Don't forget the etchback either. That removes a little bit of
dielectric from underneath the pad next to the drilled hole and adds to
the problem. 


Andy K.
Sychip Inc
Office (972) 202 8852

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary
Ferrari
Sent: Wednesday, December 10, 2003 11:25 AM
To: [log in to unmask]
Subject: Re: [DC] Manufacture adjusting Gerber's

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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