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Even with O-rings in the design, it is darn good. The cross sectional area of the O-ring is very small.

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Yes, but didn't Phil say "orders of magnitude?" A little "linear" trimming of exposed area and a little added path length doesn't seem to compensate for "orders of magnitude."

Jon

If you do not include moisture coming in from the cables (between the copper strand or between the copper and the insulation).

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
                     "If it doesn\'t fit, use a bigger hammer!"
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Neoprene O-rings can degrade faster than the materials used in coatings.

The arguments some of you dopes make are hilarious.

I work with fully ruggedized gear every day. I have seen several sealing methods, and even utilize conduction cooling in several cases.

Any clue as to why we do not have moisture entrapment or moisture attack problems?

Ever heard of hermetic pass thrus?

There are several ways of sealing off a raw circuit card assembly without a case.

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I'm just a hobbyist with modest math skills. However, I do have a feel for what "orders of magnitude" means and can compare that with linear path length changes in geometry. I'll leave it to the "big boys" to work out what is practical for specifics and watch.

Jon

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A totally hermetic housing is better, but O-rings on aluminum produces a housing that will last many years in water.

I know of systems that used a hygroscopic material in one of the optical components that have been used in the ocean for about 10 years now without a failure. The O-rings have continued to work over very wide temperature ranges and repeated changes in pressures.

These were just simple Viton, not the newer materials.

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That needn't happen if you put in metal to ceramic penetrators or connectors. You can bring signals in and out and keep the housing hermetic.

Even just stripping the middle of the the wires, soldering the section and then potting it in place reduces the rate that water gets in.

er hammer!"

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Which meets an application need. But that doesn't appear disagree with Phil's comment, since he was using "eventually" in context with "thousands of years" and "orders of magnitude." Thanks for the additional explanation.

Jon

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Simple O-ring seals, when properly implemented, are typically good down to 600' depths.

What a totally retarded methodology.

a sealed housing is not an option.

The saline moisture transport is principally through the atmosphere and whilst I have some experience of conformal coatings I am concerned about the saline content of the atmosphere, and any condensate, and its affect on the coating.

The pcb holds an optical sensor & 'tronics and its housing is transparent to visible, IR & UV. It is for outside use. This pcb is quite small (~60x55mm), is all extra low volatge, and production will be 100k+ p.a

I need a reliable dipping conformal coating that resists the salt in the atmosphere.

thanks

That small of a board at that number of production... You do not need to service it. Make 'em, pot 'em and forget about it.

Use stycast. It fully hardens, and NOTHING gets in. You conformal coat the PCB assy. Then, after a cure period, you dip it in stycast.

The conform is enough done right, but hard potting media IS what you want, and servicing them is more costly than simply making more would be. So make 110k of them hard potted.

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You are correct in that servicing is not an issue. I do not know of stycast - is this just another epoxy?

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I was giving scale to the numbers not disagreeing.

"orders of magnitude" could mean many of them.

On Mar 8, 5:40=A0pm, MooseFET wrote: [....]

BTW: One of the systems that has run a long time is housed in a titanium housing. The O-ring seal is good to the bottom of the ocean but the aluminum housing suddenly gets a lot smaller at only a couple of thousand PSI.

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e n 300 PSI! You must have a moron doing the design of the O-ring seal. Read the data sheet and follow the suggested design and you'll do better.

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Try Sylgard. The light may go through well enough.

Do you googlers even KNOW ABOUT the search side of google?

Searching "conformal coating" with quotes returns "about 163,000" hits.

Now, go and do your own homework first.

Good Luck! Rich