gEDA-dev: PCB - XY values of centrioid in BOM/DXF exproter (was gEDA-user: PCB Element for a Molex 71661-2068?)

[Dan McMahill]

Bert Timmerman wrote:
> Hi Dan and all,
> 
> For the DXF exporter I'm currently coding, I use the centroid X,Y-value (as
> in the BOM exporter) as the insertion point for the 3D model of the package
> involved, in a mechanical CAD package (for example AutoCAD).
> 
> For symmetrical and double symmetrical packages (> 99% of all packages) this
> should be no issue as it coincides with the center of gravity.
> 
> The problem lies in assymetrical [PGA, BGA, SIL, TO, connectors, ...]
> packages where the centroid (a calculated average X,Y-values of the actual
> existing pins/pads on the package) does not coincide with the center of
> gravity of the package (basically for X and Y values, since pcb has no
> notion of Z values).
> 
> This asymmetricallity arises from pins/pads missing in the pattern, or shape
> of the connector (pin/pad pattern).
> 
> IMO it would be a good practice to use the center of gravity as a
> mark/insertion point for pcb footprints/3D models.

yes.

> Now, there are a lot of footprints out there, and we do not want to break
> _everything_.
> 
> So, this [should, could] be solved by using the mark X,Y-values in the DXF
> exporter and _not_ use the calculated centroid X,Y-value.

assuming that all the footprints are fixed up.  When I made the 
decision, it wasn't clear how many footprints actually put the mark in 
the right place.

> However, a question that comes to mind is whether anyone is actually using
> the XY file for "pick and place" of components ?
> Positive feedback ? (from list members who actually use this feature).

good question!  I asked for feedback when I added (since it had been 
asked for) but didn't really get any.  I will note that gerbv (in cvs 
head) can read PCB XY files.

> 
> If not, it's not lean to _allways_ provide a centroid file, that is, there
> probably should be a checkbox to be ticked in the BOM exporter if someone
> _really_ needs this file (default set to unticked).
> 
> I think I can send a patch if needed.

I guess I'd rather see the default be checked just to avoid breaking any 
scripts or makefiles that users may have aleady.  I'm not sure what the 
harm is though of always generating the file.  I'm sure many users don't 
use some of the layers which come out of the gerber export either.  For 
example solder paste stencil is probably not used by most hobby or very 
low volume users.

-Dan