I'm not an expert in BGA reflow, but I have repaired macbook pros, macbook airs, white iMacs and aluminium iMacs, all with these nvidia chips that develop cracked solder joints, a few dozen Macs in total, with this simple "recipe" I was given:
1.- If you have air conditioning, turn it off. If you've got a fan, turn it off too. And close the windows. You don't want any currents of air where you do this.
2.- Put the pcb over the preheater set at exactly 120 degrees celsius and leave it there for at least 10-15 minutes before continuing. Check again: we don't want any air currents over the pcb/preheater cooling the pcb.
3.- The idea is that a preheated pcb only needs a bit more heat over the nvidia chip to melt its balls
We are going to leave the preheater on during the entire process.
4.- Proper timing and a bit of hot air does the rest: The "recipe" I got said blow hot air (*) @ 300 degrees celsius during 3 minutes no more as close as you can (I do it at about 5mm) without ever touching the chip, all over the nvidia chip and nothing else. You can do zig-zags, go in circles, whatever, but keep it moving over the nvidia chip and only over the nvidia chip. You will feel the hot, but no, don't turn on the AC or the fan yet or you will ruin the experiment.
5.- After 3 minutes turn off everything, the gun and the preheater, and let it cool alone. Now you can turn the AC back on :-)
6.- The first time I did it I had no faith at all. But that was two years ago and that Mac and all that have come after it are still working fine.
(*) I use a chinese gun like this in EEVblog #167 - Atten 858D, with the medium sized noozle (I think it's 12mm).