Potting calibrated components

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.

In article , markp wrote: [...]

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.

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.

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.

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

Thanks for your comments Ken.

Mark.

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

There are also potting compounds that stay resilient, but they're horrendously expensive.

Good Luck! Rich

In article , John Larkin wrote: [...]

Also: Epoxy s conductive until it completely hardens. Don't apply voltage until then. Make sure you mix it exactly right.

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

John

Personally, i would not trust such a goofy mix.

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.

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

What are the thermal properties of cubic zirconia?

Thanks, Rich

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

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.

Cool, but the stuff with thermal conductivity much better than copper has a ghastly huge grain size.

John

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

John

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

In article , Rich Grise wrote: [...]

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

What a bunch of whiners. Go look here. They mold things out of it.