Hello Kim,
    Based on what you described, I wonder if it is be the leading edge of the boards
(first few) that are blistering.  If that is the case, Try leadering, or running dummy
boards ahead of the assmeblies. Also, check your Reflow profile (short/long wave IR??)
with a "mole" or similar device.    It may be the initial thermal shock at IR refow.
Not knowing all the details of your process, I am guessing Based on the "old" days of
IR fused tin/lead boards, The first boards through heat thequickest and had a higher
surface temp.  Once in a while, we would see similar issues.
      The laminate lay-up seems pretty standard (.014" core??) for most applications of
a .062" six layer.  If the material has been stored for any length of time, either at
the vendor site or at the assembly site (dependent upon RH/temp conditions) moisture
absorption can occur.  If this is suspected, try baking the boards at approx. 270 deg.
F. for 4 hours (at temp) prior to component placement and run a couple to check..
    Just a couple of Ideas from a BOARD  BUM.   JOHN

±è´Þ¿µ Â÷Àå ÀüÀÚ ÁõÆò°øÀåÇ°Áúº¸ÁõÆÀ wrote:

>  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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