Burn In Change in Characteristics

Le Tue, 14 May 2013 04:06:58 -0400, rickman a écrit:

High K ceramics (X5R/X7R,Y5V,Z5U,...) are notorious for exhibiting high aging rates (counted in % per time decade: about 2% for X7R and worse to much worse for other dieclectrics).

Aging is reset when you throw the cap above its Curie temeprature which basically is what you do when soldering the cap.

So you are saying this is purely a function of time from exposure to the Curie temperature? What I am seeing is happening at a normal operating voltage not far above room temp. 24 hrs under power and *every* one of the boards shifts the 20 Hz attenuation from -0.95 dB (give or take a couple of 0.01 dB) to -1.05 dB (again about 0.02 magnitude variation). I didn't even notice this for the first few hundred boards tested for frequency response. But the current order is a lot larger than usual and I'm staring at these numbers all day long. So I finally realized how consistent the connection is to before and after burn in. It is virtually a perfect correlation.

I don't think this is a matter of just time as the boards have been sitting for varying amounts of time since they were soldered. But the burn in time is pretty consistent since that is the bottle neck in testing and after 23.5 hours they are outta there to make way for the next batch.

Why not use COG/NPO for the "sensitive" areas? They're not all that expensive. ...Jim Thompson

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I say that something that means -1dB at 20Hz for audio isn't terrible critical, maybe more it down to 10 to be sure

-Lasse

How big a change in C is that? Could it just be a temperature thing. Are the boards warmer after 24 hrs of burn-in? Or is the sign in the wrong direction for that?

George H.

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I read it as the caps aging in use, and reset if you bring them up to curie temperature

try resoldering the caps on a board that has been burned in and see if it goes back to start

-Lasse

I think that you will see different patterns of burn-in "drift" with different manufacturers, and most certainly with different ceramics (X7R, Z5U, X5R, Y5V, U5V, X7T, and the "gold standard" C0G). Since your correlation is so good, perhaps a sample as small as 5-10 would be sufficient.

Is it just a thermal effect, is the board still warm from the burn-in?

This is not a problem at all. I just noticed that is was very consistent. I thought this was just variation in the measurement until I realize the correlation. No, the goal is just to be better than -3 dB at 20 Hz and 20 kHz so the units are all fine. I'm just curious about the effect.

BTW, the frequency limiting parts mostly are pushing the size/capacitance tradeoff and there is no more room on the board for an extra via much less another huge cap. I think the ones on the output are 10 uF. Inboard there are 2.2 uF IIRC.

Le Wed, 15 May 2013 03:11:33 -0400, rickman a écrit:

For the part I was telling, yes. You reset the cap by soldering it, then let it rest at room temperature and cap value slowly decrease. Say 2% per decade for X7R. That's why manufacturers (at least the serious ones) have some appnotes saying you have to wait before checking caps (in circuit testers for ex.) after they've been soldered in case you need to do so.

That is 0.1dB, or 1%. This might come from either temperature elevation or another possibility is that, if your caps are DC biased, that under bias there's also a slight and _slow_ disaccomodation (takes minutes to tens of minutes to settle).

Did you changed the caps ref (smaller package)? The bias disaccomodation depends on the electric field value, so the smaller the cap the bigger it changes under the same bias conditions.

One can double or triple the capacitance,using the SAME capacitors and the SAME board space very easily; stack them. With a small change in board area, one can put the SMT caps on the pads sideways and maybe get three in almost the same area. Then stack another 3 above... Any of the above tricks require NO extra via.

BTW, there is nothing wrong in putting a via in a pad - takes ZERO extra board space.

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Yeah, more data! does the voltage change? What about T? Say is there enough room an X7R cap?

George H.

What does that mean, 2% per decade? Decade of what?

There is no reason to expect a temperature difference. The boards are pulled from a chassis where they aren't really warm anyway, and sit as they are tested one at a time. This takes about an hour and there is virtually no difference from the first to the last. I expect this is definitely related to a part being under voltage. Obviously the caps are the most likely, but I don't know much about such a subtle effect, so I'm asking here.

Changed relative to what? The only factor I see is the time under voltage. Most of the caps on this board are as small as I can get them.

What about T? There is only minimal T fluctuation. The boards are always at room temp, even in the burn in chassis, they are fan cooled and not very warm to begin with, so the ICs just don't get warm enough to even feel.

What voltage are you talking about? I don't see any reason for a voltage to change other than possibly the zeners that set the bias point of the amps. I think I mentioned them in my first post. I don't have time right now to mess with it. I'm trying to get product out the door. I was just curious if anyone knew of such an effect in caps or the zeners.

I have no reason to think there is any difference in temperature in the two measurements. It takes me an hour to test a batch of board after being under voltage. So any temp difference would show as the effect wearing off with time out of the burn in chassis. This is not elevated temperature burn in and the chassis is fan cooled being designed for much higher power boards.

I'm starting to wonder if the zeners are drifting while powered, 6.2 volt units.

The "board" is 51 daughter boards of fairly low power consumption. The burn in chassis is a production card rack with fans. So I doubt you could measure 1 degree difference from off to powered on. Then it takes half an hour to swap out the daughter cards and the testing takes an hour as each one is tested separately, one at a time. So there would be lots of time to cool down and a drift in the effect would be seen.