[Colin] uses some spare boards to lock the target board down to his bench and then uses a custom stencil and solder paste ... although the BGA could have concealed faults that would be hard ...

Most PCB manufacturers nowadays offer the option to order a stencil along with your boards, and this is convenient if you use solder paste for any SMD components, not just for BGA parts.

which was done by hand rather than using a BGA stencil. In total, 200 balls were melted on to the chip in preparation for attachment to the Pi400. The old 4GB RAM module was removed and the PCB ...