I've heard of head-in-pillow defects where the solder may touch but fails to wet the pad on the chip or board. BGA probably makes it almost impossible to nudge

I've heard of head-in-pillow defects where the solder may touch but fails to wet the pad on the chip or board. BGA probably makes it almost impossible to nudge a leg to check for mechanically solid connections. I don't know if bad joints from the factory are a problem in practice, or they usually only crack in use.