Re: Leaky SMT Ceramic Caps

I also suspect microcracking. The cracking may be very close or covered by the metallization at the end. I would suspect failure was due to mechanical stresses rather than thermal shock. Look to see if the circuit board at the capacitors has been subjected to bending. One thing I have learned over the years is that ceramic capacitors do not take too well to bending stresses.

James T. White wrote:
I was troubleshooting some boards manufactured by a contract assembler
recently and ran into a strange problem. Some of the 1206 SMT ceramic
caps conduct slightly at supply voltage but show open with an ohmmeter.

In one case, a 0.1uF cap pulled to +5 via a 4.7K resistor was
sufficiently leaky that the processor on the board wouldn't come out of
reset. Upon removal, the cap was tested and showed the correct
capacitance value. When tested with an ohmmeter it showed infinite
resistance but with 5V applied across it, the cap it would conduct >2ma
of current. Replacing the cap solved the problem. I've also seen the
problem in another part of the circuit where a 0.1uF cap is pulled to +5
by pulses from the processor or pulled down by an external load. Same
symptoms and fix.

I suspect the contract assembler used the cheapest parts he could find
but was a bit surprised by the high leakage at just 5V. Has anyone else
seen anything like this?

Thanks.

.

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