Thank you all for the numerous insights offered on the issue of circumferential voids that we observed in our 10 mil hole FR4 panels with aspect ratio of 10. I forgot to mention in my earlier note that these voids were invariably in the center of the plated barrel. Based on the analysis of the cross section and auditing of the processes, it appears that it is either a drill parameter (including debris) issue or trapped air bubble during copper/tin plating. 

I would like some advice on panel agitation to remove trapped air bubbles.

In order to remove air bubbles, I should do a good agitation of the panels during plating.
1. At what tank(s) (acid clean or acid dip or acid Cu or tin?) should I install the agitator? (The hole graphitization line has ultrasonic vibration during several key stages, and it is unlikely that graphitization did not occur).
2. Who supplies suitable vibrators for this type of job?

I would deeply appreciate any and all help.

Thanks.
Sandy