Autorouting Artwork

For most PCB designers who started their careers back in the days of Bishop tape and Exacto knives, the finished tape-up was more than just a job completed, it was a work of art.  Most designers took great pride in the way their boards looked.  To some extent, this pride in the finished product was  lost with the advent of CAD, and declined even further with the increased use of autorouters.  Autorouters tend to do an esthetically poor routing job, but  they do it in quick time.  While the pressure to get that electronic product to market in the shortest possible time justifies the use of autorouters, the esthetic appearance of the finished board need not necessarily be compromised.

Many of today's PCB designers know how to use autorouters only from the standpoint of procedure; they place the components, set a few simple parameters for number of board layers, routing direction on various layers, and similar basic requirements, then hit the "GO" button and go for lunch.  When they come back, they spend a little time in cleanup, then do Gerber plots. Done! The results are quick but messy.

However, there are a few tricks that one can use even with the most  basic autorouters that will not only produce a quick routing job, but also one that requires minimum cleanup, and yet is quite neat and organized in appearance.  An added bonus is that a neatly laid out design takes less space and performs better electrically due to shorter traces and reduced EMI.  So what can you do with your average autorouter to get a better design?

Use your autorouter to evaluate your placement.  First, go through the component placement in the usual manner, the same method that you have been using to give it your best shot.  Now set your autorouter loose on that placement and with the least amount of constraints, let it finish the routing to completion, or until it gives up.  Evaluate the tight spots, where the autorouter could not complete the connections.  Improve the placement if at all possible.  Now delete all previous routes and start fresh.  Remember, at this point, we are not concerned with via minimization or any kind of cleanup. This is the "quick and dirty" phase.  The idea is to achieve the highest percentage of completion in order to confirm that the component placement is efficient.  You may need to repeat this phase several times, until the numbers start looking acceptable. Since you are not doing any post-routing cleanup, the whole process takes a relatively short time and the end results will justify spending that time.

Once you are happy with the placement, it's time to start routing for "real".  Delete all previous routes, and if necessary, manually preroute any critical lines.  Fix these lines so that the router will not move them or rip them up and re-route them.  Now set the autorouter to do the first routing pass with maximum constraints; no vias, straight line connections only, and only in the preferred routing directions. The actual parameters will vary from one router to the next, of course, but the idea is the same.  On each subsequent pass, the constraints are gradually relaxed.  On early routing passes, you might want to set the number of vertices or bends in any given route, as well as the deviation from the preferred direction, if you have the control.  Then you can ease the restriction on the number of vias per route.  It often helps if you can do a cleanup pass after each change in routing strategy. A cleanup pass at this stage will improve routing results by yielding cleaner routes with less vias.  Continue this process until the router completes the job.  If you were able to get 100% completion at the "quick and dirty " phase, a 100% completion at this stage is guaranteed and you will be surprised at the final results.  They will be far neater, organized and efficient than what you have been getting from that same router before.  As always, run a cleanup and via minimization after the routing is complete.

If you find that this method does yield better results with your router, you may want to explore the possibility of setting up scripts, batch files, or routing strategy parameter files, that will enable you to repeat the routing procedure on future jobs.  With a little experience, trial and error, you will be turning out autorouted boards that you can be proud of.

Nandor Kerek leads PCB layout seminars and can
be reached at Systems 80 Tel:(604)596-2655