Net classes are one of those features of EAGLE that are not obvious to find and comprehend. However, they are useful in allowing you to assign certain rules to a group of nets. For example, in order to carry high current for power nets, you’ll need your nets to be extra thick. You can create a net class called POWER and then set the width of that class to be larger than the minimum width of the DRC (Figures 1 and 2).
There are a few details to keep in mind here. First, the default net class should always be set to all zeros. The reason for this is that the net classes and the DRC compete with each other, the larger of the two will always win. By default, you want all of your traces to obey DRC and that’s why the default class values are all set to zero. Whatever values you enter in the DRC will be larger than zero. Second, when you define a net class you are defining minimums. In figure 2, the power net class has a width value of 45 mils. If you draw a 50 mil trace on the board, EAGLE will still be happy because the net class defines a minimum. Finally, by clicking on the >> button you can define the specific clearance between netclasses (Figure 3).
Now you can go to the various nets of your design and assign them to the power net class. To do this, you click on the INFO icon and then click on the net (Figure 4). You can also assign net classes on the board using the same procedure. This affords a few advantages, first the DRC can now report errors on power traces that are thinner than the net class width you specified. Second, the autorouter takes these properties into account and it will route power nets using the thicker width automatically.
Net classes allow you to easily assign uniform characteristics to a group of signals in your design. In doing so you can guarantee that your traces are thick enough to carry the current in your design or that there is enough clearance to avoid noise coupling into sensitive sensor lines. They force you to consider what are the critical signals in your design. In turn. that exercise guides you on the path to a successful layout.
After this brief introduction, we are sure that you now know net classes and where you can apply them. Stay tuned for our next post in the “What you didn’t know about EAGLE” series.
CadSoft Support Team