It's to do with the plating of the barrel in a multilayer board.

When the hole is drilled, the external surfaces have complete copper
coverage and so the annular ring isn't evident until after imaging and
etching. Hence you can add the thickness of the plating to the pad size
for an annular ring requirement.

On the inner layers, imaging and etching have already been completed so
when the hole is drilled, it needs to 'see' enough copper around the
hole so that the plating of the barrel makes good contact to the
innerlayer pad. That hole size is the drilled hole, but the final hole
size is the one specified by the designer and relies on an oversize
drilled hole to be plated up. So the annular ring is specified 'worst
case', which is before plating.


Andy K.
Sychip Inc
Office (972) 202 8852

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Stan
Radzio
Sent: Wednesday, December 10, 2003 10:34 AM
To: [log in to unmask]
Subject: Re: [DC] Manufacture adjusting Gerber's

Gary,
        I was following you right up to the point where you said "for
inner layers".  I don't mind looking like an idiot asking questions that
are obvious to everyone else.  Could you explain why you are using the
drill hole size versus the finished hole size on inner versus outer
layers?  Said another way: what is the manufacturing process that you
are accommodating with this design decision?

Thanks,
-Stan


-----Original Message-----
From: Gary Ferrari [mailto:[log in to unmask]] 
Sent: Tuesday, December 09, 2003 5:12 PM
To: [log in to unmask]
Subject: Re: [DC] Manufacture adjusting Gerber's


Ted,

The board fabricator always drills with a larger drill bit to compensate
for shrinkage of the laminate. This is why the designer uses the drilled
hole size when calculating lands for inner layers. The finished hole
size is used for calculating outer layer land sizes.

The board fabricator is not requred to automatically adjust
inappropriate spacing.  They usually will contact the designer and
explain the problem. That is, if you use a decent board fabricator.

Regards,

Gary

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