Hi, Inge,
One comment on John Maxwell's good thoughts;
Regarding the fact that it is a backplane, albeit a 4mm (.150") thick
backplane; pressing a circuit board or circular connector into the
backplane with a certain amount of force, it is still possible to flex
even a thick backplane enough to cause flexor stresses on the caps.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Friday, September 12, 2008 5:35 AM
To: [log in to unmask]
Subject: Re: [TN] exploding cercaps
Mike,
your are absolutely right...the unanticipated factor may be...how do I
say without being politically incorrect...the factor may be that these
boards are made in....hmmm....a country in which quality is not the
primary policy. I will recommend someone here to move to the other side
of the planet and do some checking. Yes again, you are right, one can
be mislead to use standard analysis thinking.
Thanks for the dig.
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: den 12 september 2008 09:42
To: [log in to unmask]
Subject: Re: [TN] exploding cercaps
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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