Thanks for all the replies guys. I realise I was not very detailed in my
post, preferring to pose the question in general terms. The actual circuit I
have is as follows. 1 L298 H-bridge driving a stepper motor. The driver is
controlled by an L297 stepper motor controller. This generates the phase
sequence & PWM for the driver. The 'Ground' end of the H-bridge is connected
to ground through a current sense resistor. The net connecting the H-bridge
to the current sense resistor is the 3mm wide one. The motor current passes
through this net and the sense resistor to ground, hence the need for a wide
trace. The purpose for the sense resistor is to give current feedback to the
PWM generator. Routing the wide traces back to the L297 for feedback is not
really an option. The sense input on the generator is an opamp input. I
don't know if it is bipolar or not. Given that only short distances (>6cm
total for the line from sense resistor to the opamp input) are involved I
can use a thinner trace (0.35mm). There is no separate ground sense line.
This configuration has worked fine for me in the past.

When I route a board I usually setup the width constraint for each net,
autoroute the board and then manually cleanup. In this case the auto router
will route the net using the 3mm thickness throughout. If the part of the
net which did not have to be so wide could have its width defined seperatly,
then the autorouter would route differently and probably better. The virtual
short thing sounds interesting & I can see how it works. I'm less sure how
the from-to width rule works never having used it.

Thanks for all the feedback.


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