Jack,
Notice that the larger the PCB, the larger the fabrication allowance
required.  And, it applies to EACH side of the hole!  So, for a 12", 40 mils
(+/- 20); 18", 48 mils (+/- 24), and a whopping 56 mils (+/- 28) for a 24"
PCB!  Don't forget to add the electrical clearance to each of the above,
too.

Ray

----- Original Message -----
From: Jack Olson <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 22, 1999 9:51 AM
Subject: Re: [DC] IPC Specs - Errors?


> >From the fabricators we use, I have been told to make sure the copper is
> 13-15 mils away from the drill, keeping in mind that the drill is always a
> bit bigger than the finished hole size (maybe 5 mils?). So if that
increased
> drill size is a 2.5 mil annular ring in effect, and I am adding at least
13
> to it, I get that my anti-pad has to be a minimum of about 30 mils larger
in
> diameter than the finished hole size (15-16 mil ring).
>
> I am having trouble relating a "hard requirement" like that (from the
> vendor) to the spec, and even more trouble thinking that an unplated hole
is
> really a board edge.
>
> Do I really need to increase the anti-pads on my padstacks?
>
> Jack
>
>                 -----Original Message-----
>                 From:   Gary Ferrari [mailto:[log in to unmask]]
>                 Sent:   Tuesday, September 21, 1999 7:53 AM
>                 Subject:        Re: IPC Specs - Errors?
>
>                 Obvious to us is that we require a
>                 minimum electrical space. We must also prevent the
resulting
> space from
>                 getting too small as to create a similar problem with the
> AOI systems. We
>                 attempt to accomplish this by requiring a reasonable
> fabrication allowance on
>                 each side of the hole. Granted, in fine line and space
> applications, the
>                 spacing may become smaller than our 0.010 inch minimum.
But
> in more case than not, the result will be satisfactory.
>