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The drilled holes aren't always perfectly centered. For single sides boards this doesn't make much difference, but for plated through holes having a hole in the pads copper could mean that the pad only partly connects to the through-hole metalization if the drill doesn't match the hole perfectly.
Are through-hole pads without hole in the copper allowed?

edit
About the larger annular ring (re Leon's comment): it's not the outside I'm worried about. I've never had an annular ring so small that the drill comes outside of it. If the drill goes a little bit to the west, then possibly on the east side the copper of my pad doesn't connect to the through-hole metalization. This wouldn't be noticed when the board is tested, since there's still a (partial) connection, but it may have a higher resistance.

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I understand what you want, but it doesn't make sense: the drill diameter is determined by the hole diameter in your pad. No hole in the pad = no hole drilled.

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  • \$\begingroup\$ I'm very stoo-pid. :-/ \$\endgroup\$ – stevenvh Apr 19 '11 at 13:41
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    \$\begingroup\$ This is just wrong, the hole drilled is independent of any hole in the copper layer. Though maybe your software doesn't make this clear to you. \$\endgroup\$ – starblue Apr 22 '11 at 14:03
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The only time I know of when it's useful to have holes in the copper design is when boards are being hand-drilled. In such cases, a much-smaller-than-actual-size hole will help show where the drill should go and keep it centered. Otherwise, it's often nice to have holes shown actual size in paper plots but it's a very bad idea for production.

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If you are getting problems with off-centre holes in your pads, you should change your PCB supplier, or use a larger annular ring. It sounds as though your supplier has problems with his PTH process as the copper is deposited after the holes have been drilled.

You must have holes in your vias, it's the only way to get connectivity through the board. Holes can be filled, but it's a very expensive process and only used on high-end boards with vias in pads for BGA devices.

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  • \$\begingroup\$ Yes, I understand that the hole is needed, but I was wondering if I had to provide it in the pad's copper, or let the drill do the work. \$\endgroup\$ – stevenvh Apr 19 '11 at 11:56
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    \$\begingroup\$ You shouldn't have the holes in your Gerbers, some PCB suppliers don't like it and will send you an email saying something like "No donuts!". Others will just ignore it. The drill file is where it belongs. \$\endgroup\$ – Leon Heller Apr 19 '11 at 13:43

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