Sasha,
I don't have experience with PCB's in a vacuum system, but I do have some
experience with lots of other materials in high vacuum as I used to work
designing electronics for mass spectrometers. I'm an EE, not a materials
person, but a few insights rubbed off on me while I worked with them. The
high vacuum part of our system was around 10exp(-9) torr, so I don't know if
this is the same range that you are concerned with.
However, in this range of vacuum, the main problem was any surface
irregularities that could contain bits of material that slowly but steadily
vaporize at this low pressure level. If you inserted a "dirty" piece into
the vacuum system, you could wait days for the pressure to approach the
final value. If the part was dirty enough, you could wait a week or more
and perhaps never get there. We called this phenomenon a virtual leak.
That is, when a part has enough material with low volatility in it, the
steady outgassing acts similarly to a small leak in the chamber. Heating
the vacuum chamber and the parts inside it speeds up this process quite a
bit. Since the parts in our system were made of stainless steel, ceramics
and glasses, we could heat the system to 350 degrees C. for several days to
get everything clean and the pressures stable. Since the instrument was a
mass spectrometer, we had the luxury of being able to monitor the
composition of the outgassed material and it was usually water! Even at 350
degrees C. and high vacuum, it could take days for most of the water to come
off. Obviously, you can't do this with a PCB but heating helps get the
outgassing over with sooner.
The bottom line is that any rough or porous materials are bad news in a high
vacuum system, as it makes it very difficult to get them truly clean. That
is why I'm not surprised that boards with solder mask performed worse than
boards with no mask. We look at solder mask and think of it as a smooth
"sealing" layer, but on a micro scale, I bet it looks like a piece of
Muenster cheese. The board itself will entrap material in its surface, then
you cover it up with another porous layer that entraps more material and the
composite will outgas for a long time. Also, any uncured mask would
probably outgas badly for a long period.
If I were to guess how to improve things, I might try using the thinnest
laminate possible so entrapped material had the best chance to get out.
Beyond that, I'm on pretty thin ice. I would guess you want a resin
material that has the fewest voids and the lowest gas diffusion rate, but I
could be way off on this. Also, I don't know the properties of the copper
foils and the electrodeposited copper used in PCB fabrication with respect
to outgassing. Any materials scientists out there care to pick this up?
Regards,
Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Sasha Miladinovic
> Sent: Wednesday, November 21, 2001 10:01 AM
> To: [log in to unmask]
> Subject: Re: [TN] HELP <=> PCB vs. High Vacuum <=> HELP
>
>
> Earl, Brad,
>
> Thank you for the answers. Obviously, there is no a lots of experiences on
> this area among TechNetters or the question itself wasn't interesting
> enough. :)
> However I just want to follow up this subject with the experiences that I
> gained during the testing of the PCB in vacuum chamber. I hope
> that this is
> going to help someone in the future.
> I found that the best solution was to add two extra layers (top
> and bottom)
> to the PCBs that are only containing pads and annular rings for the plated
> holes. Pads are connected with the layers with hidden vias. That way, I
> minimised the amount of the exposed copper/ENIG on the surface of the PCB.
> There is no coating on the laminate at all, which minimises outgassing (I
> wouldn't believe this if I haven't seen it myself).
> I'm washing the flux residues away directly after the reflow, first with
> Zestron and than with not-ionised water. After this, the PCBs are
> baked and
> stored in the nitrogen atmosphere. Before mounting the PCBs in to the
> vacuum the PCBs are stored in the vacuum chamber during the 8
> hours period,
> outgassing as much as possible. When this procedure is finnished and the
> PCBs are mounted in to the vacuum, make little prayer and hope
> for the best
> when you push "ON" button. :)
>
> Sasha Miladinovic
>
>
> **********************************************************************
> Sasha Miladinovic - Production Engineering
> Amersham Biosciences, PCB Production, Umeå
> Tel: +46 (0) 90 150 232
> Fax: +46 (0) 90 138 372
> E-mail: [log in to unmask]
> **********************************************************************
>
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