Ray,

The fabrication allowance in the table is a single number that the designer
uses to adjust his calculations to compensate for fabrication process
variation. The numbers in table 5-10 (IPC-D-275) are there as a guide to cost
comparison and what is achievable. They hope to illustrate when one would be
moving into a lower yield situation. The numbers are a statistical summation
of all the variations in the process (which tend to be far greater than the
numbers in the table).

As a designer you may choose the exact numbers in the tables or select any
number that you wish. Critical is that you place SOMETHING in the formula.

There is no further tolerance applied to a standard fabrication allowance.
You need to double your annular ring requirement since you have a ring on
both sides of the land. But only double the annular ring, not the standard
fabrication allowance. The same is true for the clearance in a plane.

Regards,

Gary Ferrari
Executive Director
IPC Designers Council
860-350-9300