I’m using Eagle right now, but I’m sure my question is not Eagle-specific. When using an autorouter, what are the guidelines/tradeoffs in selecting a value for the routing grid?
Mike
When using autorouters the grid should be set quite small. I use 5 mil for the Pulsonix autorouter, as a rule, and it does a very good job. The Eagle autorouter is rubbish, BTW.
Leon
If you have a low density board (mainly DIPs), then a 25-mil grid may work. Otherwise, I'd go for a 10-mil grid, or possibly a 5-mil grid. Basically, try different grid and see what works best for your board. I've noticed that EAGLE's auto-router (AR) leaves a bunch of wiggly lines when the grid is too small. It looks like it "changes its mind" quite a bit with small grid settings. I've gotten tired of cleaning up after EAGLE's AR, so I rarely use it anymore. If nothing else, running EAGLE's AR will give you some ideas of how to manually run traces.snarflemike:
I’m using Eagle right now, but I’m sure my question is not Eagle-specific. When using an autorouter, what are the guidelines/tradeoffs in selecting a value for the routing grid?Mike
I think that most people “know” that EAGLE’s AR is lousy. There is an on-line AR that works with EAGLE and is much better, but I can’t remember the URL (sorry). I’ve played with a demo of Pulsonix and was really impressed, especially with its AR. I don’t have the money for Pulsonix ($3K - $5K) or enough freelance work to justify buying it, so I’m sticking with EAGLE for now.
Pulsonix actually uses the Electra autorouter:
It is available for Eagle. It might look expensive, but it’s much cheaper than other autorouters.
Leon
Try routing with a 5mil grid. I also find that increasing the “cost” of vias makes for a much better auto-route. That being said, I never auto route. Period.
Try the autorouter, then rip it up and try yourself. Practice and you will get better 
A 5mil grid is ok, but just make sure the board house you are working with accepts that.
Gold Phoenix is a min 7 mil grid, unless you want to pay more. So, by default, I route everything at a 7 mil grid to make sure I meet that requirement.
I don’t understand how the routing grid is relevant to the board supplier.
Leon
leon_heller:
I don’t understand how the routing grid is relevant to the board supplier.
By setting the routing grid less than the min spacing allowed by the PCB manu, there is the chance for two traces to either be spaced below this min clearance or the clearance with another part/pad could be less. Thus causing it to fail the min requirement. Such as GP’s 7 mil min.
Its just a fail-safe thing I do to prevent it.
The software I use won’t let me make errors like that.
Leon
leon_heller:
I don’t understand how the routing grid is relevant to the board supplier.Leon
I am pretty sure your right there. I have used a 5mil grid in the past many times on boards with 7 or 8 mil clearance. EAGLE is smart enough to not violate the DRC when autorouting.leon_heller:
The software I use won’t let me make errors like that.Leon
Wouldn’t a 7mil grid violate a 7mil space requirement if it didn’t know what it’s doing? 7mil grid would mean that the traces were spaced centre to centre 7mils apart, making the space between smaller.
Even a 1 mil routing grid won’t violate the 7 mil track spacing requirement. The track spacing will always override it.
Leon
If you load the .dru file before you run the autorouter (or specify the requirements in the design rule check before you run the auto router), Eagle won’t violate those requirements. You could autoroute with a 1 mil grid and the results should still pass the design rule check when you’re done.