I think you misunderstood what I meant by nominal.
I was refering to the fabrication drawing stated hole size. The size of
that hole is a whole 'nother calculation that takes into account the
lead size.

> -----Original Message-----
> From: George H. Patrick, III [SMTP:[log in to unmask]]
> Sent: Friday, April 24, 1998 10:15 AM
> To:   [log in to unmask]; [log in to unmask]; George
> Patrick
> Subject:      Re: [DC] Hole size calculations
>
> 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 (;->
>
> --
>     /\        Praegitzer Design -- Portland Design Center
>    /\/\       George Patrick, Systems Administrator
>   /\/\/\      Email: [log in to unmask]
>  --------     Voice: (503) 531-2050 x1813, Fax:   (503) 531-2051
>
>  Webmaster: http://www.aracnet.com/~gpatrick
>             http://www.dacafe.com/USERSGROUPS/cctug

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