Okay, you’re at the final stage of Package-on-Package assembly. Simply heat per the reflow profile suggested by your solder manufacturer (hopefully us!). A cage should be placed over the components if they may come in contact with curtains on the reflow oven. Other than that, reflow is pretty strait forward. Assembling Package-on-Package components with solder paste will almost always contribute a little more voiding than using a ball attachment flux, but that is the tradeoff for increased solder volume.