Dear Technetters ;

   I'm experiencing somewhat strange defects on 6 layerered PCBs.
  The defect itself is not actually strange one because I and everyone can
easily find out it as a blister.
  But the blister is only in the thin core laminate while there is not any
one between bonding layers.
  The thin core laminate is composed of two 7628 prepregs. The manufactured
quality was good.
  Please let me describe more details about the 6 layered PCB.
  The first layer contains parts of ground and signal patterns with
adjacent 2 plies of 7628 bonding
  prepreg to 2nd layer. Yes, the 2nd layer is the one copper of the thin
core laminate and
  this layer plays the signal role with seldom copper lines.
  The third layer contains large copper area and it means this layer is
power and ground.
  I watched 14 PCBs which were rejected after reflow soldering process had
blister only in the edge
  or the near point to edge. The PCBs had no blister before reflow
soldering process.
  I couldn't believe that why the blister is only in the thin core laminate
and why it happened
  after reflow process because the PCBs were noble ones that overcame
severe heat shock through
  HSAL process. Why does the blister be connected to the edge of the PCB?
  Moreover I surprised at the fact that for three times in 2 years, our
colleage has been experiencing
  the same phenomena with the same construction PCBs with the other 3 PCB
houses.
  Please save me from the 'blister Ghost'.

  Thanks for your attention and in advance.

  Tal Young Kim
  QA Dept.

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