The reason a fixed value works is interesting.

Ignoring targeting error due to stackup tolerance and front/back artwork alignment, the drill error is a function of 2 basic variables:
1. Alignment error between the drill and the artwork (x, y, scaling, and theta errors).  These will show obvious tendencies across the panel.
2. Drill "walk" and "whip" (these will be completely random, with a Rayleigh distribution)
  -"walk" - PCB drills don't have split points, which means a line is the first thing to touch down.  The drill will tend to pivot on the point at one end of the line, dragging the hole in that direction.  Big bits have long lines, so this is the primary error for them.
  - "whip" - Before the drill touches the drill stack, it is spinning real fast and the end of the tool will get a displacement due to centrifugal force.  Small bits tend to be relatively long and skinny, so this is the primary error for those.

The sum of all of these errors tends towards a constant value for a particular fabrication shop!

Wayne Thayer


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul O Connor
Sent: Tuesday, November 26, 2013 3:54 AM
To: [log in to unmask]
Subject: Re: [TN] PWB/PCB, clearance ring

On most of the CAD Design software I've worked with the spacing gap i.e (pad to copper) is a global value (gap) assigned to each layer, to reassign spacing's per pad or hole size isn't a feature of most software packages. There was/is one that did have the option but its not a common feature.
I use the copper weight of the layer to determine the clearance.

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