Have you tried the high resin 7628? It is a great compromise for giving the
desired thickness with the resin content to fill properly. It is used for
single ply construction with very good results. 
Because his blister is in the middle of the core we all have to believe the
material manufacturers use reliable materials.
My humble opinion is the blisters are caused from overheating in the reflow
process. Because I haven't seen the board, that's all I will comment.

At 10:28 PM 9/29/99 -0700, pod wrote:
>Many disagree with several reasons - in my opinion, none are good. I never
>bond anything using 7628 preg. I use only more resin rich glass glass styles
>facing copper or other 7628, except 7628.
>
>Earl Moon
>----- Original Message -----
>From: ±è´Þ¿µ Â÷Àå ÀüÀÚ ÁõÆò°øÀåÇ°Áúº¸ÁõÆÀ <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Wednesday, September 29, 1999 6:07 PM
>Subject: [TN] Any Opinion?
>
>
>> 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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