Ken,

As John Maxwell pointed out, it sounds like you may be seeing the normal
effects of capacitor dielectric aging.  It's a common stumbling block
with parts at incoming inspection, test, etc.  There's a pretty good one
page summary with a handy formula at:

http://www.atceramics.com/technicalnotes/circuit_designer.asp

Click on Dielectric Aging Phenonema.  The aging aspects of ceramic
capacitors are also covered in EIA-521.

A quick way to see if this is what's going on with your parts is to
preheat then a little bit and float a few parts in a solderpot for 5-10
seconds and remeasure them after they cool. That'll usually bring the
value up enough to confirm the issue.

Bruce Tostevin
Benchmark Electronics
Hudson, New Hampshire

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ken Bloomquist
Sent: Friday, June 27, 2008 5:14 PM
To: [log in to unmask]
Subject: [TN] Ceramic Cap Value

Happy Friday,

I need some help understanding how you test for the value of ceramic
capacitors. We build a board that has some 0603, .470 uF ceramic caps,
+/-
10%. The assembly that these parts are on has been having a higher
failure
rate at our ICT then we've seen in the past. We tracked the failure to
this
cap which had a low reading. We replaced the cap and the unit passed ICT
just fine.

We measured the cap using our HP 4262A LCR Meter and sure enough it was
reading low, around .398 uF.

We pulled some new parts off the reel and they read low also ranging
anywhere from .398 to .425. We suspected that we had a bad reel of parts
even thought they were from a respected cap manufacturer.

Being curious we tested some more reels from different lot codes and
found
the same thing. Being even more curious we tested some other parts from
the
same manufacturer and found them to be low by around 5% - 7% and in some
cases slightly below 10%. We then tested some other parts from a
different
manufacturer and they tested low also.

Now we are suspecting our test equipment so we went to an independent
lab
and had them tested on an Andeen Hagerling 2500A Bridge. Guess what, we
got
the same low readings.

After all that my question is, what are we doing wrong to get such low
readings on these ceramic caps? It just doesn't seem likely that four
different parts from three different major suppliers tested on two
different
pieces of equipment would all be that far down on the low end.

Any education in capacitor testing and reading would surely be
appreciated.

Best regards,

KennyB

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