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September 2008

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From:
Mike Fenner <[log in to unmask]>
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Date:
Fri, 12 Sep 2008 08:42:19 +0100
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IMHO most ceramic cracking problems are due to combinations of circumstances
which flip marginal pass to marginal fail, Usually the major component in
the failure chain is unanticipated stress caused by a mechanical assembly
operation on or to the soldered board. Starting with singulation and ending
with being screwed into the box.  So they all need to be scrutinised
carefully especially comparing the written procedure to reality.
Unanticipated can mean apparently unrelated changes that therefore go
unremarked. Slight change in depth of v score for example can give an extra
twisting force to singulate board. So its a question of forensic plodding to
identify them.
Component/soldering process issues like too much solder (esp. in Pb-free),
placement pressure, thermal shock with or without moisture ingress are
usually quite a long way behind - although often the first to be questioned.


Regards 

Mike 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, September 11, 2008 7:34 PM
To: [log in to unmask]
Subject: [TN] exploding cercaps

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 

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