Bulging Polypropylene Capacitors

[Mario87]

Hi all,

Not sure where to put this, so thought I would try here...we have a production run of boards with a large (rated 1000VDC / 1uF) polypropylene capacitor. This is on a PSU which steps down from 400VAC to 65VDC and the capacitor is just after the bridge rectifier to smooth out the rectified AC.

Boards are built, verification tested (NOT powered, just resistances / isolation checked), then sent for coating and now they have returned from coating we have at least 3 boards with the capacitors bulging. The boards have never been powered on and I don't believe any heat is used during coating (need to confirm this for sure incase they warm them to help dry it), but even if heat was used it can't be more than 60*c or so...not enough to cause damage.

Anyone know what could cause this failure? I have seen many electrolytics bulging in my time, but never a polypropylene cap...any suggestions welcome!

[ftg]

Could a solvent in the coating be infiltrating the plastic case of the cap?

[moffy]

Is the coating a vacuum impregnation process? and could the caps be outgassing internally?

[Mario87]

Could a solvent in the coating be infiltrating the plastic case of the cap?

This looks to be the most likely cause at the moment, however we have had 100+ boards made already with this coating and these caps, so its strange we now only get a handful with this issue

Is the coating a vacuum impregnation process? and could the caps be outgassing internally?

No vacuum impregnation, just sprayed on and dries, its an acrylic based coating if memory serves correctly.

[paul@yahrprobert.com]

I would cut one open and see what's up. If you find anything wet then it might be a fake.  Another theory is that those are polypropylene with oil soaked paper and the oil sucked up the solvent from your encapsulant.