User Guide

7.4 WhatHastobeDefinedBefore
Autorouting
DesignRules
The Design Rules need to be specified in accordance with the complex
-
ity of the board and of the manufacturing facilities available. You will
find a description of the procedure and of the meanings of the individual
parametersinthesectionon SpecifyingtheDesignRules onpage 89.
TrackWidthandNetClasses
If you have not already defined various net classes in the schematic dia
-
gram you now have the opportunity, before running the Autorouter, of
specifying whether particular signals are to be laid using special track
widths, particular clearances are to be observed, or whether certain
diameters are to be used for vias for particular signals. Please consult the
help pages (CLASS command) or the section on Specifying Net Classes
onpage 87 forinformationaboutthedefinitionofnetclasses.
If no special net classes are defined, the values from the Design Rules
apply. The value Minimum width in the Sizes tab determines the track
width, the values for minimum clearances/distances are taken from the
Clearance and Distance tabs. The diameter of vias is defined by the va
-
luesinthe Restring tab.
Grid
The Design Rules determine the routing and placement grid. The mini
-
mumroutinggridis0.02mm,whichisabout0.8mil.
Placement Grid
Although the Autorouter does permit any placement grid, it is not a
good idea to place the components on a grid that is too fine. Two good
rulesare:
•
Theplacementgridshouldnotbefinerthantheroutinggrid.
•
Iftheplacementgridislargerthantheroutinggrid,itshouldbe
settoanintegralmultipleoftheroutinggrid.
These rules make sense if, for example, you consider that it might be
possible, within the Design Rules, to route two tracks between two pins
of a component, but that an inappropriate relationship between the two
gridscouldpreventthis(seediagram).
120
EAGLEManual