Hi Ioan! Here is a reference that should assist in your efforts:
Cracks: The Hidden Defect by John Maxwell
http://www.avx.com/docs/techinfo/cracks.pdf
John Maxwell's paper is one of the references I show all new engineers as
a great summary of capacitor cracking root causes.
Dave Hillman
Rockwell Collins
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Ioan Tempea <[log in to unmask]>
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12/15/2010 09:08 AM
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Subject
[TN] Intermitent capacitor
Dear Technos,
We see intermittence in the functioning of 1206 ceramic caps after
mechanical assembly. Everything OK in Test, prior to assembly. Due to
mechanical constraints, the board is rather roughed up during integration,
so I was not surprised with the issue. But now the things get complicated:
if the caps are touched up, the intermittence goes away and everything
works again. So we've had the capacitors x-sectionned, see images at
http://ipc-technet.groupsite.com/gallery/17220
So here come the questions:
ˇ If I am not wrong, I have seen comments on this very forum about
touch-up of ceramic caps actually fixing internal damage of the component,
rather than the joints, which is misleading and just a temporary fix, the
cap will stop functioning in a short while. Do I see it right?
ˇ Although there is an 0603 in close neighbourhood of the problem
1206, see image, the damage is only on the 1206. Am I wrong considering
this normal, bigger size, higher bending, more damage?
ˇ The lab couldn't find anything wrong, neither in the joints, nor
internally in the part. I see a dark line at the interface between the
solder and the part in the right side image; is this normal, or an actual
crack?
ˇ It seems the plates of the caps are perpendicular to the PCB,
instead of parallel. This is how they came in the tape, we are not
flipping them. Would this orientation of the plates make the component
more prone to internal cracking than parallel?
ˇ There indeed does not seem to be any 45 degrees cracking of the
ceramic package at the corners (indicative of mechanical stress). Since
nothing internal has been detected either and the joints were deemed good,
what else could be the issue?
Thanks,
Ioan Tempea, ing.
Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
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