Hi all,

I am trying to update the hole/pad chart that our designers use for their
CAD tools.

To do this I have been looking at the IPC-2221 calculation (paragraph 9.1.1)
which has the age old formula:  Land size min = a + 2b + c

What is different from what I remember is "a" is now the maximum diameter of
the finished hole.  In the past I used the drilled hole diameter.

Since the internal annular ring is measured from the effective drilled hole
to the edge of the foil pad this equation seems to have something missing.
It seems like 2x the plating thickness needs to be added to the formula for
internal pads to guarantee that the annular ring requirement will be met in
the final product.  This formula looks like it will allow no annular ring,
or a hole tangent to the edge of the pad, when taken to its extreme.

Since external pads are measured from the plating the formula looks fine for
external pads.

Any comments?


Bill Dieffenbacher,
Technical Leader Product Design Engineering
Lockheed Martin Control Systems @ Johnson City, NY
Tel (607)770-2961  Fax -2056   MARCALL 8*255
[log in to unmask]

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