Re: gEDA-user: next PCB release - 1.99za vs 4.0

[DJ Delorie <dj@xxxxxxxxxxx>]

Thinking about drills...

If a cutout is a property of an insulating layer, the a drill through
the whole PCB is really a composite of 2N+1 drills through N insulating
layers and N+1 conductor layers.

That seems kludgy to me.  Drilling is a single operation, why do we
have to represent it as multiple objects ?

I'm thinking we structure the layers into "composites".  Each
composite contains some conductor/insulator layers, including other
composites, and an overall shape (outline, drills, slots).

Thus, a drilling operation is stored as a single "drill" object in the
composite's outline layer, and buried vias are just drills on a
sub-composite.

The heirarchy of composites represents the manufacturing process, so
implicitely defines the possible blind and buried vias - you either
drill through a composite or you don't.  If a composite happens to
represent copper layers 4 and 5 and the insulator between them, you
get a buried via.  If the composite is layers 0 and 1, you get a blind
via.  If it's the top-level composite, you get a regular via.  Etc.

This also solves the "which insulator is the copper on" problem,
because it's on the ones it's in the composite with.  They'd all have
the same shape anyway, you just limit the conductor layers to the
shape of the composite.

Within each composite, we'd still need to figure out how to store the
physical stackup (order, groups, etc) and do the usual top/bottom/drc
stuff.

I'm not sure how to reflect this though: consider two flex pcbs
connecting the tops/bottoms of two rigid PCBs, one with two layers and
one with four:

   -----------------------------
-----                         =====
-----                         =====
   -----------------------------

Now what about two flex pcbs that leave a rigid pcb on the same side,
one on the top and one on the bottom, yet lead to two unrelated rigid
pcbs?  Do those other pcbs need to share layer groups, or need to be
separate?  Where the two flex pcbs overlap on the one rigid pcb
they're part of the same composite, but where they leave that pcb they
become two separate things.

Or does PCB need to know that a "pcb assembly" may be made of multiple
separate PCBs assembled later?  I.e. spit out separate gerber sets for
the flex pcbs and the three rigid pcbs?


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