In your message dated Thursday 8, February 1996 you wrote :
George,

Thanks for your comments. I do not have any problem with any of your views, and 
what is fascinating after such a short time on this group is the diversity of 
experience and the range of different causes and solutions to common problems.

The question of drilling copper on inner layers is one which I am currently 
reviewing and would appreciate advice from anyone who has carried out a study on 
this subject. Is it better to drill at the optimum rate for glass or copper or 
somewhere in between.

My understanding is that the optimum feed rate for drilling glass laminate is 
3mil per rev with a cutting speed of 500-600 feet/min. The optimum for copper is 
less, perhaps 2mil per rev. This means that if you set the feed rate for copper, 
you are running below the rate for glass and vice versa. This is also the case 
when drilling normal double sided but layer separation is not a problem and 
slight burrs can be removed.

For processing reasons, ground planes benefit most when deleting NFP's because 
when they are left in place there are far more potential sites for short 
circuits and they are more difficult to inspect. Bonding to a ground plane is 
much better when there is a clearance rather than an annular ring.

   -------\_/-----\_/----            ----------\_______/-------  pre-preg
   xxxxxxxx xxxxxxx xxxxx            xxxxxxxxxxx       xxxxxxxx  copper
   ======================            ==========================  laminate  
           |       |                                |
           weak bond                            strong bond

More information on the drilling aspect would certainly be helpful and our main 
reason for preferring to remove NFP's is processing rather than drilling. I 
would certainly like to know if they are considered beneficial to the drill life 
and hole quality. Can anyone contribute to our understanding?

Thanks for making me think more deeply about our preference.

Paul Gould
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