Ceramic Capacitor Aging and Restoration

[Retrovert]

I have no idea if this is useful, or if it applies to the vintage ceramic caps we encounter, but I found this description from Johanson Dielectrics of the aging process for ceramic capacitors:

The effect of time imposes a predictable loss of capacitance when comparing Class II and IV capacitors. For X7R and X5R the loss is calculated at -2.5% per decade hour and for Y5V it is -7% per decade hour. After manufacturing, the crystalline structure of the dielectric material is reset. It is due to the realignment of these crystals after firing that the capacitor goes through a logarithmic loss of capacitance.

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This aging rate is expressed in decade hours. This means after manufacturing the loss of capacitance is calculated at 1hr, 10hrs, 100hrs, 1000hrs etc….. This means at 10 hours the capacitance changes the same percentage as it will when measured at 100 hours and 1000 hours. As time goes on the aging process slows.

Now what's interesting is that this process can be reversed:

The aging process is reversible. By heating the capacitors over the "Curie Point" (approx 125°C for Barium Titanate capacitors), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing. This process is referred to as "De-Aging". The amount of De-Aging is dependant on the level of temperature and how long the capacitors are exposed to it. Exposure to 150°C for one half hour or exposure to 125°C for one hour is sufficient to return the capacitor to its original value. The soldering process is also an effective De-Aging process.

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After the soldering process, the capacitors have essentially been De-Aged. Capacitance measurements may be erratic in the initial 10 hours after testing. This is due to the initial capacitance value, dielectric type and the time between reflow and the capacitance measurement. For this reason, it may be necessary to wait for the capacitance to stabilize after reflow before testing. In "High K" dielectrics, the capacitance may also appear slightly high after the soldering process. This is normal as the capacitance is intended to be stable after 1000 hours so that there is adequate capacitance throughout the life of the circuit.

The full technical note is available both as HTML and as a PDF.

I haven't tried this, don't know if it is typically necessary, nor do I know if it is practical to heat up a chassis up to 125 degrees and not destroy everything. My guess is that it likely isn't that far above the normal operating temperature, but baking geriatric circuity likely is not a good idea and you can't blame me if you turn a radio or TV into burnt toast.

[bob91343]

Gads don't do that! All plastic parts will suffer. That includes dials, rubber feet, dial cords, windows, pointers, and many electronic components. Most consumer grade components are rated for 85C or so, and you will destroy the capacitors at least. Any moving parts will have lubrication run out. And you can say goodbye to any labels. Most plastic cabinets soften at ordinary hot summer temperatures, and that is seldom above 40C.

[Retrovert]

Originally Posted by bob91343

Gads don't do that! All plastic parts will suffer. That includes dials, rubber feet, dial cords, windows, pointers, and many electronic components.

Ummm, no, they won't.

My posting specifically referred to only the chassis, but it seems that the rest of the case wouldn't suffer from baking.

You surely are aware that most components, such as caps, are typically rated up to temperatures higher than 135° Fahrenheit? Caps are usually good to 85° Celsius, which is 185° Fahrenheit. Carbon-film resistors are good up to 155° Celsius. One could conceive of using a heat gun upon only the ceramic caps, or doing it with NOS parts that had not yet been installed. So my comment wasn't totally outlandish, ignorant, or downright stupid as you made it out to be. I would not bake nitrocellulose, acetate or other low-softening point (or flammable!) plastics.

The fellows over at Antique Radios routinely bake radios at 180° Fahrenheit to remove moisture and improve performance:

I have done this many times even with complete radios. An hour in a pre-heated oven at 180 deg F. should be fine, even longer will not hurt.

A transformer that has been soaked in paraffin will have some dripping, one soaked in bees wax even less as bee wax has a higher melting point than paraffin. Transformers that have been vacuum impregnated in varnish should not experience any flow of the varnish.

Much above 200 deg. F will cause extrusion of pvc or rubber insulated wire, not good.

and furthermore:

I have a spare oven in the cellar workshop, connected across one leg of the 120 volt line, with a modified thermostat, which allows it to stay at between 160 and 180 degrees. I bake almost every chassis in this oven for several hours to fully dry it out after cleaning, and before I attempt electrical restoration. baking the RF cols in many early bettery sets makes for a tremendous improvemnet in performance, particularly in those sets with lorenz coile, or sets using fiber coil forms.

Given the above technical note this makes perfect sense. (As does driving out any residual moisture.)

The interesting part of the technical note, and why I posted it, beyond the explanation of aging, was this: "After the soldering process, the capacitors have essentially been De-Aged." That's interesting to me and it explains why the performance improves after baking.

Some repair shops would even soap up and hose down dirty chassis before working on them:

In the '60's I ran a TV repair shop and would often have a TV that was so filthy that the techs would not touch it. We'd take it into the alley behind the shop and shoot it down with liquid soap, and maybe bug killer (protecting the speaker of course) and then a through rinse with a garden hose. Next came the critical part - get this set dry ASAP. We used compressed air and a squirrel cage blower from an old furnace. It would seem to be dry in a half hour or so but we always ran the blower for a day on it. Never lost a set.

and

When washing chassis, I use only one tablespoon of dishwasher detergent (Electrasol), one tablespoon of Borax, and a teaspoon of Washing Soda. This does not corrode aluminum, but rather leaves it perfectly clean, and removes the yellow corrosion form Cadmium plating.

Baking the set in an oven for twelve or fourteen hours at 165 degrees does indeed get all remnants of moisture out of coils and transformers. I use a modified "Yard sale" built-in oven set up in my workshop., as the Better Half looks askance at the use of our kitchen appliances for this purpose.

So there!

And as far as baking electronics with plastics, a common trick to rejuvenate video tapes is baking them at 135-140° Fahrenheit for about two hours. The sibling site even has a page about doing this in a toaster oven. I knew a fellow who made a tidy sum, from audiophiles and business migrating tapes, by doing such baking before the technique was widely known.

[bob91343]

I guess I misunderstood. I thought you wanted to heat things up to 125C but apparently you meant 125F. Sure, that's not too hot.

[Retrovert]

Originally Posted by bob91343

I guess I misunderstood. I thought you wanted to heat things up to 125C but apparently you meant 125F. Sure, that's not too hot.

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.

[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.

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

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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.