Steve - The finger pressure test sounds a little strange, however I have
seen a similar test used for cracked solder joints. If you can't see cracks
in the caps with a microscope, I would suggest you try a penetrant dye
assuming they are surface cracks. Internal cracks or missing dielectric on
internal layers would be real hard to find except by destructive
cross-sectioning. Since ceramic does not flex, and assuming there are no
cracks, only the conductivity of the finger could possibly change the value,
unless someone else has another idea. Ed/

Ed Valentine
Electronics Manufacturing Solutions
8612 Mourning Dove Road, Raleigh, NC 27615
Phone: (919) 270-5145, Fax: (919) 847-9971
Email: [log in to unmask]
Website: http://www.ems-consulting.com

----- Original Message -----
From: Stephen R. Gregory <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, May 30, 2000 6:20 PM
Subject: [TN] Capacitor issues...


> Hi All!
>
> This is a weird one. We've been building a board for a customer for quite
> some time now. Hadn't had a problem since until recently (after going
through
> some ECO's). Customer thinks we've got a bad batch of capacitors. They're
a
> ceramic 1206 that's reflowed on, the dielectric is a X7R, and is placed by
a
> Phillips Topaz. I've looked at the cap under a microscope and don't see
any
> evidence of fractures. The cap in question is in the center of a .031"
board,
> and is in a scored 6-up panel.
>
> The customer says they can tell when there's a bad cap on the board, by
> putting it on a test fixture and putting finger pressure on the cap and
> watching for the value to change for the capacitor. If it changes, it's
bad,
> if it doesn't, it's good.
>
> Does that sound like a good test? The reason I ask is because I've read on
> one of KEMET's web page "Knowledge Edge" articles that dielectrics such as
> X7R are considered ferroelectric dielectrics, and have a common
> characteristic of piezoelectricity, so by putting pressure on the cap it
is
> going to change the values, won't it?
>
> Just trying to understand...
>
> -Steve Gregory-
>
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