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