Circuit Board Design: Auto-routing in Altium

You've finished and double checked your schematic, all the footprints have been loaded or created, and you've laid out all of your components on your board. Now it is time to route the traces.

Traditionally, board designers route traces by hand giving them complete control over everything. However, there definitely are times and places for the auto-routing tool.

When to Use Auto-Routing Tools

• Simple board designs with fewer components
• Situations where impedance matching and trace lengths are not a concern
• Beta/prototype board designs when components may be frequently changed or added in the future

We've had some fun experiences pushing Altium Designer's auto-routing tools to the limit on more simple board designs. The biggest advantage to auto-routing is the ability to add a new component on the schematic, import the changes to the PCB, click auto-route and *snap* you are done. Your new board design is ready (of course design rule checks and further scrutiny are required before submitting for fabrication).

Auto-Routing Tools

The first tools for auto-routing are the design rules and net classes.

Net classes are used to select trace routes to size trace widths for appropriate current. One unfortunate thing about net classes is that the trace width is set for the entire net. If I want my trace widths for VCC between my power source and my processor to be able to handle a higher current than my VCC trace widths to ICs, net classes alone will not work.

There are several options to get around this problem. Creating design rules for rooms and setting up rooms so that different widths on the same net are in different rooms is a common solution. The simplest may be to auto-route as much as you can and then go back and manually change widths on some areas.

Creating a fake net-class connector component to split a net into two nets is an option, although it can be cumbersome and also will probably require turning some design rule checks off. I wonder if creative designers have found any other options.

An Altium feature that I would like to see in the future would be the ability to have objects that are sub net classes. The ability set up design rules for these sub net classes would bring auto-routing to the next level.

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