Re: gEDA-user: Open Source mechanical CAD on the horizon

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Tue, 2010-03-09 at 16:54 -0800, Dave N6NZ wrote:
> On Mar 9, 2010, at 4:01 PM, DJ Delorie wrote:
> 
> > 
> >> is that true?  Is it simply generated on the fly off the pad
> >> information during gerber export?
> > 
> > That's true.
> Just in case anyone is confused by the tight snippage: It is true that there is no "real" paste layer, it is generated on the fly off the pad information.
> 
> OK, so... for a long time, I've been thinking about how to add refined paste information.  My approach would be:
> 
> 0. Extend the lexer and parser to "warn and ignore" on unrecognized keywords in footprints. This allows some backwards compatibility of pcb with new footprint keywords, although at the expense of error checking.  Maybe should have a 'strict' option to cause an error.
> 
> 1. Extend footprints to include a "paste (...)" keyword that looks pretty much like the 'pad()" keyword.
> 
> 2. If "paste()" doesn't exist for a footprint, synthesize one from the "pad()" information.  That way, old footprints work just as they do now.
> 
> 3. Add a paste layer to carry around the paste() information through all the translation/rotation of the symbol.
> 
> 0, 1, and 2 I think I can sort out relatively easily.  I have no idea how to add a layer and do all the necessary updates for footprint relocation.
> 
> -dave

I can imagine two options for file->Save time:

1. Adding an internal flag to state whether the paste information has
been derived from the pad, or loaded from the file. In the derived case,
we could skip re-saving the paste layer. (This works nicely if we
decided to have some setting to specify a shrink between pad and paste..
it would allow the shrink setting to stay editable (and update the
board) for non-manually modified pads.

2. Just save paste() layer information each time. This does, however
mean that the paste is stuck in stone with the rest of the footprint
when it is placed. Perhaps not such a bad thing.


Questions though..

What to do with a manually defined paste layer if the user fiddles with
the size of the copper pad / solder mask? (Assuming that eventually
becomes more flexible to edit).

Solder mask aperture is important as well as pad size, since the stencil
opening probably ought never include areas which are solder-masked. It
is possible (although I'm not sure how useful) to set a partially masked
pad - perhaps as some kind of heat-sink for a transistor, with a defined
mask opening.

Regards,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)
Tel: +44 (0)1223 748328 - (Shared lab phone, ask for me)



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