Inge,
I have included some comments added to your speculations based on my
experience and represent my observations only.
John
>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
(John Maxwell) Mechanical/flexure stresses are the most common root
cause of MLCC failure but these are failures on a 4mm thick board.
This related to what Werner stated in is response, large fillets lead
to failure similar to mechanical/flexure cracks.
>B Anomalous metallisation give local current issues which create
>hotspots and finally sparks etc
(John Maxwell) At best a third order effect
>C. Solder metallurgy faulty, or fatigued, giving too high serial
>resistance which gives local hotspots etc
(John Maxwell) See item B above
>D. Shorts or delaminations not likely, as the failues are always at the end.
(John Maxwell) I agree
>E. Bad Silver adhesion not very common
(John Maxwell) Cap loss is the failure mode and not burning caps.
>F. Nickel with bad pretinning = high resistivity or loss of galvanic
>contact not very likely either.
(John Maxwell) See item B above
>I know some of you have had many working days without sleep, because
>of puffing cercaps.
(John Maxwell) When I was a consultant I viewed puffing caps as my
house payment, retirement and tuition for my kids to go to school. As
a user I did loose sleep.
>Thanks in advance
>
>Inge
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