Keep in mind that the stack of components that have just been assembled are vulnerable to misalignment and oxidation. We have done some work up-front to provide Package-on-Package solder pastes and fluxes that have sufficient tack to hold components in place during typical conditions, and flux that will protect the solder from oxidation during transport and reflow. Automated conveyors are suggested, but if you are handling the stacked packages be careful. It is up to you to make sure the assemblies are not mishandled.