C0G ceramics vs humidity

I've never heard of any humidity effects on C0Gs. Fully dense ceramics don't absorb water the way plastic does.

Cheers

Phil Hobbs

Hmm I assume the polypro's I buy are sealed, but maybe there is some long time constant for water leakage?

George H.

There is. As little as hours for dipped, maybe a week+ for boxed.

Bob

Kind of what I was thinking, but the way ceramics are made it's hard to say how well sealed they are.

Bob

On a sunny day (Thu, 09 Apr 2015 07:59:03 +0300) it happened snipped-for-privacy@downunder.com wrote in :

Although I have not tried it that way, it seems to me with a Raspberry Pi (35$) and a USB sound modem (5$) on ebay, you should be able to do it all.

Here:

The Raspi runs Linux, has ax25, soundmodem I do have it installed on a Raspi IIRC, plus a whole lot of other ham software (sat tracking for example 'predict').

Writing your own AX25 kernel (I did) is not for a beginner (at least not in a resonable amount of time if you are diehard and want to try in asm on a micro). In 2013 I did it (HDLC 1200Bd) in a PIC for data transmission from my model airplane. Slightly modified encrypted protocol, FPV, on screen HUD. Some of thar PIC asm code was published here.

Why not try it?

I'd exepct no significant effect. If there is anything, it would probably be in the low parts-per-million range.

It would be interesting to measure. Any conventional c-meter would have tempcos that would make that sort of long-term measurement meaningless... even if it had enough resolution.

That's the kind of thinking I was trying to avoid :^)

That's the irony. I'm building a capacitance meter who's stability depends on 6 capacitors (it has 6 ranges). C0Gs are a natural, except it would be nice to know if changing humidity would cause drift. I seem to remember seeing hermetic ceramics at one time, but not recently.

It would make an interesting experiment.

No one has mentioned the possible effect on Leakage. May or may not be more important than the C change.

Mark

I'd try measuring a few before and after boiling them in water for an hour or two. Sort of the T/H equivalent of cold spray and a heat gun.

Cheers

Phil Hobbs

Put one cap in a constant-humidity environment, put the other in a humidity-controlled environment, hold the temperature constant, and make some comparisons.

To avoid multiple effects humidity on drifts --> PCB/SMT components coating, IMHO.

I've done it many years ago on old chassis VME's cards and is also nowadays used by many EMS.

Habib.

Not in my application, but maybe in others...

I don't have that kind of equipment anymore. My test chamber will probably involve a cup half filled with water and a paper towel over it.

Have you tried asking the manufacturer? I've gotten "inside" information by asking. Typically people who sell stuff want to be helpful. That's what I do.. try and help my customers*.

George H.

I was thinking of an AC Wheatstone bridge with four identical caps, with the output transformer coupled to some AC detector. That could be balanced to PPM resolution. Opposite-arm pairs would be in separate compartments; make it mechanically out of sheet metal or copperclad FR4 maybe, soldered up. Balance it, then put a small wet sponge in one section and a dessicant in another, and see what happens. Reverse wet/dry and compare.

An afternoon project to build, maybe a week or so to take data.

An FR4 PCB could be expected to have moisture absorption, enough to affect PCB capacitance some.

I like to get away from a screen+mouse now and then, maybe mill or bend some metal, and solder stuff. Make measurements. Like in the olden days.

The capacitance/humidity measurement would be interesting, given that sub-PPM resolution is likely needed.

If it's like most capacitance meters, the ranges are in decade multiples. Assuming a 1pF first range, the approximate capacitances would be 1, 10, 100, 1,000, 10,000, and 100,000 pF. Looks like 0.1uF (100,000 pF) is the largest value available, which should do the job.

In the distant past, I designed marine radios, where humidity, condensation, and immersion are considered normal operating conditions. Tests for humidity specify non-condensing which is somewhat unrealistic for many areas of the planet. Quite often, high humidity will produce condensation on the components and PCB as the temperature crosses the dew point. We had considerable difficulty dealing with water on the PCB. Any ionic contaminants in the PCB would cause leakage when wet. Water buildup on small value capacitors would change the value by making the water part of the capacitor plates. The eventual fix was overkill cleaning, bake dry, and various flavors of conformal coating to keep the water out. I suspect you'll find in testing that the C0G/NP0 caps are quite unaffected by humidity, but everything else around them needs protection.

Your higher capacitance C0G/NP0 caps are classified as MLCC (multi-layer ceramic capacitors). These have the unique ability to act as a microphone: The result is that if the PCB or soldering stresses the capacitors, they will change in value. Flexing the PCB will produce capacitance changes or capacitor damage. Vibration will also produce capacitance variations, which could result in an unstable reading.

MLCC caps will also crack if thermally shocked, usually with soldering iron rework instead of hot air or IR rework tools. A big crack is usually obvious, but I've also seen microcracking where the capacitor didn't short, but did change in value. If you look at the AVX data sheet above, you'll notice a "Capacitance Variation" line under each type of mechanical and thermal stress section. Some of those variations are the results of microcracking. More:

You probably won't have any problems with the lower value capacitors, but the higher and denser MLCC caps might require careful handling and soldering.