There is another problem with this rule If I am correct, it is indeed, a 
hole clearance. But true clearance will be about 3 mils less, more or less, 
because the holes are drilled oversize so that 1.5 mils of plating (1 oz) 
will bring it back to the specified size. That is a conductive reduction of 
the clearance. It can be even larger because fab houses drill within a 
range in order to meet specified hole tolerance.

In other words, if you specify 10 mils annulus, you actually get 8.5 mils 
*nominal* minus fab tolerances. Thin, too thin, usually.


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