Ray,

This explanation will be a difficult one so here goes. The left hand
illustration in figure 5-18 indicates a minimum 0.010 inch clearance between
a land and its clearance in the plane. The purpose of this minimum has to do
in part with the method of inspection used by board fabricators. Most
fabricators utilize automatic optical inspection equipment AOI. Clearances in
planes usually overlap each other resulting in an infinite number of possible
geometric configurations for the optics of the AOI cameras to interpret. The
end result is thousands of error flags for an operator to sort out. The
reduce these false errors, certain algorithms must be either turned off or
desensitized. when this is done, the AOI system has difficulty identifying
spurious copper, of a certain size, in the spaces between copper.

The input received by the committee is that the designer must design in a
larger space, no less than 0.010 inch. this will enable the fabricator to
obtain a more reliable inspection from his AOI equipment.

Now let us relate it to the other examples in figure 5-18 which refer to
landless unsupported and supported holes. 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. Those few cases of exception are
usually associated with a higher technology that utilizes equipment and
processes designed for those technologies.

Furthermore, I wish to draw your attention to paragraph 5.3.1.4 Edge Spacing.
This paragraph is concerned with copper spacing to a board edge. That edge
can be an outside edge or the inside edge of a cutout in the board. An
unsupported hole may also be construed as a cutout. The paragraph indicates a
requirement of minimum electrical spacing plus 0.016 inch. The last number
reflects a hard process allowance instead of the variable one used in our
previous discussions. This is to ensure that from an electrical product
safety standpoint, that a conductor does not come too close to a board edge.

I would also like to point out that in IPC-2221 Generic Standard on Printed
Board Design, one of the replacements for IPC-D-275 that the Standard
Fabrication Allowance table has significantly changed.

I hope that this helps clarify your concerns.

Regards,

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