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Auto-Route versus Auto-Interactive Routing--Which Is Better?

Key Takeaways

  • How automated routing can help you with your PCB design.

  • The details of both standard auto-routing and auto-interactive routing.

  • Which type of routing will be better for your design?


3D rendering of a circuit board with tight routing

A circuit board with some tight routing.


One of the keys to making workflow more efficient is to incorporate automation into the process. Perhaps the greatest example of this is in the development and manufacturing of the Ford Model T. It is said that it took 13 hours to assemble the car when it first went into production, but within five years, they were turning out a new Model T every 90 seconds. The assembly line wasn’t a new concept, but Henry Ford’s refinement of it proved to the world the benefit of an automated workflow.

While automation has proved itself over and over again as a mechanism to increase efficiency on the production line, it is also important on the developmental side of a product. Some engineering processes have become so complex that it is no longer profitable for a human operator to do the work. Here is where automation can truly shine by doing these complex calculations and operations quickly and without error.

In the world of printed circuit board design, we can see how the benefits of automation reveal themselves in the use of auto-routing technology. What used to take days of painstaking manual routing can now be done in hours with auto-routing tools. There are, however, two ways to approach PCB design auto-routing: either with standard auto-routing or with auto-interactive routing. We’ll examine both auto-route versus auto-interactive routing here to see which method will be the better fit for your design.

How Automated Routing Can Help Your PCB Design

There is no doubt about it, routing traces on a printed circuit board can be a lot of fun. Running a trace from pin to pin while avoiding obstacles and bouncing between layers can be like playing a video game. If you like to solve puzzles, then being a PCB layout engineer will be right up your alley. However, the fun diminishes when you have to route thousands of traces in specific patterns and lengths all while obeying very detailed design rules. Here is where the designer will need a little help, and that help will come from a PCB auto-router.

The most obvious benefit of using an auto-router on your PCB design will be to increase the speed at which you can complete all the trace routing. Just as important, though, is that the router will be able to quickly calculate and handle many scenarios that would slow down a designer trying to route it manually. Auto-routers also have the ability to work with the embedded design rules from the layout database, which saves time, as well as having the ability to set up their own rules as needed. They can also accomplish many other tasks as needed, such as mitering trace corners, glossing the routing, and other trace routing clean-up tasks.

There are two types of auto-routing that are available for PCB designers to use. The first is regular auto-routing, which although bound by the design rules that you set up, is directed and controlled completely by the router itself. The second is interactive auto-routing, which gives the designer control over how the traces are directed on the board. In the picture below, you can see the difference between the two with the results from a standard auto-router on the top and the auto-interactive router on the bottom. Let’s explore these two routers, starting with the standard PCB auto-router.

PCB routing software showing the differences between auto-routing and auto-interactive routing

The top images are the result of a PCB auto-router, while the bottom is from an auto-interactive router.

Some Details about Auto-Routing Technology

Auto-routers that can route the entire board for you are known as batch routers. These tools have been around for a long time and their capabilities have grown and been enhanced over time. While it used to be true that the final results of a batch auto-router were more like black-box magic than anything else, that has changed and the routing is now predictable and repeatable. These routers are fully-configurable with multiple options to give you the results that you are looking for.

Most batch auto-routers today are incorporated directly into the PCB layout design tools that they are associated with. This allows them to easily work with the design rules and constraints that are already set up for the board. While this saves a lot of time for the designer, the routers also have the capability for the designer to change those rules, as shown in the picture below. In addition, the routers can be configured for how they will run and what type of routing they will do. Some of these options include:

  • Pre Route: Only the escape traces and vias from surface mount components are routed with this option.

  • Route: This includes fully batch-routing the entire board or only routing selected nets by running several passes of the auto-router to complete the routes.

  • Clean: This removes the loops and stubs that can be leftover from the auto-routing process.

  • Post Route: This option is used to incorporate additional routing features such as shielding or test points.

Auto-routing is a great tool to use and can save you a lot of time, but you need to be aware that the results will essentially be orthogonal in their patterns. While this is helpful with some types of nets, it isn’t necessarily the best approach to routing a tight bus pattern of traces for memory routing. For that, we will look at the auto-interactive router.

Cadence Allegro’s PCB router’s rules menu

A modern PCB auto-router has a lot of rules and properties that can be set up for the design.

How Is Auto-Interactive Routing Different?

Auto-interactive routing still uses the power of an advanced auto-routing engine but combines it with the skill of the PCB layout designer to direct the path of the routing. This is done by giving a virtual canvas for the designer to work in where they can set up the parameters and direction of the routing to be done. Cadence’s Allegro PCB Designer is a good example of how auto-interactive routing works in printed circuit board design:

  • First, the nets are organized into bundles so that they can be manipulated as a group in the design. With the constraint manager in Allegro, the nets can easily be selected by their groups and assigned to a bundle from either the schematic or layout

  • Once the bundle is created, routing properties can be attached to it, pins can be swapped, and routing layers adjusted as necessary, as shown in the picture below.

  • Next, the bundle is maneuvered as needed to achieve the desired routing flow by the designer. This allows the designer to navigate the bundle around areas of conflict such as connectors or tight component placements.

  • Once the bundle is in place, the designer has the ability to straighten out twisted connections going into pins. This is done by readjusting the position of the bundle, swapping pins, or changing layers.

  • Finally, the bundle is converted into regular etched traces to be part of the finished design database.

Auto-interactive routing gives the designer the ability to manually create the tight routing patterns that are necessary for DDR or other advanced routing technologies, yet with the precision and speed of automated routing. In addition, auto-interactive routers can be used for high-speed trace tuning and other routing cleanup tasks. Now, the question is, which type of auto-routing is better for you?

Routing controls for bundles for auto-interactive routing in Allegro PCB Designer

These bundles of nets can have properties attached to them and be maneuvered on the design as needed.

Auto-Route versus Auto-Interactive Routing--Which Is Better?

The truth is, both types of auto-routers are important depending on what type of routing that you need to have done. As we just saw, routing a DDR memory bus is much more efficient when using auto-interactive routing. However, when the DDR routing is completed and there are a lot of loose wires left to connect, the regular batch router will become your best friend--especially when it comes time to clean-up the routing. It is also very convenient to use the standard router to quickly punch in point-to-point routing and save you from a lot of tedious manual routing work.

No matter what you are doing, it will pay in the end to spend some time with the auto-routers in your PCB design system in order to become familiar with them. This will allow you to more efficiently use the best router for the task that is before you. And as we have seen, Allegro PCB Designer offers some of the best auto-routing technology currently available.

You can read more about routing printed circuit boards from our E-book on routing.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts