Solder joint void limit for BGA components is 12.25 % area for lead free class 2 circuit board assemblies. Most of the voids come from out gassing of the chemicals in the flux. To reduce voids change the chemicals in the formulations that create the out gassing.

The void requirement is that the sum of all of the voids cannot equal more than 25% of the cross-sectional area of the solder joint. Anything higher than 25% should result in a reject.

Voids can be impacted by several factors. In many cases, a modification of the reflow profile can reduce voids significantly. The targeted profile for minimizing voiding would be a long, high soak and a lower peak temperature.

The long, high soak temperature is particularly critical to provide sufficient time for the volatiles in the paste flux to evaporate prior to reflow.

We have also learned that differences in paste characteristics can drive differences in voiding behavior.

If you are experiencing a significant void rate even if with an optimized profile, an evaluation of other paste products may be beneficial.