Re: gEDA-user: PCB package creation guidelines

[DJ Delorie <dj@xxxxxxxxxxx>]

> How does the automated population process work then?

I asked an insider about this.  It's different for each machine.  He
said not to bother with it unless we had a specific machine we were
targetting.  Of course, we can still keep track of relative geometry
in the layout - angle and position relative to the original symbol,
for example.  We'd use that information later as the basis for
machine-specific placement.

> > > 2) The middle of the silkscreen outline outlines the biggest
> > > outline that can occur.
> 
> The part has some nominal size and some tolerance. size+tolerance is
> where the outline should be placed. This ensures when you place 2
> parts one next to the other, they will fit on the PCB even in the
> worst case.

Except where the part outline doesn't encompass the pads/pins, but in
that case the pads/pins serve the same purpose.

The purpose of the outline is to show an iconic representation of the
part sufficient to assist in proper selection and placement of the
part.  It is not a bounding box!  IMHO outlines should actually look
like the part they represent.

However, in cases where the part extends beyond the pads/pins
(i.e. where the outline happens to be the bounding box), the
size+tolerance idea sounds reasonable.  Or, make the outline be the
size, and its thickness be the tolerance.  I always go by the "don't
let outlines overlap" rule, but that includes the thickness of the
outline's trace.

As an extremely long-term project, we could add bounding regions to
the symbols as a separate layer, separate from the silk layer.