ok let me explain a few things. Reflow is an art, and a goof up on 1 thing will ruin the card or chip or both. Been there, Done that. i have a tote full of junk xbox boards from my learning experience a few years ago.
you MUST preheat the board before attempting a reflow. In some special cases it may not be necessary, but the reason why its required is the fact of thermal shock.
Thermal shock leads to extreme warping of the board, and will cause the solder under the BGA to bridge and create a mess during a reflow. very BAD idea.
The board temperature (not heater temperature) must be around 180C prior to a reflow. Reflow of lead free solder is roughly 217c. Keep in mind some older boards may be leaded solder and that would put the bottom preheat too hot. as it melts around 170 to 180c.
Once the board has preheated, you heat the BGA with the top heater to the reflow point. Too hot, it will blister and pop. Too cold, it wont reflow, so use another temperature monitoring thermocouple right next to the chip to monitor its temperature. Once it exceeds 220c, let off. The chip will also get wiggly. You can barely vary lightly tap the side of the chip and itll bounce/wiggle and thats how you know the solder has molten. But you must be light as a feather, becuase too hard it'll jump balls and your day is ruined.
Also the speed at which you bring the chip from the preheat 180C to the reflow 220C also matters. Too fast and the chip will warp or pop. Too slow, the chip may fail from too long of heat exposure.
Also make sure you squirt some flux under the chip before you begin the process or your wasting your time.
And even with all this under your belt the odds get better, but never reach 100%. Not even for me. its always a 50/50 shot. if fail, you must replace IC, and the reason I say this as I have done reflows absolutely perfect with no bridging, clean solder, and no warping or blistering. But still be dead because the IC has failed from heat exposure, or it just failed from other causes and a reflow didnt fix it, so thats it, 50/50 shot with original IC and not damaging the board.
Once you get the art under your belt, it makes it easier to fix more expensive boards. If the IC fails, such as a GPU, you can remove it by hitting the reflow temp. Once the chip is wiggly, you can remove it with a suction device.
After the board cools, you can clean the board up of all solder so the pads are clean and flat and this can be tricky because if you pull the wick too hard or let it cool down to fast or let the soldering iron get the pads too hot, it can cause the pads to rip right off the board very easily, Game Over, throw the board away and get a new one if that happens.
Once cleaned up flawlessly, Flux the area, and heat in a brand new balled chip and bingo. If your reflow process is good, itll bond itself back onto the board and its back up and running again. I have done this on numerous occasions.
Main PC: Intel core I7 920, MSI x58 platinum, Radeon4850
PB: tibook G4, ibook G4, Lombard, 160, 165, 180, Duo 2300x2, Duo 270c x2, 520cPPC, 3400c, 1400c
Desktop: G3AIO, 5260/100 x2, SE, SE/30, 512k, plus, LCIII, 7100, iMac G5 iSight, 6400/225