Epoxy and parasitic effects on circuit performance ...

On a project, I have an enclosure packed with electronics. After calibration and getting things just right I pour the epoxy (about .5 ounces).
When the epoxy sets off the units are consistently out of calibration.

My next step is to do some empirical testing and have a few wires sticking out of the epoxy that normally wouldn't be sticking out so that I can calibrate the circuit externally. Then with THAT value I can incorporate it into the full epoxied design.

I think I can fix this, but I wanted to ask if there was a preferred epoxy, or potting compound readily available that may not have the parasitic effects I am observing. I do not believe this is a stress issue due to deformation as the epoxy sets off, nor is it a heat issue as much as a dielectric capacitance issue with the epoxy.

Reference:
The epoxy I am using is from "bondo" the label says "FIBERGLASS RESIN" .... easy to use and measure. One drop of hardener for every 1/10th ounce of resin

Epoxy has a significant dielectric constant (in the range of 4 but depends on specific epoxy).

Epoxy also shrinks up to 10% by volume (between 0 and 10% depending on exact formula) during the curing process. That change causes a change in dielectric constant of the epoxy (resin dielectric constant is different than the cured solid epoxy).

Many epoxies have fillers (typically inorganic). From mica, glass fibers, steel or iron powders/dust/granules all minimize the shrinkage during curing but also change dielectric constant or magnetic properties.

What is your circuit and what property is drifting after the conformal coating is applied.

Interesting.

You do not say what type of circuit you are potting in epoxy.

I would not expect a low frequency amplifier to be affected very much by epoxy, but an HF amplifier or an oscillator I would expect to see a change.

The relative permittivity of FR4 circuit board is about 4.4 (Wikipedia), which is a lot larger that that of air which is 1. So any interwiring capacitance is going to be 4.4 (ish) times greater.

The only circuit that I have ever potted was a replacement EHT multiplier for an oscilloscope.
I cannot remember the compound that I used, but it was not epoxy and it was specifically intended for circuit potting.

JimB


Editing
Having just said that I did not use and "epoxy", I probably did.
In my head, epoxy was limited to two part adhesives like "Araldite", but apparently it is a much wider group of materials.

The potting compound I used for my voltage multiplier was packed in a similar manner to this:
https://uk.rs-online.com/web/p/potting-compounds/0164337/
What the composition of my compound was I cannot remember, it was at least 25 years go.
I do remember dire warnings about some of the fumes that it could give off while curing, but I survived.

JimB said:

I do remember dire warnings about some of the fumes that it could give off while curing, but I survived.

Click to expand...

Yeah, but think about how many years it removed from the end of your life. All that time lost with family and friends, all that time lost for...
WAIT, stop wasting time here on ETO and go annoy your family and friends!

The circuit is here....

The Great Circuit Challenge!
Post #45 ... LVDT to PWM
Post #47 has some video

... Once the coil is positioned, the only adjustment is on the 10k-10k voltage divider feeding the Op-Amp on pin 5. The "upper" (+5V leg) is adjusted to about 7k.

Note: I designed this circuit about 25 years ago when I was in the field of robotic research & development, and never had the need to place it within epoxy. It works quite nicely, but recently I have an industrial use for it that needs to be sealed and as a result have run into the issue mentioned in this thread.

BTW) As far as fumes ... 1/2 an ounce of epoxy doesn't put off much in the way of fumes, but I have a 30ft x 50ft barn with adequate ventilation.

gophert said:

WAIT, stop wasting time here on ETO and go annoy your family and friends!

Click to expand...

Wow!!!! What's Jim done...

You might look for a low dielectric potting compound such as designed for RF circuits.

Would a conformal coating suffice instead of potting? Methinks less bulk of material would have less effect on the circuit performance.

That is what I would suggest, a conformal coating. I personally like Urethane based coatings, they are easily removed for troubleshooting purposes, but there are many formulations depending on requirements.

This web page will give you some pointers:

https://www.miller-stephenson.com/c...MI7evQ99_z2gIViMDACh2UKwGdEAAYAiAAEgINT_D_BwE

Would there be any milage in coating with a thick-ish layer of something with appropriate dielectric properties - to keep the epoxy away from the sensitive bits - and then backfilling the rest of the box in epoxy if you need it for mechanical/environmental reasons? Just an idea.

All coatings will have a different dielectric constant than air.
I would not know which one is better, say, urethane or epoxy, but what I did see a lot in the past was the use of wax to secure and protect RF coils.

Thanks for all of your input ... CFC isn't enough by itself because of the fragile nature of the coil, so it needs to be completely sealed. After some empirical testing inside the epoxy with key wires coming out for testing/tuning I have come to the realization that I need to change my design slightly before I move forward with this project. The next design will be more of a true LVDT output where a "balanced" output will produce a 50/50 duty cycle.

one possible solution would be to increase the capacitances and decrease the resistances (by a factor of 10 or more), which will maintain the same time constants, but swamp the capacitance changes.