The best thing about waking up isn't Folgers (or Maxwells) in your
cup, but realizing Chuck Norris hasn't killed you while you slept.
At 06:34 AM 9/12/2008, Whittaker, Dewey (EHCOE) wrote:
>That's why his designs are good to the last drop.
>Dewey
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
>Sent: Friday, September 12, 2008 5:38 AM
>To: [log in to unmask]
>Subject: Re: [TN] exploding cercaps NTC
>
>Inge:
>[Could be John is descended from the coffee branch of the family]
>
> :)
>
>Regards
>
>Mike
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
>Sent: Friday, September 12, 2008 11:47 AM
>To: [log in to unmask]
>Subject: Re: [TN] exploding cercaps
>
>Thanks for the input, John.
>Your knowhow in this matter is unquestionable. I really appreciate that
>you
>gave our problem a minute.
>(In fact, not our problem, I do consulting for another big company).
>Inge
>PS. It's Friday, and I feel for a little 'Maxwell' test: John, what is
>the
>basic equation for the magnetic flux in a electromagnetic coil? As
>son-son-son of famous Maxwell, you must keep the magnetic
>tradition....he-he
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
>Sent: den 12 september 2008 01:44
>To: [log in to unmask]
>Subject: Re: [TN] exploding cercaps
>
>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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