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 ################################################################ DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE DesignerCouncil <your full name> To unsubscribe: SIGNOFF DesignerCouncil ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################