FR4 PCB under transformer oil?

[daqq]

Hi guys,

For a high-ish voltage project we are thinking about making a high voltage isolation transformer using two PCBs with a little electronics. We are thinking about submersing the whole assembly into transformer oil rather than put in into epoxy or a similar encapsulant.

There should be several advantages - no particular attention should be needed for the casting, no bubbles to worry about and also it can be removed and modified/serviced at will. Also it's an improvement over an airy construction, since the gaps need not be very large.

There are disadvantages - it's wet, so it needs to be sealed.

My question is: Are there any non-obvious drawbacks to this? Does FR4 like transformer oil? Won't it dissolve or something ? What about solder masks?

Thanks,

David

[CM800]

Hi guys,

For a high-ish voltage project we are thinking about making a high voltage isolation transformer using two PCBs with a little electronics. We are thinking about submersing the whole assembly into transformer oil rather than put in into epoxy or a similar encapsulant.

There should be several advantages - no particular attention should be needed for the casting, no bubbles to worry about and also it can be removed and modified/serviced at will. Also it's an improvement over an airy construction, since the gaps need not be very large.

There are disadvantages - it's wet, so it needs to be sealed.

My question is: Are there any non-obvious drawbacks to this? Does FR4 like transformer oil? Won't it dissolve or something ? What about solder masks?

Thanks,

David

Looking around online, it seems the consensis is that FR4 will absorb hydrocarbons and will grow.

http://www.electronicspoint.com/threads/fr4-in-oil.227674/

Seems a kapton or even ceramic PCB might be the solution.

[Zero999]

I wonder is a conformal coat of urethane or silicone rubber would help?

[CM800]

I wonder is a conformal coat of urethane or silicone rubber would help?

I imagine it would, but any hole in it and you'll have a bad day. I'd bet it cheaper just to build your PCB on ceramic if it can be one-sided, else just keep it out of the case.

[amyk]

I remember from Mike's X-ray machine teardowns they had what looks like regular FR4 immersed in insulating oil (along with the transformer and other HV electronics.) Maybe it depends on the oil?

[TerraHertz]

My question is: Are there any non-obvious drawbacks to this? Does FR4 like transformer oil? Won't it dissolve or something ? What about solder masks?

Is it a one-off, for your own use?
Then why not do development versions on FR4 anyway? You can experiment (and read) on different oil types in the process. If it turns out to be a problem, redo on some more resilient substrate.
Surely there must be some oil types that are compatible with FR4. How about silicone oil? Synthetics?

Plus, we'd like to hear your results. It's something I'd like to know too.

[Jay_Diddy_B]

Hi,

There is no problems with using FR4 in transformer oil. In the past I designed HV tanks for medical X-ray machines. Just used regular FR4.
I used two oils:

voltesso 35

and

Shell Diala

I considered these oils interchangeable and mixable.

Silcone Rubber is not very good in oil, it swells up.

Teflon wire is fine. Normal magnet wire is fine.

Regards,

Jay_Diddy_B

[CM800]

Hi,

There is no problems with using FR4 in transformer oil. In the past I designed HV tanks for medical X-ray machines. Just used regular FR4.
I used two oils:

voltesso 35

and

Shell Diala

I considered these oils interchangeable and mixable.

Silcone Rubber is not very good in oil, it swells up.

Teflon wire is fine. Normal magnet wire is fine.

Regards,

Jay_Diddy_B

Seems I'm wrong... Glad I am. That is quite good for future projects I have in mind.

[N2IXK]

I have worked on HV power supplies for broadcast transmitters that used FR4 boards for the rectifier diode stacks immersed in a tank of transformer oil. No problem with the board material deteriorating.

[SeanB]

Most transformers use SRBP ( the brown phenolic paper) as both insulators and structural members, and also I have seen a lot of GRP boards in there as well.

You just need to make the traces a little wider ( do not go for a thinnest possible trace, make them wider so there is better adhesion to the copper), make the clearances generous and most importantly do not use any soldermask on the board, unless you are doing SMD, where you will make the mask as small as possible around the parts that need it. You can use certain printed labels on the board, just not those that are done with a solvent based single component paint, you want an all epoxy 2 part mask and silkscreen. You can either use ENIG or tin plate, though the ENIG will not corrode in the oil if it gets moist.

Most important is to make the housing able to withstand either being baked at 130C for a few hours to dewater the oil, or it must be strong enough to not buckle with a vacuum applied to do the same at room temperature.

Your connections need to be oil tight, I would recommend using a machine shop to turn up a few dozen ( as required) pass through connectors, in PTFE, with threaded  rod and washers to make the connections, or to solder some feedthrough capacitors for lower voltage stuff. Or use transformer connector kits, as those will have the whole set of Neoprene seals and parts to make an oil proof high voltage connector.

Inner wiring sleeve with woven glass fibre sleeving, using PTFE wire or even solid bare copper wire.  Make the cover such that it has a decent neoprene rubber sealing washer, and bolts every 50mm to clamp it down.

[daqq]

Thanks guys,

So, FR4 should be OK, but there will be a price to pay in terms of a more complicated enclosure.

Thanks.

[SeanB]

Yes, and remember as well you cannot use components with rubber seals, so most electrolytic capacitors are out, and many film capacitors as well. A trick is to use motor run capacitors in metal cans, and simply use them as is, or use a tubing cutter to decan them to use under the oil bare. You can use polypropylene caps as well, but they will change value slightly as the oil wicks into them.