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Vacuum compatable poting for pcb

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Linas

Mon Nov 18 2013, 01:32PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Hello,
I need to make stepper motor driver for XYZ control, and since vacuum connectors are so expensive, my solution is to use only UART/power/programming pins to connector, and rest (limit switches and stepper drivers ) hold in vacuum (only one db9 connector is multiple times more expensive that all electronics, in order of 450EUR)

Since steppers will have to be moved only 0,001% of time, i will not have much heat in electronics. But still, i want to put it in aluminum box, and use some kind potting compound. Original potting compound is very expensive, but can i use something less expensive ? if yes, what ?

In my opinion epoxy can create gases, same story with polyurethane. Since i going too use only ceramic packages with ceramic capacitors/resistors, electronic will be stable at that pressure, well, as far as i know.

Ash Small

Sat Nov 23 2013, 02:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm not sure if I fully understand the question, but if you are planning to pot it and then use it under vacuum, any potting compound will outgas under vacuum.

We need more information please.

Epoxy can by used under moderate vacuum, but even the best still outgas. Why do you need to pot it? cooling?

Conundrum

Sat Nov 23 2013, 04:05PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Have had some success with grey gasket compound sold in car accessory shops.

Not sure if it will work in this case but the insulation resistance is excellent and it is non corrosive.

testtest

Sat Nov 23 2013, 07:00PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

I have used Permatex Cold weld epoxy on a quick KF25 to ISO63 adaptor for my turbopump. Machined the gap to minimize the epoxy exposure to the vacuum and sanded out any excess. Been holding for four years. BUT do not bake the system. Can reach better than 10^-7 torr in the system but keep in mind the epoxy in right next to the pump. Outgassing is not that bad after about 2 or 3 weeks in the system. I waited about a week for full curing under a 60W lamp. Overall compares to an unbaked vacuum five liter system.

Conclusion: doable and OK for mid vacuum.

Cons: not bakeable. Could break if stressed or develop leaks with temp differences when applied to stainless steel components.

Linas

Sat Nov 23 2013, 08:38PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Well, it will be a for cooling.

Problem with my setup that it will contain some expensive optics inside chamber, and i don't want to produce any gases that can be deposed on mirrors and other optic equipment. Also not to damage turbo-molecular pumps (less possible).

Potting also should protect optic components if my electronics decide to make magic smoke, sine it will be used for Stepper drivers, some small current will be flowing, but shit happens

Idea is to put all electronics to aluminum box,and pot it. so small amount of potting will be exposed to vacuum. also i don't want any expansion of potting material for obvious reasons

2Spoons

Mon Nov 25 2013, 05:03AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Could you make your own vacuum connectors out of valve bases?

testtest

Mon Nov 25 2013, 07:54PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Linas wrote ...

Well, it will be a for cooling.

Idea is to put all electronics to aluminum box,and pot it. so small amount of potting will be exposed to vacuum. also i don't want any expansion of potting material for obvious reasons

How about doing the inverse.
Put the electronics in the Al box and leave the inside of the box at atmospheric pressure. Then use either vacuum feed-troughs for the wiring and seal the box cover with epoxy. I had good success with drilling a small hole for each needed wire and a dab both inside and outside with epoxy to seal the individual wire. I used enameled magnet wire for the transition. That was about 10 years ago and it still works. I roughed up and used acetone in the area where the epoxy would go. This was a preamp right next to the CCD mounted on a cold finger of a LN2 Dewar. Only needed mid 10^-6 torr however. Electronics is near room temp on the outside wall but in the vacuum. System is pumped to about 1E-5 torr and then the LN2 takes care of the pumping for the next 12 hours.

Linas

Wed Nov 27 2013, 04:59AM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

we ordered special vacuum connectors, so i don't need to build one. Original idea was to put electronics outside, but that will use 3x db9 connectors, and by putting electronic inside i need only one. In that case we will save them for other tasks

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