My understanding is the additional 2% Ag adds tensile strength to the joint as its primary benefit. I do not believe it will affect voiding rates. Some voiding is considered to be beneficial, the question I think is whether current rates of voiding are causing failures or simply concern.

There are systems available that will allow the soldering to be done under a vacuum reducing voids to levels close to zero. However the cost is high and there are added issues that need to be considered.

Changing from Sn63 to Sn62 should have minimal impact on the amount of voiding. The degree of voiding is far more dependent on solder paste flux chemistry and the reflow profile.

Selecting the right solder paste for low voiding can have a huge impact on the amount of voiding since wetting speed and solvent choice in the solder paste can greatly influence the size and amount of voids. If you need specific advice on a low-voiding solder paste, it is suggested to directly contact your solder paste vendor and inquire.

An optimized reflow profile can also reduce voiding. In general, a longer & hotter soak temperature and a lower peak temperature have been found to reduce voiding in most applications. If you need advice regarding your specific reflow profiling challenges, you can contact the technical service staff at your solder paste vendor.