Constraints for fanout

Hi, 

I want to create some rules for Allegro PCB when it creates fanout for my BGA. I have defined two types of VIAs and I want to force Allegro to use one VIA - and thick track width -  for Power& GND pads and other VIA - with thinner track width - for signals. Where can I define such rules?

Any suggestions are welcome.

Hossein