This is something you may have run into if you’ve ever manually dipped and placed PoP components. Yesterday while trying to hold a conversation about the package-on-package process, I lost track of the very same process I was discussing. I neglected to dip one of the components into solder paste before placing it on the board. This is a picture of what happens. The stack itself soldered well, as would be expected, but fell off when the board was lifted from the conveyor at the end of the line.
If you’re wondering what a small error like this costs:
1 hour lost time + price of the board and components.
Luckily the other 14 PoP stacks can still be used for cross-sectioning and learning more about the PoP paste that was being evaluated. If this was a production board I would be able to simply dip the stack and re-place it on the pads, send it through a second reflow, and test the final assembly for functionality – but you just can’t get away with that during evaluation.