh, and for record, we have to operate our unit with all the water on it. We're trying to pass the DO-160 condensing waterproofness test.

Any thoughts? Is there a silicone potting option?

 

[JohnnyB21]

I did a search for my field of work and came across this thread, so I'm not too sure if you found a solution to your problem yet.

Silicone potting would not be a good alternative due to an adhesion unless you were to use a silicone based primer to bond it together.

Try looking into a polyurethane potting solution. I'm not too sure if you will see this thread, but if you do please inform me. I may have possible solutions that may meet your requirements.

 

[patprimmer]

I am sure you mean non flammable, not inflammable (the curse of the complications from the inconsistencies of the English language)

My money is also on PU. Second choice would be epoxy.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[IRstuff]

Parylene has been often used for such applications:

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

TTFN

FAQ731-376

 

[JohnnyB21]

I agree with patprimmer, a 2 part epoxy would also be a good 2nd option.

 

[DesignerGuy16]

If you need a vented box, try getting a sealed box and using a product like Gore Protective Vents.

http://www.gore.com/en_xx/products/venting/protective/index.html

They're a PTFE membrane vent that will only let air in, but let air and water vapor to escape. Depending on what other chemicals it may see you may need a different material to protect against things like oils, etc. They're relatively cheap as well.

James Spisich
Design Engineer, CSWP

 

[IRstuff]

Are you doing this yourself? Have you consulted a military conformal coating house? Parylene is a mil-qualified conformal coating:

http://advancedcoating.com/techinfo/gen.html

Or you can see what's been qualified to meet MIL-I-46058:

http://qpd.daps.dla.mil/qpd/quick_search/default.aspx?qpl=QPL-46058

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=25144

TTFN

FAQ731-376

 

[patprimmer]

This is not for Airbus air speed indicators is it. ;-)

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[IRstuff]

Those need to be ICE resistant. By the time water gets there, it's already too late...

TTFN

FAQ731-376

 

[patprimmer]

Well ice is solid water.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[DonkeyPhysics]

Hi folks,

Thanks very much for all the replies. I did some more research on this subject after I posted here, and found that parylene seemed like a miracle cure.

Since parylene is pinhole free waterproof at .001" (or even (.0004", depending on who you read), completely coats all surfaces, even sharp angles, doesn't crack, has good thermal transmissivity (read no heat problems) is corrosion resistant, fungus inert and NON-flammable... this is like the second coming, especially after reading about the potential drawbacks of all the other options.

The primary inhibition with parylene is the cost to entry (since the process requires vapor deposition in a vacuum, which means expensive new equipment and training). I think ultimately my company may invest in this, but for now, we found that one of our current board suppliers already has a certified parylene setup, and the cost was workable (particularly compared to the absurd labor charges we were looking at with the other solution some folks in our company had to use once).

One other downside... it's also apparently hard to do repairs on. However, our board supplier says they can do that too for now, so yippy for me.

Ultimately, we decided to go for it, and we passed D0-160E with Parylene.

 

[DonkeyPhysics]

PS: For the record, I admit that I used the wrong word when I said inflammable. I was actually aware of that particular problem, and have in fact been amused for years that the words "flammable" and "inflammable" mean exactly the same thing. Too bad I type too fast sometimes and forget to proof-read.

PPS: I promise I do not work for Airbus, and no air speed indicators were harmed in the making of this thread.