Some people do underfill PoP components with capillary underfill. This is done in one application step, there is no distinction between underfilling the top and bottom packages. This works out well, and is made possible by the design of the bottom component. The lower components in a PoP stack are designed to be low-profile by design – to limit stack height. The intermediate packages are so thin that underfill applied to the side of the stack flows between each component in unison.