Package-on-Package processing steps can create delays in getting to the reflow process. If you are new to stacking components, consider the mainstream approach to board-level PoP assembly: 1) Print SMT paste 2) Place bottom component 3) Dip second component in PoP paste or PoP flux 4) Place second component 5) Repeat steps 3 and 4 for any additional packages that are going to be stacked 6) Reflow In a well structured line this is no problem, however many engineers report that their process is a series of fragmented steps. When fragmentation of the line occurs, boards end up sitting around with paste and flux on them – sometimes for hours or even days! Time to reflow should be minimized, because long durations before reflow may impair the flux’s ability to reduce oxides on the solder powder and pads. Chemistries that are chosen to become Package-on-Package flux vehicles are more resistant to delays, although ‘air-time’ is obviously harmful for any flux.