> -  No more than 5 balls per square inch allowed
> -  Solder balls cannot be greater than 5 mil (0.005in)
> -  Solder balls cannot be less than 5 mil from the nearest land or trace

This sounds a bit arbitrary so if you want a neat metric rule why not:

-  No more than 1 ball per 100mm^2
-  Solder balls cannot be greater than 100um
-  Solder balls cannot be less than 100um from the nearest land or trace

David Whalley