I've been working with some old PTH padstacks lately, and thought I'd see how they "stack up" against the IPC-7251 recommended patterns. Really at first I was looking at the spreadsheet generated by the LP Calculator, both from PCB Matrix. Then to see where the LP Calculator came up with its numbers, I've been consulting the draft IPC-7251. I think I've found an inconsistency in hole and land diameter relation that in some circumstances causes insufficient annular rings, by IPC-2221/2222 standards. In section 3.1.3, paragraph 1 it says, "Each dimension given is considered part of the equation which takes the maximum hole size, twice the annular ring requirement, and an appropriate manufacturing allowance." That's the minimum land size = a + 2b +c from IPC-2221 sec. 9.1.1. So far so good. So we move to the series of tables 3-1 through 3-9, where the PTH lands are defined based on the maximum lead size. We see the numbers from IPC-2222 Table 9-3 (as cited by Jeffrey) for defining the _minimum_ hole diameter. In the next row, the values in "Int. & Ext. Annular ring Excess (added to hole dia.)" correspond to IPC-2221 Table 9-1 minimum standard fab allowance + double the Table 9-2 minimum external layer annular ring, which should be added to the _maximum_ hole diameter. But as far as I can tell, this value is instead intended to be added to the minimum hole diameter from the previous row. The LP Calculator adds this value to the minimum What seems to be missing is some accounting for IPC-2222 Table 9-5, "Minimum Plated-Through Hole Diameter Tolerance Range". Without it, it's as if drill sizes have zero tolerance, which we know isn't the case! I've been searching validation (or otherwise) on the Internet but to no avail. I'm not sure if I'm on to something or if I'm missing one key point, or if I've just totally lost myself in a cloud of tables. This seems to be the place to ask... Cheers, Kim --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------