Chris, There are a lot of potential causes for voids. I'll list a few below: a.) cleanliness (or lack there of) b.) Via in pad where said vias are not filled c.) reflow profile design d.) incompatible materials
My knee jerk reaction would be to check the reflow profile. My personal opinion about voiding is that it tends to come from flux in your solder paste that hasn't had a chance to fully volatize prior to reflow. If you're using a "straight ramp" style profile, you may wish to consider using a soak profile. If you're already using a soak profile, try to extend your soak time. Your solder paste manufacturer should be able to give you an idea of what the maximum time soaking should be. Try to push your profile to their recommended limit. Maximizing the time in soak tends to have the greatest impact on driving off materials that will cause voids when the alloy changes to a liquid state. I'm not by any means saying that this is a "fix all" but it's a good place to start.