TECHNET Archives

September 2008

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Inge <[log in to unmask]>
Date:
Thu, 11 Sep 2008 11:33:55 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (57 lines)
Hi all,
I have solved numbers of cercap issues, some easy tasks, others very 
qualified. Now, in order to find the right track asap, I thought ; why not 
utilize TN expertise? Alzo, my question:

What is the failure mode most likely to be?

FR4 backplanes, 4 mm thick, lots of pressfit connectors. Transient 
protection by means of ceramic capacitors, size approx 8x6x1mm (LxWxT). SMT 
60Sn. The catastrophic failures ALWAYS start at the end of the cap, from 
small microfumaroles to half ceramic body vaporized. From tiny spots of soot 
on the FR4 to something that reminds of what happens when you are careless 
with your gas torch welder!  The board is warped, about 2 mm along 200 mm. 
The board is torqued down flat with several bolts. May not seem to result in 
a lot of tension, but ceramic capacitors are very sensitive to axial/bend 
forces. I have cut out samples about 25x100 mm, populated with 4-8 caps, 
then polished them until only half thickness remains, then bent the samples 
slightly ('simulate' torque down). Out of a hundred, some caps developed a 
obvious crack CLOSE to the end metallisation (Ag;Ni). The cracks are always 
in parallel with the end metallisation and across whole width of cap body. 
EDS on the solder joints gives that the solder surface is irregular and 
grainy, and contains small amounts of both Silver and Nickel. The end 
metallisation is unusually thick with a 60-80 um Silver layer. On outside of 
this is some 2-5 um Nickel. The meniscus is normal, which means very 'fat' 
in my opinion (most of us don't realise yet, the benefit with meagre 
fillets). The failure occurs stochastically in field use. Forgot to mention 
that the BaTiO is beige and it's a 100k MLC. One and the same manufacturer.

My speculations are:

A. Mechanical force from curved/straightened board is root cause
B  Anomalous metallisation give local current issues which create hotspots 
and finally sparks etc
C. Solder metallurgy faulty, or fatigued, giving too high serial resistance 
which gives local hotspots etc
D. Shorts or delaminations not likely, as the failues are always at the end.
E. Bad Silver adhesion not very common
F. Nickel with bad pretinning = high resistivity or loss of galvanic contact 
not very likely either.

I know some of you have had many working days without sleep, because of 
puffing cercaps.

Thanks in advance

Inge 

---------------------------------------------------
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
-----------------------------------------------------

ATOM RSS1 RSS2