Re: gEDA-user: pcb: Track routing strategies and tips

[Colin D Bennett <colin@xxxxxxxxxxx>]

On Tue, 10 May 2011 21:58:57 -0400
gene glick <carzrgr8@xxxxxxxxxxxxx> wrote:

> Kai-Martin posted that placement is more important than routing.  I'd 
> say they are equally important.  The best layout guy in the world
> can't fix a lousy placement.  Bogus layout guys throw more layers at
> the problem.  So yeah, take the time to plan it out before routing.

One problem I have with placement is guessing how far apart to place
components.  If I do the routing and then realize I could shrink the
board, it is really painful to do so since all the traces (lines) will
not scale or move usefully with the components.  So basically if I move
a component I need to then re-route a significant part of all the traces
connected to it.  (It would be fantastic if pcb could adjust traces
dynamically as components are moved.)

> > Does anyone have any tips on how to plan a layout for easy and clean
> > track routing?  In particular for 2-layer boards.
> 
> No substitute for experience here.  But, partitioning the design by
> type may help : analog, digital, low-speed, high-speed.  Try to think 
> beyond blindly connecting the parts.  Sometimes swapping gates,
> adding parts or other strategies become clear as you route.  This is
> a huge benefit when you route your own board.  Layout guys just
> connect the pieces together.

Rather than a strict two-step process of (1) schematic capture and
(2) PCB layout, I have recently found an iterative-design process loop
of "do { edit_schematic(); edit_pcb(); } while (!satisfied);" to be very
helpful, for instance when there is a choice of connector pinout or
microcontroller I/O pin usage.  I often find that if I switch the MCU
I/O pins used for a connection it really cleans up part of the layout.

> > One strategy that I have seen and recently tried is to use the top
> > layer for all horizontal trace runs and the bottom layer for all
> > vertical trace runs, or vice-versa.
> 
> 2-layer is tough.  You also have to account for power and ground.
> The parts themselves also crowd routing area. 2-layer is not
> particularly suitable for high-speed anything.  Seems good for power
> supply design, and some audio work (I've seen a lot of audio ref
> boards on 2 layer). You can make good designs with 2-layer, just is
> more work.  Cost difference to 4-layer is not bad.

At least for DorkbotPDX/pcb.laen.org, 4-layer is double the cost of
2-layer per unit area. However I guess you could do the board in a
smaller area with 4-layer so the final cost would actually be less than
double that of the 2-layer design.

> > Do you ever study other people's PCB designs to learn from them?  
> Yeah, a lot. You will find good and bad.  There's a whole world of 
> opinion out there - and you know what they say about opinions :) 
> SI-LIST is a great place to exchange ideas on layout. Several
> industry experts frequently post.

I am checking out SI-LIST now.  Sounds interesting.

Regards,
Colin


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