Re: gEDA-user: General Layers questions

[John Doty <jpd@xxxxxxxxx>]

On Mar 16, 2011, at 4:24 AM, Stephan Boettcher wrote:

>> Ok. So "via" should be a circle element on "hole" typed layer.
> 
> No.  A Via is a composit, consisting of a circle on the hole layer, and
> various circles on copper layers, and circles on mask layes, and
> thermals.

The "layer" concept should be physical, not a metaphysical abstraction. Objects in a layer may contain holes, but a "hole layer" is nonsensical, a toxic conceptual shortcut. An "outline" layer is similarly bad: the insulating layers may all have the same shape sometimes, but not always.

Trying to model things that aren't layers as if they were layers is one common mistake in this kind of tool. Equally common is leaving out layers: the insulating layers in a PCB are just as important as the copper, and have their own properties (shape, thickness, material, etc.). They're a critical part of the layer stack.

The description language needs to be able to express "feature p in layer x is aligned with feature q in layer y" in order to build up composites. This is the geometrically sensible way to describe the result of drilling through several layers. But the geometric description language should not be tied to any particular fabrication procedure.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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