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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 1 Jul 2008 10:42:17 -0500
Content-Type:
text/plain
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text/plain (183 lines)
It can be failure to derate the capacitor (or resistor, for that matter)
for aging and humidity. The voltage rating should be looked at as well
as the resistance/capacitance and the part derating should take into
consideration the supply voltage the component will see. If the supply
voltage is at the extreme low end of the chip's operating range, a
certain derating percentage would be lower than if the component was
expected to operate at the high end of its voltage range. The typical
operating environment is also used to factor the derating percentage. If
a known decrease in resistance/capacitance is associated with the parts
over time, then that should also be factored into the derating
calculations. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Tuesday, July 01, 2008 10:18 AM
To: [log in to unmask]
Subject: Re: [TN] Ceramic Cap Value

Hi Ken,

We've seen this phenomenon twice in the past 4 months, high rate of ICT
failure on ceramic caps and had the same issue, replacing the parts
sometimes worked and sometimes didn't. We even have a painful open case
on this matter.

I need more clarifications from the group regarding this issue and how
to get around it.

The paper on aging asks to take the phenomenon into account when
designing the circuitry.
Does this mean the high failure rate Ken and I witnessed is borderline
design?

Can it be marginal testing method also?

Can it be anything else that design or testing?

Thanks,

Ioan


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bruce Tostevin
Sent: Monday, June 30, 2008 9:37 AM
To: [log in to unmask]
Subject: Re: [TN] Ceramic Cap Value

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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