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Eagle CAD

Last update: 31/07/2015

Using the autorouter of Eagle for the realization of a pcb it is really important to establish the size of the tracks. Using the DRC function (on the right side of the menu) we can change a lot of parameters regarding the layers, the path, the distance between tracks, the shapes and so on, but unfortunately these parameters regard the whole pcb.

Very often, almost always we can say, in a single pcb tracks must have a different size. While could be normal, for a signal track, a size of 0.3-0.4 mm for instance, in many cases the size of the tracks must be increased a lot, particularly for power supply lines or for any other track that needs a wider shape for instance.

The problem can be really easily solved using the "net classes" function of  Eagle: here follows a sample, using a simple schematics:

  We are going to realize a pcb according to the following schematic: a simple voltage regulator, just to give an idea.
  ... and this is the simple component layout, the pcb we are going to realize.
  Pressing the autorouter button all tracks are automaticcaly generated according to the size established on the DRC menu (track size, drill size, distances etc.). The DRC menu allows to change the size of the track but in this case the new rule applies to the whole pcb. Very often we need to create only few tracks with a size different from the others: normally a 0.3-0.4 mm size could be perfect for a signal track but could be considered too small to provide all current necessary to the circuitery. To change the track size individually we have to select on the menu: edit - net classes

We will create a new class for the track simply giving the new attributes: name and size (if '0' value is left the value will be accordingly the DRC menu). In this case we create a new class: name - Ground (any name is ok) size - 24 mils.

Cliking now with the right button of the mouse on the schematic on the wire we wish to change the size ...

  ... we will assign to the net GND the net class nr1: Ground

Clicking on the autorouter now all the track relative to the net 'GND' will have a size of 24 mils.

this is a really important matter: if tracks are drawn using the autorouter, it is really important to establish the right size of the tracks, particularly for the supply nets.

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