Re: gEDA-user: New Column: From the CAD Library

[John Griessen <john@xxxxxxxxxxxxxx>]

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


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