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We designed a microchip to sense high pressures under water. The chip senses pressure through a thin diaphragm that deflects and a corresponding piezoresistor circuit that measures the deflection. For every pressure, the chip spits out a corresponding voltage output.

The chip contacts are soldered directly to a FR-4 PCB and then the whole thing is potted in urethane.

The chip works great outside of water, and underwater, but the transition causes problems. Say the chip is placed in a water tank and pressurized to 500 psi for a few hours. It measures 500 psi no problem. But when we depressurize the tank and remove the chip, the chip outputs 20 psi, or some other random number, when it should measure 0 psi.

We think that the FR-4 material is swelling from the exposure to high pressure water. This swelling would stress the pressure sensing diaphragm and cause the erroneous reading.

What are good PCB materials that swell less when exposed to high pressure (~500 psi) water?

Or, any other hypotheses as to what is happening?

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    \$\begingroup\$ Ceramic PCBs are available, though whether that helps or not I have no idea. It may be something more mechanical though like a permanent deformation of the diaphragm or some such. Also urethane is not 100% water proof depending on the type. You would be better to pot with an appropriate epoxy. \$\endgroup\$
    – Trevor_G
    Commented Nov 5, 2017 at 1:14
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    \$\begingroup\$ expose an unpotted module to the pressure. it can be unpowered. use distilled water for less contamination. then check if the drift has occured (once it is dry) \$\endgroup\$
    – jsotola
    Commented Nov 5, 2017 at 1:28
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    \$\begingroup\$ I am skeptical of your explanation because FR4 doesn't retain much water. Similar to what jstola suggests. Take a dry, unpotted piece of FR4 and weigh it. Also measure its dimensions carefully with calipers. Pressurize it down to 500psi for the same amount of time as your working boards. When it comes out, pat it dry, and remeasure and re-weigh. If it has swollen or retained water (as evidenced by weight gain), then your original suspicion is confirmed (or at least it is not disproved). But if not, you need to look for another explanation. \$\endgroup\$
    – user57037
    Commented Nov 5, 2017 at 3:08
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    \$\begingroup\$ Do the same thing with a piece of cured potting material. If anything, I bet the polyurethane is swelling. \$\endgroup\$
    – user57037
    Commented Nov 5, 2017 at 3:08

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You might look at PTFE based materials like Rogers 5880.

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  • \$\begingroup\$ Or aluminum substrate, frequently used for LED boards, and therefore might be less expensive. \$\endgroup\$
    – Ale..chenski
    Commented Nov 5, 2017 at 3:08

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