Ken,

Is the failure immediate after assembly, or after use?

I had a metallization migration problem many years ago with a custom built polyprop cap. It was used in a resonant conversion
circuit, 360nF, 500Arms @ 200kHz switching. Turned out that 40% of the available free electrons were being used.

Halfing the value suggests metallization might be missing, or very thin and migrating at one end of the cap.

Best Regards
 
David Greig

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: 27 April 2007 16:29
To: [log in to unmask]
Subject: Re: [TN] Metalized Poly Caps

Thanks Steve for putting up the pictures, now here's the situation.

We are having a higher than expected failure rate on these capacitors.
They are supposed to be .22 mfd and are testing at around .12 mfd after assembly.

The manufacturer is saying that it is because we are getting water inside of them during the cleaning process. A possible cause
is that we are breaking the "seal" on the end or possibly putting them into the cleaning process while they are still hot from
the wave solder.

We've built assemblies using these types of parts for decades and haven't seen this sort of problem in the past.

We pulled a couple of these caps apart and that is what the photos are showing. It appears that the metallization is missing on
part of the film. They say that this is due to the wash process. The wash is an inline wash system using DI water, no
saponifier. The contractor got a new wash system a few months ago but it's a typical inline wash using the same temps and speeds
that they used on the old system.

Have any of you been seeing problems with these caps lately or have any insight into this problem? See the photos below.

http://stevezeva.homestead.com/files/cap1.jpg 

http://stevezeva.homestead.com/files/cap2.jpg 

Thanks,

KennyB

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