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I've been asked if a design of mine can be immersed in transformer oil. (And still function properly!) The circuitry is mostly SMT Qs, ICs, Rs, ceramic Cs all of which I think are OK but I'm unsure about a couple types of components - Aluminum Electrolytic Caps and Connectors.

Oil type: I don't have that info and it could vary between installations but I assume paraffin or silicone based oil.

The components I am most unsure of are:

Aluminum Electrolytic capacitors such as United Chemi-Con EMZF500ADA101MJA0G.

Connectors, specifically Molex 22-23-2041 and On Shore Technologies OSTOQ045450.

The boards are conformal coated with polyurethane.

Any advice on component evaluation or cautionary advice would be greatly appreciated. Thank you!

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  • \$\begingroup\$ It sounds like a feasible approach with non-flammable thermal conducting properties and anti-foaming agents. \$\endgroup\$ Commented Jul 28, 2020 at 16:07
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    \$\begingroup\$ pugetsystems.com/submerged.php These guys have PC motherboards running for 7 years in mineral oil. It all depends on the materials. Rubber seems to degrade. \$\endgroup\$
    – Aaron
    Commented Jul 28, 2020 at 16:21
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    \$\begingroup\$ Aluminium electrolytics often have a rubber plug or seal or base. That and anything else rubbery and some thermoplastics may soften, swell or dissolve in oil. \$\endgroup\$
    – user16324
    Commented Jul 28, 2020 at 22:46

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Why are you immersing your assembly in oil? The only reason I can think of is that you intend to submerge it. So, I'll work on that assumption.

First, any component that has an airspace is a problem. All those airspaces need to be filled with the fluid to balance pressure.

Knowing this, and knowing how most electrolytics are made (they're layer stacks rolled up in a case), I would not trust any electrolytic cap to be submerged in oil, let alone high pressure, unless the manufacturer rated it that way for the specific fluid you intend to use.

All your plastics - boards, connectors, other assemblies - need to be compatible with the fluid, again you need to check with the manufacturers. For connectors there are specific types marketed for pressure-balance immersion that are compatible with oil.

Finally, I wouldn't trust conformal coating to protect your board if it's under pressure. Trapped air bubbles in the coating could collapse. It also makes the assembly impossible to do component-level test and repair. You might not care, but the entity who has to maintain the equipment will.

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  • \$\begingroup\$ Customer has a transformer with taps that we connect to. The transformer is submerged in oil for cooling and for a hazardous environment. Customer wants our stuff in the box with their transformer. \$\endgroup\$ Commented Jul 28, 2020 at 20:37
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Dielectric constant of oil will be much higher than air, 2-3x more probably. If your circuit has high frequency tuned circuits or fast edges the additional capacitance and cross-talk might have some negative effects.

That may affect some circuits. I would be particularly worried about the electrolytic capacitor seals. In fact Chemi-Con says:

enter image description here

As @Hearth mentions, connector plastic bodies are another type of material that might be affected by contact with oil. I would be suspicious of the longevity of any plastics including connectors, spacers, washers, wire insulation, transformer insulation etc. Ceramics, metals and epoxy (and other thermoset plastics) are probably okay.

Silicone oil is probably better than hydrocarbon oil, but it needs to be considered.

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    \$\begingroup\$ You'd probably want to also check any plastic connectors or mounting hardware and make sure you don't have any that would dissolve in the oil. Most plastics are resistant to solvents and oils aren't generally great solvents in the first place, but you don't really want to find out the hard way that your system includes that one rare incompatibility. Oil getting soaked into PCBs might also change their dimensions and will definitely change dielectric constants, which may have detrimental effects if you have controlled-impedance lines on them. \$\endgroup\$
    – Hearth
    Commented Jul 28, 2020 at 16:17
  • \$\begingroup\$ Spehro, I came across that statement on the Chemi-con website just before I checked here for answers! Maybe I'll have to use ceramic caps instead. \$\endgroup\$ Commented Jul 28, 2020 at 20:51
  • \$\begingroup\$ Hearth - yes, thanks for that info. I have a lot of work ahead of me for this. \$\endgroup\$ Commented Jul 28, 2020 at 20:52
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    \$\begingroup\$ @CareyFisher Beware of pressure causing cracking. Potting is a whole can of worms, I had to look into it deeply for a recent project. \$\endgroup\$ Commented Jul 28, 2020 at 20:58

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