gEDA-user: PCB: proposale for solder paste and adhesive spec in footprint

[Dave N6NZ <n6nz@xxxxxxxx>]

I keep revisiting the idea of adding better paste information to the 
footprint.  My current thought is that it could be generalized to 
account for anything that needs to be dispensed, such as the adhesives 
that are sometimes used for SMT assembly processes to hold big parts in 
place.

I propose a "Dispense" keyword.  Dispensing is associated with the 
element, not the pad.  This allows for a glob of glue to hold the 
package down that is nowhere near a pad, or a field of solder paste dots 
that are associated with a single large pad.

Form:
Dispense [ x1 y1 x2 y2 volume "material" "flags" ]

x1 y1 x2 y2 - the material should be evenly dispensed along this line. 
If x1=x2 and y1=y2 you get a dot.

volume is specified in some convenient unit in the neighborhood of 
picoliters. A post processing tool is responsible for turning that into 
something usable, such as CNC for a "flying syringe", or stencil 
apertures that yield the appropriate amount of material in whatever 
process is being used.  It would be nice if PCB had some reasonable hid 
for exporting a gerber for paste stencil built in.  I could also see a 
CVS hid and an XML hid for exporting dispense information.

material is an arbitrary string.  Presumably there are a couple of 
well-known names like "solder" and "adhesive", but it could be anything.

flags -- just seems consistent with PCB philosophy :)

I think all PCB needs to do with the data is carry it along through x/y 
translation/rotation and then make it available via report generators in 
the hid.  For backward compatibility, I would propose that absent a
'Dispense [ ..."solder".. ]' clause in an element, the paste layer 
gerber should use the current method of simply copying the pad. 
Otherwise the gerber hid should use a user-provided "stencil thickness" 
to compute the aperture that yields the required volume.

Comments?

-dave

p.s. Will be out of town over a few days, so after today will catch up 
on any discussion when I get back.


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