From the replies I have seen to this thread it seems that the
IPC spec includes size tolerance and the  over drill of a fab house.
Fabrication Allowance seems to include any locational tolerance.  To
summarize using the new spec values (except for hole size tolerance
which I couldn't find but used our normal limits, +/-.003):

        Max hole size =
          nominal + size_tol + over_drill
        Pad size =
          max_hole + FA + 2(annular_ring)

           or (for a level B Board)

        Pad size =
          (nominal + .003 + .005) + .010 + 2(.002) =
          nominal + .022"

        A comment about the new spec:
        The table of fabrication allowances is mucho different.  There
are no longer different values depending on the size of the board.
Level B allowance is 0.25mm (.010") whereas in D-275 it was between
.016" and .024" depending on the size of the board.  That's a big diff
in our world of mils.

George W Uptain, Jr.
Austin Product Center of Schlumberger Oilfield Services
(512)331-3183
>

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