Stacking Package-on-Package components is one of the easiest parts of the assembly process. Although each Package-on-Package solder paste and Package-on-Package flux is different, I have not found any issues using up to 20psi of pressure during placement of a second component. (Please let me know if anyone out there has had problems with higher pressure stacking.) Basically stacking placement is just like normal SMT placement. Just be sure your placement machine will tolerate the slight change in height.