Multilayer Ceramic Capacitor (MLCC) failure root cause analysis

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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