PCB under vacuum

[amdx]

On 11/7/2019 1:35 PM, Jon Elson wrote:

On Wed, 06 Nov 2019 16:44:18 -0600, amdx wrote:


As long as I'm here with experienced people, is there another
vacuum pressure kit that is cheaper, or works in a vacuum?
MKS makes a range of vacuum sensors, Pirani and other types. We use one
of their modular type that attach to a standard fitting on the outside of
the chamber. It wasn't much more expensive than the full kit you linked
to. Wide-range Pirani sensors are prone to damage from pump oils and
other contaminants in the vacuum system, we already had to change out our
sensor element once.

Jon

In the FWIW dept,
I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.
Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.
> https://webshop.demaco.nl/torr-seal-113-gram-45-120-c.html?gclid=Cj0KCQiAno_uBRC1ARIsAB496IX6hMKSX0rQr9sFKe_dqHWavhuezEVjqkiykhIzm94RyvK-t2X1BKYaAsQ3EALw_wcB

Can they make a good seal on a ribbon cable through a slot?

Will the ribbon cable leak?

Is there any difficulty sealing a -7 Torr slot passing a ribbon cable?
He mentioned maybe a 1/2mm slot width.

Just want to put some ideas in his head to think about if anyone has
experience with this.

Thanks, Mikek

 

[Bill Sloman]

On Friday, November 8, 2019 at 10:51:58 AM UTC+11, amdx wrote:

On 11/7/2019 1:35 PM, Jon Elson wrote:
On Wed, 06 Nov 2019 16:44:18 -0600, amdx wrote:


As long as I'm here with experienced people, is there another
vacuum pressure kit that is cheaper, or works in a vacuum?
MKS makes a range of vacuum sensors, Pirani and other types. We use one
of their modular type that attach to a standard fitting on the outside of
the chamber. It wasn't much more expensive than the full kit you linked
to. Wide-range Pirani sensors are prone to damage from pump oils and
other contaminants in the vacuum system, we already had to change out our
sensor element once.

Jon


In the FWIW dept,
I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.
Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.
https://webshop.demaco.nl/torr-seal-113-gram-45-120-c.html?gclid=Cj0KCQiAno_uBRC1ARIsAB496IX6hMKSX0rQr9sFKe_dqHWavhuezEVjqkiykhIzm94RyvK-t2X1BKYaAsQ3EALw_wcB

Can they make a good seal on a ribbon cable through a slot?

Will the ribbon cable leak?

Is there any difficulty sealing a -7 Torr slot passing a ribbon cable?
He mentioned maybe a 1/2mm slot width.

It depends a bit on the insulator on the ribbon cable. Most flexible plastic cables depend a "plasticiser" in the plastic to stay flexible. If you pump them for long enough, the plasticiser evaporates off (degrading your vacuum in the process) and the plastic insulation gets brittle.

PTFE (Teflon) insulation doesn't have this problem, nor does Kel-F.

https://en.wikipedia.org/wiki/Polychlorotrifluoroethylene

For a half millimetre slot width he might have to go to flexible printed circuit rather than a cable.

You can buy D-type connector vacuum feed-throughs. For high vacuum work they can incorporate glass to metal seals. They weren't cheap in the late 1980s.

--
Bill Sloman, Sydney

Just want to put some ideas in his head to think about if anyone has
experience with this.

Thanks, Mikek

 

[George Herold]

On Thursday, November 7, 2019 at 6:51:58 PM UTC-5, amdx wrote:

On 11/7/2019 1:35 PM, Jon Elson wrote:
On Wed, 06 Nov 2019 16:44:18 -0600, amdx wrote:


As long as I'm here with experienced people, is there another
vacuum pressure kit that is cheaper, or works in a vacuum?
MKS makes a range of vacuum sensors, Pirani and other types. We use one
of their modular type that attach to a standard fitting on the outside of
the chamber. It wasn't much more expensive than the full kit you linked
to. Wide-range Pirani sensors are prone to damage from pump oils and
other contaminants in the vacuum system, we already had to change out our
sensor element once.

Jon


In the FWIW dept,
I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.
Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.

Ribbon cable and vacuum seal.. Threre's the question of gas leaking
along the cable. Good vacuum seals are mostly metal to epoxy/ insulator.
plastic leaks.

George H.

https://webshop.demaco.nl/torr-seal-113-gram-45-120-c.html?gclid=Cj0KCQiAno_uBRC1ARIsAB496IX6hMKSX0rQr9sFKe_dqHWavhuezEVjqkiykhIzm94RyvK-t2X1BKYaAsQ3EALw_wcB

Can they make a good seal on a ribbon cable through a slot?

Will the ribbon cable leak?

Is there any difficulty sealing a -7 Torr slot passing a ribbon cable?
He mentioned maybe a 1/2mm slot width.

Just want to put some ideas in his head to think about if anyone has
experience with this.

Thanks, Mikek

 

[Jasen Betts]

On 2019-11-07, amdx <nojunk@knology.net> wrote:

On 11/7/2019 1:35 PM, Jon Elson wrote:
On Wed, 06 Nov 2019 16:44:18 -0600, amdx wrote:


As long as I'm here with experienced people, is there another
vacuum pressure kit that is cheaper, or works in a vacuum?
MKS makes a range of vacuum sensors, Pirani and other types. We use one
of their modular type that attach to a standard fitting on the outside of
the chamber. It wasn't much more expensive than the full kit you linked
to. Wide-range Pirani sensors are prone to damage from pump oils and
other contaminants in the vacuum system, we already had to change out our
sensor element once.

Jon


In the FWIW dept,
I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.
Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.

What are theses negative numbers? torr is an absolute scale.

--
When I tried casting out nines I made a hash of it.

 

[Bill Sloman]

On Friday, November 8, 2019 at 4:01:03 PM UTC+11, Jasen Betts wrote:

On 2019-11-07, amdx <nojunk@knology.net> wrote:
On 11/7/2019 1:35 PM, Jon Elson wrote:
On Wed, 06 Nov 2019 16:44:18 -0600, amdx wrote:


As long as I'm here with experienced people, is there another
vacuum pressure kit that is cheaper, or works in a vacuum?
MKS makes a range of vacuum sensors, Pirani and other types. We use one
of their modular type that attach to a standard fitting on the outside of
the chamber. It wasn't much more expensive than the full kit you linked
to. Wide-range Pirani sensors are prone to damage from pump oils and
other contaminants in the vacuum system, we already had to change out our
sensor element once.

Jon


In the FWIW dept,
I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.
Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.

What are theses negative numbers? torr is an absolute scale.

People tend to talk about pressures of 2x10^-7 torr, and that can get truncated to -7 by people who weren't influenced hard enough by their instructors early on.

--
Bill Sloman, Sydney

 

[Martin Brown]

On 07/11/2019 23:58, amdx wrote:

On 11/7/2019 4:14 PM, George Herold wrote:
On Wednesday, November 6, 2019 at 5:10:22 PM UTC-5, amdx wrote:
My son, in his college senior year is doing some research, a machine
failed today while he was using it. They have 5 defective pcbs, so
apparently it has happened before.
   The pcb is placed in a -7 tor vacuum as I understand (I might not)
The 1-1/2" x 2" pcb is $600. (smd parts)
   So immediately, I think well I can fix that!
So other than overheating parts in the vacuum what parts would be
suspect on a pcb used in a vacuum when not designed for a vacuum.

   OK, just got off the phone, he's working with an ion trap.
The defective board is a 905 MicroPirani™ Vacuum Pressure Transducer.
https://www.mksinst.com/f/905-micro-pirani-vacuum-sensor-kit
   He told me except for the resistors, the parts have no markings, so
I'm no at all confident about any repair.

   They are suspecting the vacuum is the problem and are looking to get
the pcb outside of the vacuum.
Yeah get it out of the vacuum!  Any idea of the voltages involved?
There's a sweet spot in pressure where it's easier to get
'HV' breakdown of the gas.  The other thing you can do is turn
off the voltage till the electronics is in high vacuum...
(Is it going to high vacuum?  Some thing more than a mechanical pump.)
If the part is always sitting around 7 torr then you are stuck with
taking it out

George H.

 See my recent post, they are getting the pcb out of the vacuum.
I'm a getting a little confused as he is working two different machines,
pretty sure the ion trap does have some high voltages, but they have
been doing that, so I don't think that is a problem.

                             Mikek

Pirani gauges are essentially a hot filament in a bridge that gets a
whole lot hotter in a good vacuum. No high voltages involved. They run
coolish at atmospheric pressure and can stand a fair bit of abuse.

Typically they are used to determine when it is safe to switch the hard
vacuum pumps and power to the more sensitive hard vacuum Penning gauge.

One other thing to be aware of in high vacuum stuff with high voltages
is that every now and then a flashover can occur when an unfortunate
emission of ions shorts out the bit at HT to ground. All bets are off
when that happens - you tend to hear it go crack if near enough.

--
Regards,
Martin Brown

 

[Martin Brown]

On 08/11/2019 04:32, Jasen Betts wrote:

What are theses negative numbers? torr is an absolute scale.

Implicit prefix of "10^"

People tend to talk about hard vacuum as -N or even N
(since the vacuum bit is already implied by the kit)

Its a bit like pH but with the log10 bit unstated.

--
Regards,
Martin Brown

 

[Martin Brown]

On 07/11/2019 23:51, amdx wrote:

In the FWIW dept,
 I did get a little more info from my son.
The outer chamber does have up to -7 torr and yes, it uses
a different type sensor. On the internal chamber, they run -2 Torr after
the gas is allowed in.

OK. That is a fair bit more in line with the gauge capability.

  Their plan to reposition the pcb by cutting a slot in an access cap
and run a ribbon cable through the slot to the sensor. Then use Torr
seal to plug the hole.

Worth talking to the manufacturer. I wouldn't expect having any flexible
plastic in a hard vacuum enclosure to be a good idea. The plasticiser
has a non-trivial vapour pressure and will coat everything internally.

https://webshop.demaco.nl/torr-seal-113-gram-45-120-c.html?gclid=Cj0KCQiAno_uBRC1ARIsAB496IX6hMKSX0rQr9sFKe_dqHWavhuezEVjqkiykhIzm94RyvK-t2X1BKYaAsQ3EALw_wcB


  Can they make a good seal on a ribbon cable through a slot?

Probably not. Especially not when you bake the enclosure.

> Will the ribbon cable leak?

Almost certainly. I thought from a quick scan that they offered a
connector to allow the sensor to be connected to a feed through.

 Is there any difficulty sealing a -7 Torr slot passing a ribbon cable?
He mentioned maybe a 1/2mm slot width.

Just want to put some ideas in his head to think about if anyone has
experience with this.

I'm not up to date with modern hard vacuum practice.
But ISTR the only plastic we would ever use in hard vacuum was PEEK and
even then we rejected some batches as too gassy.

--
Regards,
Martin Brown