Tim,
I'm with Ray on this one.
If I have enough breakout on the outer layer to short the Innerlayer
then I would definitely have to scrap it. As a matter of fact
I definitely wouldn't pay for it.
What would be the use of a land being smaller then the outer layer pad
then. Either way you would have roughly the same amount of anti-pad
beyond the outer pad dia.. if your information is correct.
I myself don't use lands on plane layers, and was approached
by the board house to increase the clearance 20 over the outer
pad dia.. I asked why?
I could only get that it was specified by IPC spec (it actually
only specs this for padstacks with lands) but in either case
the drill should never get that close to cause shorting.
If the drill were off enough to be on the edge of the land
(due to the pad size of the land being smaller than the outer pad)
than it shouldn't matter, since the pad is, for the most part unconnected.
If the drill is any further out it would beyond most of our "hole to pad
center"
requirements and therefore would be worthless.
Can you convince me/us (in use of an example) differently?
Chris Robertson
PCB Designer
Lockheed - Martin Services, Inc.
[log in to unmask]
Ph/Fax (256) 722-2626
ICQ# 13541566



Ray, I believe the formula is correct.
If you  poll some of your pwb vendors I think you will
find that they will request , if not require, additional clearance
on the inner plane layers. If , due to a miss drilling you have
tangency or breakout on an outer layer pad if may not render
your boards to scrap.
But if your plane clearance ( anti pad )is the same as your outer pad
and you have break out, you will have shorts to the inner plane layer.
The additional clearance of the anti pad will provide a better yield for
your pwb vendor.



Tim Easterling
Cadd Manager Commercial Engineering
SCI Systems Inc.
8600 S. Memorial Parkway MS 200
Huntsville, Al. 35802
(256)882-4426 (ph)
(256)882-4700 x4426 (voicemail)
(256)882-4304 (fx)