I have been repairing Xbox 360 Consoles since they first started having the RROD issue, and also worked on orginal Xbox’s.
The first thing I do is diagnose the exact problem with your system. The secondary error code tells me what part of the system is affected from the hardware failure. I always do a reflow of the entire board but i will focus on the most affected part more to ensure that the fix works.
I use a re-flow profile for lead free solder. I heat the bottom of the Motherboard for 5 mintues at 350 F , then use a re-work station on each BGA chip on the top of the board, after running no clean lead free flux under them.
After I am done with the re-flow I replace the X-Clamps with M5 screws and a combination of steel and nylon washers. I only use 1 washer on the bottom of the board to keep the height down and keep the board from flexing more after being screwed down to the metal cage.
That is all the information I want to give out about the process, that is a simple breakdown of what happens.