Danny;

I fear that your interpretation of the IPC-D-275 is incorrect. The clearance
in a plane, aroud a via land is a minimum of 0.010 inch. You are not required
to add a standard fabrication allowance to the ckearance. So, your
calculation would be pad size + 0.020 inch. Please note that this is exactly
what you have recommended. The reason a standard fabrication allowance isnot
needed, is simply that the plane and the land are both part of the same
imaging step, with no individual tolerance, except for etch factors.

Where a standard fabrication allowance enters the picture is with holes that
pass through a plane, and possess  no specific land. Since the hole and the
image is seperately located, they are subject to opposing manufacturing
tolerances, and must be compensated for, eg. standard fabrication allowance.

The minimum spacing dimension was derived from many fabricator members, who
provided input to the IPC-D-275. Therefore, I would expect your fabricators
not to complain.

Regards,

Gary Ferrari
IPC Staff

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