Ceramic Capacitor Aging and Restoration

[jeyurkon]

As the article mentions, you have to heat it above the curie point (about 125°C for the ceramic in question) for any crystalline structure changes to take place. In the direction you want anyway.

Originally Posted by Retrovert

180° F, which is what the radio boys bake their chassis at, is about 85°C. I should have made all of this clearer. (I didn't realize it was going to be controversial.) An oven at 180° F should be close enough to 257° F (the aforementioned 125° C; it's 70% of it) for government work when it comes to reforming the ceramic capacitors, if this is what accounts for the increase in performance noted in those postings.

My supposition is that it is the temperature + duration is what that matters, and that the specific temperature is less critical. I don't know if it's a phase transition, which I doubt because the temperature is pretty low for changing a ceramic. Many reactions occur much slower at lower temperatures, but they do happen.

I don't know; I'm not a materials scientist. I just thought it was interesting.

But it is clear to me that baking radios at 180° F for a few hours not only won't hurt anything, but should improve performance and reliability.

[Retrovert]

Originally Posted by jeyurkon

As the article mentions, you have to heat it above the curie point (about 125°C for the ceramic in question) for any crystalline structure changes to take place. In the direction you want anyway.

Ahhh, I didn't grasp the significance of moving past the curie point for barium titanate. (I was thinking in terms of its melting point of 1625 °C, which is huge compared to 125°C. Less than 8%!) This chart explains it perfectly:

That's one mighty steep dropoff. A brief return to the Interwebs for more information.

Now this presentation on "Drift (due to Moisture) in Multilayer Ceramic Capacitors" is interesting (PME is Precious Metal Electrodes and BME is Base Metal Electrodes, usually nickel, a shift in the nineties):

Samples of X7R and Y5V PME and BME MLCCs were exposed to two 100% relative humidity (RH) environments: room temperature and 120ºC (autoclave) for over 6 months.
– Parts were optically inspected for cracking. No cracking was observed.
• Capacitances were well below nominal values at the end of the test period (data on the following slide)
– The PME parts had less capacitance degradation than the BME parts
– The BME parts exposed to the autoclave degraded more than the room temperature parts.
– The BME parts degraded below their tolerance limits.

...

Capacitors can be de-aged by heating above the Curie point (125°C for 4 hours or 150°C for 1 hour).

...

Capacitance degradation is theorized to be caused by ionic motion within the body of the capacitors. Since degradation effect is driven by surface conditions, each new generation of miniaturized high value capacitance BME
multilayer capacitors must be assessed for susceptibility to humidity degradation.

But this applies to modern caps, since the older ones were apparently PME.

I think that pretty much sums it up: high humidity is bad for ceramic caps (we knew this as a general rule about electronics) and de-aging (a different phenomenon than water-induced changes) requires heating past the curie point (and the point at which the wires and plastic parts melt).

Since soldering does de-age, one could potentially heat the caps with a soldering iron and get the same effect. (Except for the potential issue of thermal shock, but that appears to apply to surface-mounted devices, although I read about how it is best to not overheat the traditional leaded ceramic caps during soldering.) The earlier links I posted about baking make a lot more sense in that all of the water is driven out of the caps.