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Comments on PCB as a wall of an underwater enclosure

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PCB as a wall of an underwater enclosure

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Can a circuit board be waterproof enough to form a wall of a waterproof enclosure?

Regular PCBs made of FR-4 with solder mask is what I have in mind. But I'm not barring less common PCB materials and processes, although I'd prefer something with moderate cost in moderate quantities.

10m depth in river and sea water.
Temperature range between +4°C and +40°C.
An ability to withstand freezing temperatures isn't required. But it's desirable, because that would let me field-test the device year round where I live.

Why would I want to expose a PCB to water? The malice aforethought is that the PCB has electrodes on the water side and a connector on the dry side. Here’s a rough and naïve sketch of what I have in mind.

front with electrodes rear with a connector

Solder mask overlaps the edges of the electrodes, and prevents water from getting under the electrodes.

Is ENIG plating enough to provide corrosion resistance for the exposed parts of the electrodes?

Vias are tented (or plugged, or via-in-pad, if necessary).

My findings so far:

I asked several PCB suppliers at a trade show about waterproofing PCBs, and one of them mentioned in passing that solder mask is waterproof. The common solder mask materials are epoxies (from what I read on the web), and epoxies can make good water resistant coatings. Two layers of solder mask, if coverage defects are a concern.

The wiki about FR-4 says that it has “near-zero water absorption”.

PCB
posted 9 months ago
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misk94555‭
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5 comment threads

Conformal coatings? (1 comment)
FR-4 and soldermask will slowly absorb moisture (1 comment)
re: corrosion - if there is a path for current to another wetted metal also in contact with seawater,... (1 comment)
What's the application? (5 comments)
Vias? (5 comments)
Vias?
Lundin‭ wrote 9 months ago · edited 9 months ago
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How would the signals get to the SMD connector without vias? Some sort of blind vias? Are they waterproof and who says so? Anyway, to be honest this is such a bad idea from the start. Not just because of water, but because of salt, galvanization and probably a bunch of other problems too. Separate the PCB from the outside environment. You need to involve a good mechanical engineer with experience of maritime development. Salt water is really nasty business.

misk94555‭ wrote 9 months ago · edited 9 months ago
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Lundin‭ Vias. I can tent the vias with solder mask. I would pick small via diameter (say 0.15-0.20 mm) which would mean that solder mask would plug the vias entirely. That would be the easiest solution, because it’s a bog-standard PCB fab process. If that’s not enough, the PCB fab can plug the vias. It would cost me extra, but it’s still a common PCB fab process. (No throughole components, obviously.)

misk94555‭ wrote 9 months ago
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Olin mentioned at the end of his answer that he’s doing an immersed capacitive level sensor, and it works. I bet there are vias, and they aren’t causing too many issues. (At the same time Olin’s situation is different, because he’s not trying to hold water with a PCB.)

misk94555‭ wrote 9 months ago
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Lundin‭ Corrosion. That’s an interesting question. The electrodes will be ENIG plated. If ENIG isn’t enough, I could do hard gold plating (like PCIe board edge connectors). The metals involved (from top to bottom) are gold, nickel, copper. Gold is a noble metal, so it shouldn’t react with seawater. Nickel doesn’t corrode in seawater. Copper reacts with water, so I need to seal copper against water. If solder mask overlaps the edges of the electrodes, then it protects the edges of the copper foil. Say the metal pad of the electrode is 5mm wide, and the solder mask opening is only 1mm wide. The only materials exposed to seawater would be silk screen (epoxy), and gold. These materials may be inert enough. That’s my hypothesis.

Lundin‭ wrote 9 months ago
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The slightest scratch will mean that the solder mask is leaking however. Solder mask is literally what it says: a means for preventing solder to end up on top layer traces. Using it for anything else (including isolation) is questionable practice. Anyway, the standard practice for anything in tough environments is: mould the whole thing. Or as a means to save a failed design, apply silicone all over the place.