I am looking for information on the possible causes of circumfrential voids (occuring inside the hole barrel), that I am seeing in the cross sections of multilayer FR4 boards with 10 mil holes with aspect ratio of 10. After drilling, these boards go through Blackhole graphitic carbon / Cu flash / image / Cu pattern plate with Sn as etch resist / strip dry film, etch Cu and strip the Sn/ LPI and HAL.

I would like to find out the cause for these voids. I deeply appreciate any valuable advice on how to fix this problem.
 
Thanks.
Sandy 
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