The answer John gave you is really bad news because he i
Leland,
The answer John gave you is really bad news because he is absolutely
correct. The only test we've found to detect cracked ceramic chip
capacitors that works is to subject assemblies to 85C/85%RH without any
bias for 24-48 hours and then take them out of the humidity chamber and
power them up without any current limiting. If there are cracked
capacitors the moisture in the crack will cause a spectacular exothermic
short-circuit across the crack. The only problem with the test is that
the spectacular exothermic reaction usually melts the ceramic capacitor
and the copper pads on the PCB and also burns a hole in the PCB making
it non-repairable. The only consolation is that the cracked capacitor
was found before shipping and not during the customers use.
Regards,
George
George M. Wenger
Andrew Corporation Wireless Network Solutions
Senior Principal FMA / Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office] (732) 309-8964 [Cell]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Monday, March 05, 2007 8:28 PM
To: [log in to unmask]
Subject: Re: [TN] Chip Capacitor Reliability Question
Leland,
I don't know where to start first, the bad news or the really bad news.
Lets start with the bad news first.
1) Capacitor cracking seldom results in an open part unless the part is
specifically designed as such
2) They all crack such that the cracks are vertical or angle back
towards the termination or end of the chip
3) Virtually all cracks are not visible but hidden under the termination
4) Failure needs two things to go short (sometimes spectacular
exothermic events in telecom power supplies)
a) DC bias.0.86V across the ends
b) Humidity in the air even diffusing thru the conformal coat
5) Time and accelerated by DC voltage and humidity
The really bad news is x-ray isn't going to help and I know of no
electrical test to detect ever increasing leakage currents.
1) Those leakage currents grow exponentially with respect to time
starting in the pA range and ending sometimes in catastrophic events
2) X-ray won't work because with the exception of NP0 ceramics they all
contain barium and that my friend is a great x-ray absorber on the order
of lead
3) Even if the x-ray system could "see" through the ceramic, crack
widths are measured in a couple of microns, not much help
4) Acoustic microscopy is probably your best hope to find cracks on the
assembly but required a good operator to properly analyze the results.
Regards,
John Maxwell
>Everyone,
>
>I'm trying to find out a couple of things...
>
>First of all, has anyone conducted any studies on the probability of a
>fractured chip capacitor moving from an open to a shorted condition?
>
>Secondly, if an open condition cannot be detected during electrical
testing,
>and the unit has been sealed with adhesive around the enclosure (unable
to
>open without having to scrap), is there a method available to check if
the
>component has seen any damage? We have a Feinfocus X-ray unit, and the
>results we've been able to obtain have not been encouraging. The board
is in
>an aluminum and zinc frame and has been conformally coated with acrylic
>material.
>
>Thanks in advance for your help!
>
>Leland Woodall
>
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