On 02/06/2011 09:24 AM, Peter Clifton wrote:
Imperial parts are not a problem for a sufficiently fine metric grid.
+1 I heard Tom Hausherr talk last year about grids, land pattern generators. For his suggestion to just "go metric", it works with old inch spaced protoboard layouts just fine if enough resolution is used. It's always going to be valuable to combine the old with the new, where you might have very fine line spacings in the new on a metric grid, the base drawing being a metric grid, and placing the inch stuff on a calculated-from-metric "semi" inch grid only means some holes are off by 0.025mm or less if using a 0.05mm grid. Meanwhile, any scripted tool we can write to help make parameterized footprints from datasheets will be one of the biggest aids to pcb layout. Much of our discussion about ways to vote on or log successful uses of footprints in production of boards seems invalidated by the industry adoption of most nominal and least density styles of footprints where the IPC specifies extension of pad area beyond a part lead in mm. The manufacturer's suggested footprint is pretty much ignored by such a standard, yet deriving footprints from lead shapes for 2 terminal chips and gull wing leaded parts is what the usual commercial tools (and thus the usual commercial layout folks) do. When it comes to chip scale packages I didn't see any suggestion to change the MFRs land pattern. Hausherr does suggest BGAs get the three density levels, but the denser ones are only used on throw away products. IPC recommendations seem to take BGA shapes as published by MFRs and make them a little larger for robustness. The writeup on http://blogs.mentor.com/tom-hausherr goes over the same material I heard in person last year about "fairways" between microvias put in BGA ball lands planned for many tracks in inner layers of 8 and up layer boards used for dense products these days. Figure 20 and 21 are for board layouts, but look like chip layouts of the 1990's! Blind buried microvias here we come? John -- Ecosensory Austin TX _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
