Re: BGA repair?
Can you get access to the ball?
If you have a badly reflowed BGA joint, then you can suspect there will be more high impedance or partially collapsed ones.
As a really, not recommended, last possible measure, you could add additional flux to the device, heat it up to rework temperature while applying minimal force to the surface of the device (pushing it down). Forcing a ball collapse. Shorts is a BIG risk here and the temeprature profile needs to be constant across the device.
On some rework stations you can use the remove tool if it has a vacuum pickup. Place a small PTFE block on the device so the pick ups pressure spring applies the pressure to the device but instead or doing a remove cycle, just turn the heat off after a timed period.
But if your PCB has initially had OSP or entech finish, and not been reflowed properly to begin with, then getting a good flow to an old joint would be near impossible without full rework as the pad will most likely be too tarnished.
Again, this is NOT a recommended method, but a desperate act type of scenario.