| Author |
Message |
markp
Guest
|
 Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Potting calibrated components |
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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.
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Ken Smith
Guest
|
| Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Re: Potting calibrated components |
|
|
In article <3vo368F172lejU1@individual.net>, markp <map.nospam@f2s.com> wrote:
[...]
| Quote: | 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?
|
Electronics grade RTV is unlikely to cause stress problems but there could
be other things.
If the calibration includes the setting of RC time constants, I'd worry
about what type of capacitors you are using. The SMT film ones may be
effected.
If the calibrated parts are in a small area on one side of the PCB, I
would put RTV on the back of the PCB as well in the area where the
calibration parts are. If the epoxy expands, it will press on the PCB.
If it can press on one side and not the other, it may make the PCB try to
cup.
If the frequencies are high, I'd also worry about the electrical
characteristics of the RTV changing the stray capacitance.
--
--
kensmith@rahul.net forging knowledge |
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Ian Stirling
Guest
|
| Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Re: Potting calibrated components |
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John Larkin <jjlarkin@highnotlandthistechnologypart.com> wrote:
| Quote: | On Wed, 7 Dec 2005 12:34:32 -0000, "markp" <map.nospam@f2s.com> 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.
Epoxy can apply extreme stresses to parts as it cures. Consider a
softer potting material; polyurethanes are good.
Standard RTV, with acetic acid, is death to electronics.
|
I've had good results with standard acetic acid RTV, mixed with tin
oxide.
Non acidic, sets fast, cheap.
|
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John Larkin
Guest
|
| Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Re: Potting calibrated components |
|
|
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.
|
Epoxy can apply extreme stresses to parts as it cures. Consider a
softer potting material; polyurethanes are good.
Standard RTV, with acetic acid, is death to electronics.
John |
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markp
Guest
|
| Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Re: Potting calibrated components |
|
|
"Ken Smith" <kensmith@green.rahul.net> wrote in message
news:dn6t6t$cvl$1@blue.rahul.net...
| Quote: | snip>.
If the calibration includes the setting of RC time constants, I'd worry
about what type of capacitors you are using. The SMT film ones may be
effected.
|
The calibration is for more or less DC type signals, actually pressure and
temperature measurement in an engine compartment, none of which are fast
changing quantities and I use caps primarily to remove high frequency
components and prevent oscillation. I calibrate only to remove DC offsets
and scaling variation, this is done by software.
| Quote: | If the calibrated parts are in a small area on one side of the PCB, I
would put RTV on the back of the PCB as well in the area where the
calibration parts are. If the epoxy expands, it will press on the PCB.
If it can press on one side and not the other, it may make the PCB try to
cup.
|
Yes, in my situation there are components on either side of the board anyway
(plus vias) so I was going to put a thin layer on either side.
| Quote: | If the frequencies are high, I'd also worry about the electrical
characteristics of the RTV changing the stray capacitance.
|
Indeed, but for me it's all DC, relatively speaking!
Thanks for your comments Ken.
Mark. |
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Ken Smith
Guest
|
| Posted:
Thu Dec 08, 2005 2:36 am Post subject:
Re: Potting calibrated components |
|
|
In article <ko2ep19lrsdft7batk6ti06t18pmg4egla@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
[...]
| Quote: | Epoxy can apply extreme stresses to parts as it cures. Consider a
softer potting material; polyurethanes are good.
|
Also: Epoxy s conductive until it completely hardens. Don't apply
voltage until then. Make sure you mix it exactly right.
--
--
kensmith@rahul.net forging knowledge |
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Rich Grise
Guest
|
| Posted:
Thu Dec 08, 2005 9:02 am Post subject:
Re: Potting calibrated components |
|
|
On Wed, 07 Dec 2005 14:56:29 +0000, Ken Smith wrote:
| Quote: | In article <3vo368F172lejU1@individual.net>, markp <map.nospam@f2s.com
wrote: [...]
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?
Electronics grade RTV is unlikely to cause stress problems but there could
be other things.
If the calibration includes the setting of RC time constants, I'd worry
about what type of capacitors you are using. The SMT film ones may be
effected.
If the calibrated parts are in a small area on one side of the PCB, I
would put RTV on the back of the PCB as well in the area where the
calibration parts are. If the epoxy expands, it will press on the PCB. If
it can press on one side and not the other, it may make the PCB try to
cup.
If the frequencies are high, I'd also worry about the electrical
characteristics of the RTV changing the stray capacitance.
|
There are also potting compounds that stay resilient, but they're
horrendously expensive.
Good Luck!
Rich |
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Robert Baer
Guest
|
| Posted:
Thu Dec 08, 2005 9:35 am Post subject:
Re: Potting calibrated components |
|
|
Ian Stirling wrote:
| Quote: | John Larkin <jjlarkin@highnotlandthistechnologypart.com> wrote:
On Wed, 7 Dec 2005 12:34:32 -0000, "markp" <map.nospam@f2s.com> 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.
Epoxy can apply extreme stresses to parts as it cures. Consider a
softer potting material; polyurethanes are good.
Standard RTV, with acetic acid, is death to electronics.
I've had good results with standard acetic acid RTV, mixed with tin
oxide.
Non acidic, sets fast, cheap.
Personally, i would not trust such a goofy mix. |
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|
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Robert Baer
Guest
|
| Posted:
Thu Dec 08, 2005 9:35 am Post subject:
Re: Potting calibrated components |
|
|
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.
So-called thermally conductive epoxy is a joke; they are only |
slightly more "thermally conductive" than the ordinary stuff.
They are a fairly good insulator like the ordinary epoxies.
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. |
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John Larkin
Guest
|
| Posted:
Thu Dec 08, 2005 5:35 pm Post subject:
Re: Potting calibrated components |
|
|
On Thu, 08 Dec 2005 09:27:36 GMT, Robert Baer
<robertbaer@earthlink.net> wrote:
| Quote: | 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.
John |
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Ken Smith
Guest
|
| Posted:
Fri Dec 09, 2005 12:57 am Post subject:
Re: Potting calibrated components |
|
|
In article <pan.2005.12.08.03.06.31.839540@example.net>,
Rich Grise <richgrise@example.net> wrote:
[...]
| Quote: |
There are also potting compounds that stay resilient, but they're
horrendously expensive.
|
....for smallish values of horrendous. Getting a thermally conductive one
that is resilient can be a real problem though.
--
--
kensmith@rahul.net forging knowledge |
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Ken Smith
Guest
|
| Posted:
Fri Dec 09, 2005 1:01 am Post subject:
Re: Potting calibrated components |
|
|
In article <obngp1pu5t33ucc019blg9jnqd2lucrkul@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
| Quote: | 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/
--
--
kensmith@rahul.net forging knowledge |
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Rich Grise
Guest
|
| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
|
|
On Thu, 08 Dec 2005 08:22:54 -0800, John Larkin wrote:
| Quote: | 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? On last night's "CSI", |
they spotted a big fake diamond because it fluoresced. According to the
script, real diamonds don't fluoresce. But I have heard that they're the
best thermal conductor known; I've also heard that beryllia is an
excellent thermal conductor/electrical insulator, but if you grind it
or machine it, you need a full environmental (HazMat) suit. :-)
Thanks!
Rich |
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Rich Grise
Guest
|
| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
|
|
On Thu, 08 Dec 2005 08:22:54 -0800, John Larkin wrote:
| Quote: | 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?
Thanks,
Rich |
|
| Back to top |
|
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John Larkin
Guest
|
| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Potting calibrated components |
|
|
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? On last night's "CSI",
they spotted a big fake diamond because it fluoresced. According to the
script, real diamonds don't fluoresce. But I have heard that they're the
best thermal conductor known; I've also heard that beryllia is an
excellent thermal conductor/electrical insulator, but if you grind it
or machine it, you need a full environmental (HazMat) suit. :-)
Thanks!
Rich
|
Material Thermal conductivity (298K)
W·m-1·K-1
Diamond 895-2300
Carbon Nanotubes 1400
Silver 429
Copper 386
Gold 317
Aluminium 237
Of course, we use isotopically pure diamond which has a thermal
conductivity about 20x the cheap stuff shown above.
John |
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