Autorouting - FAQs
10 Key Points to remember using an Autorouter...
Users of autorouters ideally want a tool that does everything perfectly
for them. The router has do to it quickly without leaving much for
the user to cleanup. When you dig deeper into what the user wants
you discover that what they really need is a tool which can be easily
directed by the designer in a variety of modes allowing the designer
to control the design completion process. This becomes crucial as
designs increase in density and congestion. The McCAD Trailblazer
autorouter offers the designer both unattended routing as well as
fully interactive routing in a variety of modes, thus making it
possible to tackle the simple and complex routing problems with
one easy to control software tool.
These suggestions which follow apply to all autorouters however
designers can significantly improve their results with Trailblazer
by observing the following key points:
Enough can not be said about how important placement is. This
can make or break the design. Designer’s need to position
and orient components within the mandatory constraints of the
physical requirements as best as they can and in such a way as
to minimize crossovers as well as localized congestion. When you
are done with placement, can you answer “YES” to “Could
I route this layout manually using the constraints that will be
imposed on the autorouter?” If the answer is “NO”
then in all likelihood neither will the autorouter.
- Select an appropriate strategy:
Though the standard default strategy will handle better than 95%
of your designs, there are more than 15 other strategies which
you can select to handle the more rare and unique situations.
If none of these are ideally suited you can select the one which
is closest to your needs and then fine tune the particular control
aspects. Selecting or fine tuning the strategy will determine
how much cleanup by the designer, if any, may be required.
- Maximize board Utilization:
Set a default grid/spacing/trackwidth combination which is suitable
for your project and which will maximize available interconnect
pathways. For example if you specify a routing grind of 8 1/3
mils, and use track widths of 8 mils, and spacing of 8 mils, the
resulting maximum attainable board utilization figure would be
8 + 8 + 8 = 24 mils channel requirement versus the router's selected
channel width multiple of 8 1/3 x 3 = 25mils.
This results in upto 24/25 => 96% utilization of available
board space. The higher the utilization figure the more likely
you will get 100% completeion by an autorouter.
- Locate impossible spacing constraints:
Perform a DRC check, looking for pin to pin spacing constraints
which may prevent proper entry or exit of signal traces from/to
these pins. Depending upon the design, you may have to perform
local routing using a different routing setup for those specific
- Set the direction of routing:
Set the direction of routing to move from a dense area towards
a less dense area. This is what a designer would do if he were
manually routing the board. For example connectors are usually
dense areas. Routing away from the connector is usually a much
easier task than vice versa. Following this practice will reduce
or eliminate route blockage.
- Clear the congested areas first:
If the design has a number of highly congested areas, they should
be cleared first through the use of restricted route area windows.
You want to push these congested areas out into the less congested
areas. Doing so will avoid potential route blockages when routing
from the less congested area into a very congested area.
- Change the setup or layout:
If the router boggs down, this usually is a sign of some layout
issue dealing with spacing constraint which is making the completion
of an interconnect difficult or impossible. Stop the router and
examine the area in question and adjust strategies or spacing
constraints as necessary to resume effective routing.
- Use pre-routed devices when appropriate:
In some cases where you have extreme densities and difficult escapement
paths such as with PGAs or BGAs, prerouting a specific fan-out
pattern will dramatically improve completion results.
- Spend extra time in net setup:
Though most designs do not require this additional time before
routing, very complex designs, in excess of 50,000 connections
and higher, will have substantially improved results when nets
are assigned a specific order of routing as well as being assigned
to specific routing layers. In one specific instance, an end user
reduced his route time from 27 hours to 2 hrs and raised his completion
level to a point where he only had 30 partial unroutes left in
a design that had more than 180,000 unrouted connections at the
start. He was then able to finish the routing using Trailblazer’s
- Use the interactive routing mode to finish:
Any good autorouting tool will provide you with an interactive
mode where the designer can complete any routes that the autorouter
failed to complete on it’s own. In this operational mode
you can push and shove traces around as you guide the autorouter
in placing the final routes. While doing this the autorouter will
move and carefully space the shoved traces as you lay-in the final