Re: gEDA-user: PCB polygon & rectangle practicalities

[Harry Eaton <bumpelo@xxxxxxxxxxx>]

Dave N6NZ wrote:

> In fooling with various practice layouts (partial layouts, actually) I 
> think I have the basics of polygons and rectangles sorted out.  Now 
> I'm wondering about the practicalities is adjoining poly's.
>
> Here is the situation: In my design, there wants to be a polygon patch 
> of analog ground that will have nothing routed through it, although 
> there will be a few clearance vias and thermal-relief joined vias.  
> This polygon will be on the solder side (2 sided design) and will be 
> an island in what I hope will be a fairly continuous chunk of digital 
> ground plane, although I'm planning the solder side as my Y-axis 
> routing layer, so there will be lots of traces cleared out through it.
>
> Anyway the point is, while the analog ground patch is easy to draw as 
> a poly, the digital ground wants to be a simple rectangle with an 
> island in it. I presume that the best way to make that happen is to 
> lay down the AGND poly, and then draw several rectangles/poly's for 
> GND until the composite is the shape I want.  So... do I need to 
> overlap the GND poly's so that they will join up? Or can I just turn 
> on the grid and draw to the snaps and count on "touching" to be enough 
> to actually join them?  Are there some gotcha's here that I haven't 
> thought of that I am going to trip over? Manufacturability issues?

Touching is sufficient for the polygons to make contact. However, there 
is no harm in overlapping them a bit. The new polygon clipping code will 
dice up any polygon that has holes in it into smaller ones that don't 
have holes and they just touch, they don't overlap.  You should be able 
to create the larger digital ground plane from only two drawn polygons.

Another trick you can use would be to just draw a single rectangle over 
the board, and use clearing lines (and/or arcs) to isolate the analog 
ground island. If you are routing a lot of tracks through the ground 
plane, chances are that you will create other unintentional islands too. 
You can use the "MorphPolygon()" command to make new polys from the 
islands (including the analog ground one, and if you do this then after 
the morph you can remove the tracks originally used to create the 
isolation). It will be up to you to connect any islands that don't end 
up connected.

I don't think you'll run into any manufacturing issues.

harry





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