gEDA-user: pcb: strategies for 2-layer polygon planes? (chopped up polygons...)

[Colin D Bennett <colin@xxxxxxxxxxx>]

I tried to use a nice big ground plane floods on my 2-layer PCB design
to provide a low impedance ground and to implement the reference
design for the RF IC (Atmel AT86RF231) and PCB antenna designs I was
using.

I know you can't have a perfect ground plane on a 2-layer board when
you are routing tracks on both layers, but I figure that if there
are some islands without ground plane, that's fine.  Perhaps my
concept is wrong.  Anyway, here's the process I used:

1. Import schematic into pcb, disperse components.
2. Lay out major components (radio transceiver IC, antenna, data
   connector).
3. Create ground plane rectangles on both top and bottom layer.
4. Route tracks.

Now after a while of routing tracks, there were so many tracks that my
ground plane polygons were chopped to bits and pcb started drawing only
the left half of the polygon, for instance.  This obviously created
problems since nets that were already successfully routed got broken.
Furthermore, I couldn't edit my ground plane polygon easily since pcb
doesn't draw any indication of where the vertices are, and the polygon
itself was half missing.  It looked like the logical thing for pcb to
do would be to draw two halves of the polygon.  In fact it looked like
the missing half was the larger half, so it seemed a really bad choice.
I tried to patch up the missing parts with more rectangles, but it was
not great.

It was such a mess that I had to delete my ground planes and
start over.  The second time, I didn't create one large rectangle, but
instead inserted 3 or 4 smaller rectangles on both top and bottom
layers in specific areas where I needed the ground plane.  This worked
better but seems cumbersome, especially in the early layout when you
don't have components and tracks already figured out, so you don't know
where the ground plane polygons might fit.

Does anyone have any strategies or tips for general design of ground
planes in 2-layer PCBs?  Do you do a ground flood, or do you remove all
extra copper?

Regards,
Colin


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