Chapter 7
Autorouting and Autoplacement
©
National Instruments Corporation
7-17
Strategies to Achieve Better Routing Results
Speed and completion may be trade-offs.
Many of the suggestions for
routing completion will greatly increase the time needed to complete
routing. While it may be tempting to try to guarantee completion by using
all possible completion tricks for all layouts, many non-dense boards can
be completely routed without them, and they will be routed much faster.
Set alternating layer biases.
By default, each new Ultiboard project will
have trace biases that alternate between horizontal and vertical with each
successive layer. This setting greatly increases routing speed and
completion rates and should not normally be changed. The exception is if
you have placed several powerplanes. Because the router does not route
traces on powerplane layers, these layer biases may need to be adjusted to
preserve this alternating pattern so that it skips over those powerplane
layers.
Try gridless routing.
The default setting for routing is grid-based, which
is the fastest routing algorithm. If this fails to achieve completion, then next
step should almost always be to try gridless routing.
Make sure there is no via grid.
The via grid increases routing speed by
limiting the costly via placement calculations, but it can decrease routing
completion since vias may not be placed in crowded areas of the board.
Try auto adjust trace width.
If your design permits, set a smaller
minimum width for some nets and turn on the “Auto Adjust Trace Width”
feature.
Check your minimum/maximum lengths.
Sometimes traces may not be
routed because the pins are too far away to be routed with your specified
maximum length, or too close together to be routed with your specified
minimum length (the router can only increase the length of a trace by about
sixty percent over optimum). Similar routing failures may occur with
differential pairs when one leg of the pair is much longer than the other. In
these cases, you should either change your minimum or maximum length
settings, or route these problematic connections with Ultiboard’s
follow-me router to achieve the desired length.
