[from mailing list #2910]

>. kicad have clearance to board border which pcb-rnd doesn't have
>  kicad a cu border outside that clearance, don't know what that is 
>  good for

Correct. I didn't know KiCad did this, but I knew about the generic idea 
(I did a lot of pcb fabbin research the past few years, for our new data 
model).

Data model background: our objects are per layer (except for subc and 
padstack, but...) thus our clearing is calculated on a per layer basis 
(... even for subc and padstack, because we just calculate what they look 
like on the layer the polygon is on). 

Boundary and some mech layers (slots, outlines) have global effect (they 
cut through all layers) but that doesn't really do anything else, they 
don't interfere with layer objects in any way:

- we don't cut objects crossing a drill/slot/rout/cut

- we don't do a clearance on copper layers based on these

What they are used for in the industry: there is a recommendation that you 
should not have copper near the edge of the board. Different fabs will 
have different margins, the largest values typically 1..2mm. Rationale: 
they will be routing/cutting the board with the copper on, and if copper 
extends to the cut, you risk peel-up. 

In pcb-rnd's philosophy this is a DRC job: since this affects not only 
polygons but any copper, pads, traces, etc. I also think it is relevant 
for the outer layers, not the inner ones.

So my (long term!) plan on this: 

- once we have the programmable drc, we can write a rule file that checks 
for copper features too close to the edge on outer layers.

- plus we may introduce no-export attributes (I wanted to have them for 
other reasons too): if you have a poly close to the outline and want to 
handle the keep-away margin with poly clearance, you will need to redraw a 
section of the outline on your layer (... e.g. just copy the objects from 
the outline) and mark those copper objects no-export - that way they have 
the clearance you wanted but they won't show up in your gerber.

I know it sounds like more hassle than automatic "outline clears copper 
polygons", but I don't want special cases and ugly hacks, but a nice, 
constructive tool that does exactly what you ordered it to do, with 
minimzed amount of side effects.