It has been a year or so since I worked on this issue, but if memory serves
me correctly the max is 3 mils.  We had an issue with that and contacted
the manufacturer of the component to get that information.  Any 3 balls
dictate a plane, so the highest (largest 3 balls) will dictate your plane,
then the max that any others can be from that plane is 3 mils.  We actually
had some that were 6-7 mils from that plane.  You may want to call the
manufacturer of the BGA and see what they use for a spec.  One of the
reasons we had such large coplanarity problems was due to the part
absorbing moisture and delaminating in the middle causing the corners to
appear curled up.  Thus the corner balls were the worst with 6-10 mils from
planer.

Brad Kendall
Hella Electronics

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