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September 2008

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Subject:
From:
Guy Ramsey <[log in to unmask]>
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Date:
Thu, 11 Sep 2008 21:25:36 -0400
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Assembly method? 
Mechanical damage from placement?
Thermal damage?
Cleaning damage, mechanical from high pressure water jets? 
Handling damage? Stacked boards in the factory? 

Are the press fit pins installed after the caps? Board flex! 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, September 11, 2008 2:34 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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