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Multilayer Ceramic Capacitor (MLCC) failure root cause analysis

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#66894

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 27 July, 2012

I am posting this here because I think it may have something to do with PCBA:

I have a simple battery-powered consumer product (LED light) that we have been manufacturing for some time. All of a sudden in one batch, we have about a 4% rate of a single individual capacitor on the board: Failure mode is that it allows a severe leakage current, in the range of 0.1 mA to 4 mA, thereby draining the battery before product arrival.

The cap is a 10 uF MLCC, Y5V dialectric, manufactured by Samsung. There are three of the same part on the board, but only one specific cap seems to be failing. So it seems like something that is damaging that one cap during/after PCBA or FA.

We DID observe poor ESD control procedures (missing wrist straps) on the production line in this production run, but I doubt that is the cause since a Human Body Model 150 pF could barely generate a few volts on a 10 uF cap.

The only thing I’ve ready about that could cause failures like this are microcracks forming in the capacitor, either from thermal or mechanical stress. The problem is we haven’t changed anything in our design, and this has never happened before in 4-5 batches of production. It is possible that the oven temps in the PCBA reflow were slightly different, who knows.

One interesting thing is that upon initial testing of the failed devices, we immediately tested quiescent current, because the failures presented themselves as dead batteries out of the box. Upon replacing the battery we noticed no problem (normal or nearly normal quiescent current, maybe 5 uA higher than usual). But then after leaving a battery plugged into the devices for 30 minutes or so, the quiescent current begins to creep up. After a day or more it can reach as high as 4 mA flowing through the cap (biased at 6.6 V).

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#66918

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 30 July, 2012

I'm with you on microcracking. The best way to confirm this is through sectioning of the affected capacitors. You're fortunate to be seeing these failures showing-up early in your product life. Of times these failures don't appear for years.

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#67070

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 22 August, 2012

I would look at your de-paneling process. How are you cutting the boards? This could be inducing mechanical stress on the component; especially if the part is close to the edge of the pcb and perpendicular to it.

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#67071

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 22 August, 2012

They are using the method where scored boards are separated using a rotating wheel / blade. I realize this is not the best process, however these capacitors are more than 10 mm from the edge of the board. From some reading online I think that means that the de-paneling would not cause this.

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#67072

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 22 August, 2012

Most likely the depaneling is not your issue, but I do concur that there is some mechanical stress occurring somewhere. What about any fixturing for FT/ICT?

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#67073

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 22 August, 2012

I also forgot to mention that the pizza-slicers you are using can get out of spec and may need periodic calibration. By cutting too close, these can over-stress your pcbs as well. Do you only see this problem at one cutter or does it matter? Also, how are the boards being stacked?

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#67079

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 23 August, 2012

"The problem is we haven’t changed anything in our design, and this has never happened before in 4-5 batches of production."

If nothing has changed (same parts, same oven profile, same de-paneling, etc.), could the parts themselves have changed? I haven't seen any ceramic caps warn about baking to remove moisture content, but could the micro-fractures come from the part absorbing moisture? How were the parts stored?

Sorry, no answers, just questions...

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#67108

Multilayer Ceramic Capacitor (MLCC) failure root cause analysis | 27 August, 2012

Moisture should not be an issue with ceramics, but mechanical or thermal stress definitely is.

Has there been rework on this capacitors? So check for handling /temperature of your soldering irons (better:use hot air). Check oven temperature/profile (was the correct one used?). Someone new in the production line, who might not handle the boards as carefully? Fixtures can avoid handling issues within subsequent proceses (cleaning, testing,..). Regards, Thorsten

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