Potting calibrated components

M

markp

Jan 1, 1970
0
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.
 
K

Ken Smith

Jan 1, 1970
0
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?[/QUOTE]

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.
 
M

markp

Jan 1, 1970
0
Ken Smith said:
<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.
Click to expand...

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.
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.
Click to expand...

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.
If the frequencies are high, I'd also worry about the electrical
characteristics of the RTV changing the stray capacitance.
Click to expand...

Indeed, but for me it's all DC, relatively speaking!

Thanks for your comments Ken.

Mark.
 
J

John Larkin

Jan 1, 1970
0
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.
Click to expand...

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
 
I

Ian Stirling

Jan 1, 1970
0
John Larkin said:
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.
Click to expand...

I've had good results with standard acetic acid RTV, mixed with tin
oxide.
Non acidic, sets fast, cheap.
 
R

Robert Baer

Jan 1, 1970
0
Ian said:
I've had good results with standard acetic acid RTV, mixed with tin
oxide.
Non acidic, sets fast, cheap.
Click to expand...
Personally, i would not trust such a goofy mix.
 
R

Robert Baer

Jan 1, 1970
0
markp said:
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.
Click to expand...
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.
 
J

John Larkin

Jan 1, 1970
0
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.
Click to expand...

But every time I ask the guys in the shop to machine diamond
heatsinks, they whine about how hard it is.

John
 
R

Rich Grise

Jan 1, 1970
0
But every time I ask the guys in the shop to machine diamond heatsinks,
they whine about how hard it is.
Click to expand...

What are the thermal properties of cubic zirconia?


Thanks,
Rich
 
R

Rich Grise

Jan 1, 1970
0
But every time I ask the guys in the shop to machine diamond heatsinks,
they whine about how hard it is.
Click to expand...
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
 
J

John Larkin

Jan 1, 1970
0
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
Click to expand...

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
 
D

Don Lancaster

Jan 1, 1970
0
markp said:
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.
Click to expand...
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: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
K

Ken Smith

Jan 1, 1970
0
Rich Grise said:
There are also potting compounds that stay resilient, but they're
horrendously expensive.
Click to expand...

....for smallish values of horrendous. Getting a thermally conductive one
that is resilient can be a real problem though.
 
J

John Larkin

Jan 1, 1970
0
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.
Click to expand...

Then I'll be careful in the future to not wind any wire on them, as it
may constrict their motion.

John
 
K

Keith Williams

Jan 1, 1970
0
What are the thermal properties of cubic zirconia?
Click to expand...

I'm not about to give one to my wife to find out how hot she gets!
 
T

The Real Andy

Jan 1, 1970
0
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.
Click to expand...

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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