Autorouting - FAQs

10 Key Points to remember using an Autorouter...

Users of autorouters ideally want a tool that does everything perfectly for them. The router has do to it quickly without leaving much for the user to cleanup. When you dig deeper into what the user wants you discover that what they really need is a tool which can be easily directed by the designer in a variety of modes allowing the designer to control the design completion process. This becomes crucial as designs increase in density and congestion. The McCAD Trailblazer autorouter offers the designer both unattended routing as well as fully interactive routing in a variety of modes, thus making it possible to tackle the simple and complex routing problems with one easy to control software tool.

These suggestions which follow apply to all autorouters however designers can significantly improve their results with Trailblazer by observing the following key points:

  1. Placement:
    Enough can not be said about how important placement is. This can make or break the design. Designer’s need to position and orient components within the mandatory constraints of the physical requirements as best as they can and in such a way as to minimize crossovers as well as localized congestion. When you are done with placement, can you answer “YES” to “Could I route this layout manually using the constraints that will be imposed on the autorouter?” If the answer is “NO” then in all likelihood neither will the autorouter.
  2. Select an appropriate strategy:
    Though the standard default strategy will handle better than 95% of your designs, there are more than 15 other strategies which you can select to handle the more rare and unique situations. If none of these are ideally suited you can select the one which is closest to your needs and then fine tune the particular control aspects. Selecting or fine tuning the strategy will determine how much cleanup by the designer, if any, may be required.
  3. Maximize board Utilization:
    Set a default grid/spacing/trackwidth combination which is suitable for your project and which will maximize available interconnect pathways. For example if you specify a routing grind of 8 1/3 mils, and use track widths of 8 mils, and spacing of 8 mils, the resulting maximum attainable board utilization figure would be 8 + 8 + 8 = 24 mils channel requirement versus the router's selected channel width multiple of 8 1/3 x 3 = 25mils. This results in upto 24/25 => 96% utilization of available board space. The higher the utilization figure the more likely you will get 100% completeion by an autorouter.
  4. Locate impossible spacing constraints:
    Perform a DRC check, looking for pin to pin spacing constraints which may prevent proper entry or exit of signal traces from/to these pins. Depending upon the design, you may have to perform local routing using a different routing setup for those specific areas only.
  5. Set the direction of routing:
    Set the direction of routing to move from a dense area towards a less dense area. This is what a designer would do if he were manually routing the board. For example connectors are usually dense areas. Routing away from the connector is usually a much easier task than vice versa. Following this practice will reduce or eliminate route blockage.
  6. Clear the congested areas first:
    If the design has a number of highly congested areas, they should be cleared first through the use of restricted route area windows. You want to push these congested areas out into the less congested areas. Doing so will avoid potential route blockages when routing from the less congested area into a very congested area.
  7. Change the setup or layout:
    If the router boggs down, this usually is a sign of some layout issue dealing with spacing constraint which is making the completion of an interconnect difficult or impossible. Stop the router and examine the area in question and adjust strategies or spacing constraints as necessary to resume effective routing.
  8. Use pre-routed devices when appropriate:
    In some cases where you have extreme densities and difficult escapement paths such as with PGAs or BGAs, prerouting a specific fan-out pattern will dramatically improve completion results.
  9. Spend extra time in net setup:
    Though most designs do not require this additional time before routing, very complex designs, in excess of 50,000 connections and higher, will have substantially improved results when nets are assigned a specific order of routing as well as being assigned to specific routing layers. In one specific instance, an end user reduced his route time from 27 hours to 2 hrs and raised his completion level to a point where he only had 30 partial unroutes left in a design that had more than 180,000 unrouted connections at the start. He was then able to finish the routing using Trailblazer’s interactive mode.
  10. Use the interactive routing mode to finish:
    Any good autorouting tool will provide you with an interactive mode where the designer can complete any routes that the autorouter failed to complete on it’s own. In this operational mode you can push and shove traces around as you guide the autorouter in placing the final routes. While doing this the autorouter will move and carefully space the shoved traces as you lay-in the final paths.