EAGLE Autorouter Exploration

Most people who get into PCB design will automatically be intrigued by the concept of an autorouter.  It would make sense to the outside observer that an algorithm would be able to process millions of iterations of routings that finds the best balance of short traces, few vias, and well-organized traces. Similar to cracking a random password – it is only a question of time before the 'correct' layout solution is found.

According to the reviews of some experienced CAD junkies, most autorouters are very seldom helpful.  What's more, the cases that autorouters are recognized as useful belong only to those who have laid out countless boards and are able to amplify that expertise via the tool.  These reviews would make an autorouter seem like a specialized space drill: It can be used by novices on earth, but its not the best tool for a 'normal' job.

But to hell with reviews. It is true that an autorouter will totally ruin circuits like switchers, high-speed or high-current busses, sensitive analog circuits, etc...  However there is a lot of value in trying tools out first-hand.  This article will help readers to tinker with EAGLE's autorouter and decide for themselves if it is a useless tool or not.  Cadsoft includes it with the free version of EAGLE, so there's no reason not to take a swing or two!

Step 1: Start with a board on which part placement is complete.

Note that part placement is much more important than the routing so doing a stellar job here will pay dividends.  Shame on anyone who throws parts down and thinks, 'Ah, the autorouter can figure it out.'  If there are critical routes like switchers, sensistive analog paths, or high speed sections it might be a good idea to route those by hand now to make sure they are done correctly.

Step 2: Set the Design Rule Check (DRC) rules.

Generally, DRC rules should be based on those set by the specs of the board house (example) and assembly house(example).  It is best to add a buffer if possible to limit the chance of errors, for instance specifying 8 mil route and 8 mil space for a board house that is capable of 6 mil route/space is good practice.  EAGLE helps users understand each specification by showing an image of the dimension being edited in the window.

Step 3:  Setup the Autorouter.

When a user presses the 'Autoroute' button, a multi-tab window appears.  For the first pass, only the 'General' tab need considered.  The preffered routing direction must be set; note that '*' indicates no preference while selecting N/A will result in that layer not being routed at all.  The routing grid is also a significant consideration. It needs to be set small enough so all component pads can be resolved, but setting it too low drastically increases the required routing time  (and computer hardware requirements).  All other tabs should be left as default for this first autorouting experiment.

Step 4: Watch it run!

Feel free to laugh at the crazy routes it thinks up.  Sometimes it changes it's mind!

Step 5: Finish the job and double check

There will undoubtedly be silly things that the autorouter does, or one may notice some corrections are required (fattening a trace, preventing an unnecessarily long trace, etc...).   Remember, it isn't electronic design logic that laid this board out; a standard algorithm is what decided how to run the routes. Sometimes the autorouter cannot complete all traces, perhaps because of the routing grid setting mentioned earlier.  It is generally easiest to make the completions and corrections by hand instead of attempting to teach the autorouter to do it.

Here there are a couple incomplete routes that were not completed by the autorouter.

The board should be completed!  How is the quality of the layout?  Including setup, troubleshooting, and manual checking, was there a significant time savings?  Was it an overall better solution for the design?  For users that have experience with other autorouters, what have you noticed? Any favorite tools and/or features? Let the community know in the comments!

Obviously, this is only an introduction to the autorouter.  For those that want to have a complete picture of the tool, EAGLE's user manual has some excellent information on page 187, including the settings that were not discussed here.