Re: gEDA-user: PCB: Rat's nest to best routing... strategies ?

[DJ Delorie <dj@xxxxxxxxxxx>]

> Does anyone have any strategies for placing components while still
> in the ratsnest stage that will make for a really nice compact auto
> routed board ?  Or is it just trial and error ?  I've always
> wondered this...

I like to think in terms of important signals, and connectors.  Place
the connectors first, if they have a need to be at a specific
location.  Then think of the flow of signals, and place the key parts
next (cpu, memory, interfaces, etc).  I usualy hide all the power nets
for this.  The "nest" should be showing some form.  Sometimes at this
point I go back and rearrange pins to get a cleaner nest (less
crossings, etc).

Next is power.  For a two-layer board, I put power on the bottom and
signals (when possible) on the top.  For a four layer board, signals
are on the outside and power on the inside.  I place and connect all
the bypass caps, crystals, and other length-sensitive nets.

Once power is done, I autoroute and see how bad it is.  Sometimes I'll
undo and manually route a few key nets (ones that should have been
direct, but ended up all over the place) and try again.

After I'm happy with the autoroute results, I optimize, then go
through and manually "neaten" up the board.  This means moving traces
across pins/vias, making them land directly on pins, etc.  I also
double check footprints and hole sizes at this point, using a 1:1
printout of the board.

Then I apply teardrops, often undoing and re-fixing traces that don't
land centered on pads, and send it out to be made.



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