Stan,

I believe that Andy Kowalewski described the reasoning very well. However,
the laminate material shrinks back into a drilled hole resulting in a smaller
diameter that required. If one were to plate the hole, the finished size would
be smaller than desired. Therefore, the fabricator overdrills the hole so that
when the laminate material shrinks, it shrinks to a level that will still
produce the minimum desired hole size.

If the designer does not allow for this overdrill in his land pattern size,
it is very possible that the drill will completely remove all the copper in the
land resulting in no land at all.

Typically fabricators adjust line widths to compensate for etch factors. They
rarely compensate lands unless they discover a serious problem and only after
notifying the designer of how they propose to fix the problem. By
compensating land sizes they are subject to the risk of creating a greater number of
spacing violations. This results in far too many fixes on the fabricators part
unless they elect to charge for the service.

Regards,

Gary

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