Uptain, George wrote:
>
>         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.

I see a couple of potential problems with this.  From your example, it would
appear that the standard calls for holes that are .008" larger than the nominal
size of the lead, meaning you have .004 from the nominal edge of the lead to
edge of the hole.  This does not seem to take into account the tolerance of
the lead diameter, nor the tolerance of the placement method.  For example,
assume you have a part with a lead size  tolerance of +/- .003, a positional
tolerance of +/- .003, and an insertion machine with a placement tolerance of
+/- .002.  A worst case stack-up of that part shows that you are bending the
leads.  This may not matter for hand inserted parts, but the standard needs to
allow for real-world tolerances.

I don't have a copy of the new standard, so maybe a "level B" board has no
auto-inserted leaded parts (;->

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