John,
Excellent input. Thank you for candor.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Monday, March 05, 2007 6:28 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
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text
in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at:
http://listserv.ipc.org/archives
>Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
>-----------------------------------------------------
>
>
>  
>

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text
in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------