When you need to reball a BGA chip in a pinch, sometimes you are left with the most ghetto of options. A soldering iron, flux, alcohol, tweezers, solder balls and… a 25 USD heat gun.
There is one addendum I’d like to make to the technique.
Applying flux to a BGA chip helps to stick the balls in-place on the pads. It is necessary to apply just the right amount of flux. Too little flux, and the balls don’t stick; too much flux, the balls start to attract each other and escape the BGA pads. It’s kind of a headache to deal with.
If you do add too much flux, then apply a few drops of alcohol or acetone to the BGA complex. This will dissolve some of the flux, and also distribute the rest of it over the surface of the chip. It shouldn’t disturb the balls because these solvents have weaker surface tension.