Why PCB Design Auto-Interactive Routing is Not a PCB Autorouter

Created: October 2, 2017
Updated: December 25, 2020

Picture of an AT-6 in flight
Editorial credit: Santiparp Wattanaporn / Shutterstock.com

A while back I was given the gift of a flight in an AT-6, a WWII era airplane used for training fighter pilots. There couldn’t have been a better gift for an aviation enthusiast, and I spent the next 5 months eagerly anticipating it. Finally, the day arrived and I couldn’t have been happier as we took off into the wild blue yonder. Then the pilot demonstrated an aileron roll, and my Top Gun expectations were crushed as I spent the rest of the flight with an airsick bag held up to my face. I was so disappointed to find out that I didn’t have the stomach for high-speed acrobatics.

I felt the same kind of disappointment the first time I used an autorouter on a PCB design. I expected that the PCB autorouter was going to route my PCB design with the same proficiency that I would. Unfortunately, when I got my design back from the router it looked terrible. Even though the autorouter had completed the routing, it was going to take hours or days doing manual clean-up to get it presentable.

Recently, however, auto-interactive routing technology has made it possible to take the power of automatic routing and put it into the hands of the designer. Auto-interactive routing is different from autorouting and is in many ways superior to it. Before we get to the benefits of auto-interactive routing, let’s look at the basic differences between an autorouter and an auto-interactive router.

Auto-interactive routers and PCB autorouters, what’s the difference?

They may sound similar, but in actuality autorouters and auto-interactive routers are as different as night and day. Both are route engines, of course, but the autorouter tries to do the thinking for you whereas the auto-interactive router lets you be in charge.

Autorouters have been around for a long time as stand-alone applications. Even though they now interface with PCB design software, auto-routers still need their own design rules in order to operate. These rules can be setup manually or imported from the layout software. When engaged, the Printed Board Circuit autorouter attempts to complete routes for all enabled nets in the design. It runs these routes through a series of passes with preset conditions for different routing strategies. When it is finished, the imports the autorouted trace information back into the layout application to replace the existing routing. The amount of usable trace routing that the autorouter produces is totally dependant on the setup of the circuit, however, the results may not look the way you expected them to.

By contrast, the success of the auto-interactive router does not depend on any extra set-ups made by the PCB designer. Instead, since the auto-interactive router is integral to the layout application, it uses the existing design rules. These are the rules that you have already employed for standard manual routing. The commands for an auto-interactive router are also easily accessible from the existing routing menus of the layout tools. The simply selects a net or group of nets for the auto-interactive router to work on and then engages the router. Auto-interactive routers give the user control over the routing as if they were manually routing the board but with the speed of automated routing.

3D motherboard trace routing
Setting up an auto-router can be very complex

Why auto-interactive routing is not autorouting

An autorouter requires a lot of setup to work correctly. Since it is going to do all of the routing for you, you must train it to route the way you want it to. In order to train the auto router, you need to load it with design rules and routing strategies. Although design rules, like net classes and topology constraints, can be imported from the PCB layout software, they may still need to be fine-tuned for peak performance in the auto router. However, the real challenge is in setting up the different auto-routing strategies. These strategies determine how a trace is to be routed and how many attempts should be made before giving up on it. They will also include wrong way routing distances and how many routing cleanup attempts that you want the autorouter to make. Setting up the auto routing strategy is difficult and takes experience to understand how the autorouter will work in different circumstances.

On the other hand, the auto-interactive router gives you the ability to direct the path of the routing without the cumbersome strategy setup steps. Since the auto-interactive router is only going to route the nets that you have selected—instead of the entire design—it doesn’t need the strategies that an autorouter does. Once you select the net or group of nets to be routed, you can engage the auto-interactive router. Here, you have the option to either allow the router to choose its own route path or to follow a path that you manually draw for it as a template. Drawing the template for the route path yourself allows you to direct where the routing will take place while the auto-interactive router does the actual heavy lifting of putting the traces in.

Trace routing on a blue circuit board
Auto-interactive routing can give you uniform routing patterns like these

Auto-interactive routing does not mean your design has been autorouted

Using an auto-interactive router will not produce the kinds of problems that come from using an autorouter. An autorouted board can create a lot of undesirable results. You may see traces with annoying little jogs, corners, and stubs, as well as traces that have wandered all over the board. This happens because the autorouter rips up and retries trace routing in its efforts to complete all routes. The autorouter is also attempting to route the entire board. To accomplish this its focus is on completing those routes instead of creating uniform routing patterns. As a result, bus routing and other routing patterns may be broken up into areas of messy individual trace routing.

The focus of PCB Design vs PCB autorouting is very different. The auto-interactive router will only route the nets that you have selected for routing instead of trying to route the entire board. By only focusing on the selected nets, the auto-interactive router will route traces in tight uniform routing patterns. You also have the ability to specify the path that the selected group of traces will follow. This is a big difference from an autorouter where the routing decisions are made by the router and not the circuit.

Auto-interactive routing gives you the precision of manual routing with the power of a PCB autorouter.

Altium Designer’s ActiveRoute® can give you these auto-interactive benefits. Would you like to find out more about the PCB design routing solutions that Altium Designer offers its users?  Talk to an expert at Altium Designer.


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