i think Lloyd's assessment here is pretty spot on

the router prefers an empty board, depending on your level of fussiness
it may not be suitable for your work

Lloyd Johnson wrote:
> 
> In my experience the autorouter works well when starting from scratch 2 or 4
> layer boards. You have to get the design rules right for the density you are
> hoping to accomplish.
> 
> I have not had good luck invoking the auto-router to just route a few new
> added parts to an existing high density design. So I have made small
> incremental changes to the design manually. If I had to do a major makover,
> like 25% or more of the design, I would probably save the old design and
> then delete all existing tracks and vias and let the autor-router start over
> from scratch. It is very fast. After the first auto-route, look for any high
> density areas and move the parts around a bit to make a little more room in
> that area, then start the router over from scratch again.
> 
> Don't expect the auto-router to route every route the way you would do it.
> Most of what is called "clean-up" is cosmetic but stupid looking tracks.
> 
> I use 8 mil lines and spaces with 30 mil vias with 15 mil holes in vias.
> High density surface mount 4 layer boards. Usually on 6 layer designs 2 or
> more layers are just ground planes anyway. P99SE SP5 on Win 98SE no crashes
> for 1 year.
> 
> Good Luck,
> Lloyd Johnson
> DSS
> 


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