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From:
"Wenger, George M." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wenger, George M.
Date:
Tue, 6 Jun 2006 06:39:50 -0400
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Ioan,

As you can tell from all the responses ceramic capacitor cracks are
problematic.  Thermally induced cracks if they are large enough can seen
or detect them with x-ray or acoustic microscopy provided they aren't in
the termination area where there is a curved surface.  John Maxwell's
advice about not touching a ceramic capacitor with a soldering iron is
good advice.  You'll do nothing but induce more problems.  The real
problematic cracks are mechanically induced "Flexure Cracks".  They
happen when mechanical stress during operation like de-panization,
insertion of press-fit connectors, mounting screws, handling, or other
operations are exerted on attached capacitors.  The stress concentration
is highest at the inside edge of the capacitor termination.  Since this
is area is under the capacitor you can't see the crack.  The flexure
cracks propagate upward and outward.  If the crack doesn't propagate
across adjacent electrodes in the capacitor they are benign.  Even if
they do propagate across adjacent electrodes, when capacitors crack they
"crack open".  The internal electrodes aren't short-circuited.  The
parallel plate area removed by the "open" is typically small and not
detectable by capacitance measurements.  So big deal, why care if you
can't see them or detect these small cracks using acoustic microscopy.
Your product should pass test and work.  The horrible nature of "flexure
cracks" is that they are a reliability issues that can cause problems
during use.  Without a crack the electrodes are separated by the ceramic
dielectric.  However, the crack provides a path between adjacent
electrodes.  The dielectric in the crack is now air instead of ceramic.
You can detect these cracks using THB (i.e., Temperature Humidity Bias)
techniques but it is difficult to monitor and find very small leakage
currents and they need to be done insitu.  If you take capacitors out of
a THB chamber and test them externally what typically happens is you
apply a measuring voltage and there may be an increased leakage current
when the measuring voltage is apply but this leakage current causes a
local heating in the crack that drives out the moisture in the crack
causing an increase in dielectric and the leakage current stops and is
no longer detectable.

Back in 1987 when I participated as a member of the AT&T "Crack Busters"
Team we had a serious problem with products being returned with "blown"
capacitors.  They were typically high capacitance 1812 size ceramic
capacitors used as by-pass or filtering capacitors across 24 or 48VDC
power supplies or regulators.  What we learned was that we induced
"flexure cracks" during manufacturing and handling that were not being
detected and worked as long as the humidity in the environment was
relatively low.  However, whenever our products were in and environment
where the humidity would increase (e.g., thunder storm, rain shower, or
even a janitorial service mopping the floors next to operating
equipment) the 24 or 48VDC applied power would short-circuit in the
"flexure crack" causing the capacitor to blow-up.  Out of this work came
the use of humidity to find crack capacitors.  We'd put products in
85C/85%RH (no bias) for 24 hours to inject humidity into the crack and
then take the products out of the humidity chamber an immediately turn
them on (i.e., apply the 24 or 48VDC).  The instantaneous current draw
between electrodes in the crack would cause the capacitors to blow-up.
There were any times when we didn't want to wait 24 hours so we hurry
the process by putting the product in boiling water for 30 minutes.

I won't suggest using the humidity/power/blow-up technique as a process
quality screen for "flexure cracks" because when capacitors do blow-up
they usually cause PCB damage.  However, this technique is effective in
finding out that you have a flexure crack issues and allows one to
determine if product/process modifications are effective in eliminating
"flexure cracks".

Sorry for the long winded answer.  
 

Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Monday, June 05, 2006 2:22 PM
To: [log in to unmask]
Subject: Re: [TN] Finding cracked capacitors

So dear colleagues,

there is not much I can do with a plain thermal shock or thermal
cycling. I absolutely need humidity too. Bad news, I am not equipped for
that.

George, the boiling water thing is interesting. What do you mean by "put
a power supply across them", connecting a certain voltage or amperage
directly to the terminals of the caps?

Thanks,

Ioan 

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