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Fri, 24 Apr 1998 09:09:17 -0500 |
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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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