In production manufacturing solder paste is generally applied to BGA pads. The amount of solder in the balls is less than the ideal amount for making a good joint, and the solder paste also helps to hold the component in position.

That said, I have ‘successfully’ soldered small BGA components by just applying a sticky flux and then reflowing with a hot air gun. It can work fine for prototypes, but it’s not really how the packages are meant to be soldered.