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Re: gEDA-user: new footprint guidelines

I am curious about the reasoning for picking values of design rules. I have not found the assembly houses to be very useful for this sort of info. They seem to be willing to work with whatever they are sent and will only give feedback when something causes real trouble for them. 


At 12:51 PM 9/24/2010, you wrote:


Yes, the old library parts are pre-hires and the pads can be "way off"
and should be fixed.  Thanks!

If we're hand-coding footprints, we could use "0.5mm" instead of
"1965" and preserve the *meaning* of the units.  We lose some
compatibility with older PCBs, but if the purpose is to update the
current distribution that shouldn't be a problem.

We should probably go with build-time generated footprint files,
rather than continue to use the m4 runtime generation.  That allows us
to use more than just m4, too.  Makefile rules for standard %.whatever
to %.fp conversions...

My general rules:

Mask should be 3 mil away from copper, and slivers should be at least
6 mil wide.  That means, if there's less than 12 mil between pads you
go with a gang-opening.
Where did you get these numbers? Did a manufacturer give this as their capability limit? 



Silk should not overlap the *mask opening* and should be 3 mil away at
least.  5 mil min silk lines.


Same here, who's rules are these?



Origin and license should be stored in element attributes, not file
comments, so they're copied into schematics.
IPC has developed a set of rules for designing footprints to match parts of all sorts and has even provided a library of data for this. They provide three standard sets, Most, Nominal, Least which differ in the amount of land protrusion. Armin's footprint is likely a "Most" catagory footprint from his description. IPC-7351 seems to be very widely adopted and would be a great starting point for any footprint library. 



It would probably be a good idea to have more than one design for each
footprint; one for reflow'd boards and one with longer pads for hand
soldering.

All QFN parts should have some visual aids to centering :-) On my last
board, I added four diagonal lines on the silk layer to align each
corner (like a big X), that worked out well.

Refdes should be properly placed and sized but I'm not sure what's
best.  For example, on every single RESC1608N part I place I have to
make the refdes smaller and move it off the pads.  Getting size right
is far more important than position; it's easy (and often needed
anyway) to move things around in only-text mode.
I don't bother with putting the refdes in any particular location for a library part. The times a default location would work out is so seldom, that it just isn't worth the effort. I put the refdes in the middle of the library part and move it to suit the design.
Rick 


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