| Author |
Message |
John Fields
Guest
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 Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
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On Thu, 08 Dec 2005 20:53:04 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Thu, 08 Dec 2005 08:22:54 -0800, John Larkin wrote:
On Thu, 08 Dec 2005 09:27:36 GMT, Robert Baer <robertbaer@earthlink.net
If you must remove heat, use a copper or aluminum slab to the
outside; if you need even better thermal conductivity and excellent
electrical insulation (even at high temperatures) then use a diamond
slab.
But every time I ask the guys in the shop to machine diamond heatsinks,
they whine about how hard it is.
What are the thermal properties of cubic zirconia?
|
---
Thermal conductivity = 31.8 W/M°K
--
John Fields
Professional Circuit Designer
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John Larkin
Guest
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| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
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On Fri, 9 Dec 2005 01:01:09 +0000 (UTC), kensmith@green.rahul.net (Ken
Smith) wrote:
| Quote: | In article <obngp1pu5t33ucc019blg9jnqd2lucrkul@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 08 Dec 2005 09:27:36 GMT, Robert Baer
robertbaer@earthlink.net> wrote:
If you must remove heat, use a copper or aluminum slab to the
outside; if you need even better thermal conductivity and excellent
electrical insulation (even at high temperatures) then use a diamond slab.
But every time I ask the guys in the shop to machine diamond
heatsinks, they whine about how hard it is.
What a bunch of whiners. Go look here. They mold things out of it.
www.skeleton-technologies.com/
|
Cool, but the stuff with thermal conductivity much better than copper
has a ghastly huge grain size.
John |
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Don Lancaster
Guest
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| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
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markp wrote:
| Quote: | Hi All,
I recently potted some electronics using an epoxy with a filler to increase
thermal conductivity and reduce the thermal expansion co-efficient. However,
I had a problem later with some calibrated circuitry drifting. I did not
conformally coat the board before potting, which I think was a mistake!
There are issues with glass temperature and thermally induced stress in
surface mounted components too that I'd like to address. Operating temp
is -40degC to +80degC.
I don't want to risk any of this happening again, and so I'm considering
potting in a polyurethane based compound with a low Tg this time,
conformally coating the board, and to be extra safe putting a thin layer of
neutral RTV silicone over the top of the calibrated components. Any views on
whether the RTV itself might cause problems? Is it necessary? Any other
comments?
Thanks for your help, any comments welcome.
Mark.
Potting ferrite beads can be an absolute disaster. |
As was found out the hard way at an aerospace firm many years ago.
The ferrite bead MUST be able to magnetostrict and change size or its
characteristics will wildly change.
--
Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com
Please visit my GURU's LAIR web site at http://www.tinaja.com
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John Larkin
Guest
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| Posted:
Fri Dec 09, 2005 9:14 am Post subject:
Re: Potting calibrated components |
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On Thu, 08 Dec 2005 17:13:35 -0700, Don Lancaster <don@tinaja.com>
wrote:
| Quote: | markp wrote:
Hi All,
I recently potted some electronics using an epoxy with a filler to increase
thermal conductivity and reduce the thermal expansion co-efficient. However,
I had a problem later with some calibrated circuitry drifting. I did not
conformally coat the board before potting, which I think was a mistake!
There are issues with glass temperature and thermally induced stress in
surface mounted components too that I'd like to address. Operating temp
is -40degC to +80degC.
I don't want to risk any of this happening again, and so I'm considering
potting in a polyurethane based compound with a low Tg this time,
conformally coating the board, and to be extra safe putting a thin layer of
neutral RTV silicone over the top of the calibrated components. Any views on
whether the RTV itself might cause problems? Is it necessary? Any other
comments?
Thanks for your help, any comments welcome.
Mark.
Potting ferrite beads can be an absolute disaster.
As was found out the hard way at an aerospace firm many years ago.
The ferrite bead MUST be able to magnetostrict and change size or its
characteristics will wildly change.
|
Then I'll be careful in the future to not wind any wire on them, as it
may constrict their motion.
John |
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Keith Williams
Guest
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| Posted:
Fri Dec 09, 2005 9:35 am Post subject:
Re: Potting calibrated components |
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|
In article <pan.2005.12.08.20.53.13.696438@example.net>,
richgrise@example.net says...
| Quote: | On Thu, 08 Dec 2005 08:22:54 -0800, John Larkin wrote:
On Thu, 08 Dec 2005 09:27:36 GMT, Robert Baer <robertbaer@earthlink.net
wrote:
If you must remove heat, use a copper or aluminum slab to the
outside; if you need even better thermal conductivity and excellent
electrical insulation (even at high temperatures) then use a diamond
slab.
But every time I ask the guys in the shop to machine diamond heatsinks,
they whine about how hard it is.
What are the thermal properties of cubic zirconia?
|
I'm not about to give one to my wife to find out how hot she gets!
--
Keith |
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Tony Williams
Guest
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| Posted:
Fri Dec 09, 2005 11:08 am Post subject:
Re: Potting calibrated components |
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In article <3vt4ueF16sh2iU1@individual.net>,
markp <map.nospam@f2s.com> wrote:
| Quote: | Yes, there is a micro and I do use software calibration already,
the problem was drift. Funny enough, baking the unit for many
hours at 100 deg C fixed the problem, so I assume it was an
interaction between the potting compound and the PCB.
|
We used to temperature-cycle precision potted modules,
after curing, for at least 5x full cycles of the
operating temperature range. My theory was it relieved
the static stress on components that had been created
by the shrinkage of the potting compound when cured.
I also found that potting in small sections (of about
5 cu-inch per section) reduced the total stress. It
also gave a chance of tweaking the calibration after
early part-pots.
Room temperature potting compounds were the worst for
shrinkage (and resultant stress), even to the extent
of pulling electrolytic capacitors apart. We settled
on Emerson and Cummins 2651MM, which needed to be heated
to about 70C to start the cross-linking and send it off.
--
Tony Williams. |
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The Real Andy
Guest
|
| Posted:
Fri Dec 09, 2005 4:11 pm Post subject:
Re: Potting calibrated components |
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|
On Wed, 7 Dec 2005 12:34:32 -0000, "markp" <map.nospam@f2s.com> wrote:
| Quote: | Hi All,
I recently potted some electronics using an epoxy with a filler to increase
thermal conductivity and reduce the thermal expansion co-efficient. However,
I had a problem later with some calibrated circuitry drifting. I did not
conformally coat the board before potting, which I think was a mistake!
There are issues with glass temperature and thermally induced stress in
surface mounted components too that I'd like to address. Operating temp
is -40degC to +80degC.
I don't want to risk any of this happening again, and so I'm considering
potting in a polyurethane based compound with a low Tg this time,
conformally coating the board, and to be extra safe putting a thin layer of
neutral RTV silicone over the top of the calibrated components. Any views on
whether the RTV itself might cause problems? Is it necessary? Any other
comments?
Thanks for your help, any comments welcome.
Mark.
|
Is there a micro involved? If so, use software calibration. If no
micro, then you need to find alternative methods of calibration after
potting. |
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markp
Guest
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| Posted:
Fri Dec 09, 2005 4:28 pm Post subject:
Re: Potting calibrated components |
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| Quote: |
I'm not about to give one to my wife to find out how hot she gets!
|
Thought we were in alt.sex for a second there... :-) |
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markp
Guest
|
| Posted:
Fri Dec 09, 2005 4:31 pm Post subject:
Re: Potting calibrated components |
|
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"The Real Andy" <will_get_back_to_you_on_This@> wrote in message
news:61mip192k9t83m2fqef8roq8370vtcvvfp@4ax.com...
| Quote: | On Wed, 7 Dec 2005 12:34:32 -0000, "markp" <map.nospam@f2s.com> wrote:
Is there a micro involved? If so, use software calibration. If no
micro, then you need to find alternative methods of calibration after
potting.
|
Yes, there is a micro and I do use software calibration already, the problem
was drift. Funny enough, baking the unit for many hours at 100 deg C fixed
the problem, so I assume it was an interaction between the potting compound
and the PCB. |
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| Back to top |
|
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Keith Williams
Guest
|
| Posted:
Sat Dec 10, 2005 12:14 am Post subject:
Re: Potting calibrated components |
|
|
In article <3vt4o8F16n8trU1@individual.net>, map.nospam@f2s.com
says...
| Quote: |
I'm not about to give one to my wife to find out how hot she gets!
Thought we were in alt.sex for a second there... :-)
Nope, not at all! Rather, alt.sleeping_on_the_couch or perhaps |
alt.law.divorce-court.
--
Keith |
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