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Design rules are rules set in your PCB CAD software that will do one or more of two things. First, they set the limits of the capabilities of the PCB fabrication company you'll use to make your blank PC boards. For example, if your fab house has a minimum trace and space of 5 mils, your design rules, and a design rule check, will ensure that none of your traces or spaces are too small for their processes.
Next, they can be used for rules-based design. Some software will automatically size traces based on the design rules you set ahead of time. With this type of software, if you want all of your power and ground traces to be 16 mils, you set a net class with a width of 16 mils, and assign the power and ground nets to that net class. The software would automatically make any power and ground nets 16 mils wide.
Your design rules will be a crossover between what you need and what your fab house can produce. Some more primitive CAD software will require that you manually pick the width for a via or copper trace, if you want it to differ from the default. Other software, as noted above, will let you set up a series of design rules, and will pick widths based on those rules.
For example, you might specify that power and ground traces are 16 mils in width and standard signal traces are 8 mils wide. The CAD system would then default to those widths when you picked a net in that net class. The net is the connection, like from Pin 7 of chip U2, to Pin 1 of capacitor C3. The net class would be the type of net, like power or ground, or signal.
You’ll need to refer to your fab house, both to ensure that they can build what you need, and to get the final say on their design rules. Some fab houses even have downloadable design rules for various CAD packages. That makes setting things up a lot easier.
Duane Benson is the Chief Technology Champion at Screaming Circuits, a prototype PCB assembly electronic manufacturing company in Canby, Oregon.