Vacuum-drying of cleaned PCBs?

Question

De-ionized water is generally regarded as an effective cleaning agent for PCBs, as shown here and here. However, removing that water means compressed air, oven, and/or lengthy drying-time. Even if all these are used, water can still be present under (and inside) some components (i.e. transformer housing), possibly contributing to crevice corrosion and other un-desireables.

Similar to the question regarding Vacuum dessication of MSL components, what would the efficacy be of subjecting a washed PCB to low-pressure conditions for accelerated drying?

Most discrete components are solid and homogenous, so should be unaffected. But what about:

• Electrolytic, polymer, polystyrene capacitors
• Sealed non-vacuum items (fuses, relays?)
• Sealed vacuum items (thermionic tubes, neon lamps, etc)
• LCD displays
• Battery cells

Can electrolytics survive vacuum, or will they vent? What about a glass fuse? Are Nixie tubes safe in a vacuum? Would an LCD display be altered at all?

Batteries are interesting. One video reports that certain 18650-sized cells may be unaffected by vacuum, while LiPo swells but remains functional. I'd not trust one after this, nor consider coin cells, or vented liquid-electrolyte types. Another video shows a GoPro camera and Fieldpiece temperature probe both operating well under vacuum conditions, however that was of short duration. Real vacuum moisture removal would take an hour or so, depending on ambient temperature.

Answer 1

Well, this becomes a complex question with a very easy answer, if we'll let ourselves be tempted to be so short and uncomplicated:

This varies between suppliers and exact types, and only your manufacturer on a part by part basis can tell you.

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However, from experience on small scale (Note that most of what I do in my lab is upto proto-series of 20 pieces, so luck is not highly likely, but also not impossible), I can say many component types fare well under a 70 kPa (0.7 bar) drop of pressure.

Many Electrolytics get specified to vent at hundreds of kPa of pressure differential, if not close to one or two MPa, because the reason for venting is not having it blow up, but small amounts of pressure should not immediately cause destructive failure to the entire device.

Generally the advised threshold for operation, if I am not mistaken, for wet electrolytic capacitors of modern design is 10kPa absolute (in most locations that's -85kPa -or deeper- relative).

The only document I could quickly find is this one stating 3kPa absolute somewhere on page 11. (I'm sure I could find more information from more suppliers if I put the time in).

For Neon lights and the like, these should definitely operate down to pressures like 10kPa absolute, unless their construction is especially shoddy (which is a decent risk these days, possibly even with semi-branded stuff).

But basically anything specified to be used on high-flight-path system in none-pressure-compensated areas should already be vacuum qualified, since your vacuum should build up gradually and usually those parts are specified for pressure-shock and reverse-pressure-shock at low ambient pressure.

This is also a point of attention: The speed at which you build up the vacuum may be even more important than the final pressure, if you let it dip nearly instantly, you may cause a shock allowing something to percussively break from the inside (boil-off or low grade plastic containing micro cavities). But you'd need a very impressive system to be able to do that, I would estimate.

For batteries, well, that is something I'm not going to be touching, as there are a million and one manufacturers, especially lithium types, and not even half of them really consider outside mechanical shocks other than commercial grade testing requirements (if even that!).

Answer 2

I'm sceptical.

Most SMD parts need special handling and storage to remain moisture-free. This is because the plastics and ceramics can absorb small amounts of moisture which may turn unto steam in a reflow oven, causing the parts to crack. And like ESD, this can cause subtle damage that won't manifest itself clearly at first inspection.

This is why drying/baking such components involves extended periods of time at an elevated temperature, to give the moisture a chance to leave the parts gradually.

If reflow soldering can cause this, I don't see why boiling off water using a vacuum couldn't.