FBGA 256 ultiboard autotrace

[CPU_6502]

I am making a PCB with an altered Cyclone IV with FBGA256 as a footprint.

My PCB maker does not fall below 4/4 mils (width / space) for tracks and so I set these parameters for my tracks on the PCB.

However, the autoroute function, fails to trace the lines of the FPGA, forces me to go down to even lower values.
Can I change some other parameter to try to make the utotrace work well?

[alamanna]

Hi,
in the following exhaustive tutorial

Best Practices in Printed Circuit Board Design
http://www.ni.com/tutorial/6894/en/ 

there is the link to the document

Best Practices in PCB Design: Routing
http://www.ni.com/tutorial/6880/en/ 

Moreover, in the section of the Ultiboard online help about Autorouting,
http://zone.ni.com/reference/en-XX/help/372065N-01/TOC60.htm 

you might find useful the following page:

Strategies to Achieve Better Routing Results
http://zone.ni.com/reference/en-XX/help/372065N-01/ultiboard/strategiestoachievebetterroutingresults/ 

Regards,
Alessia