Is it a good idea to reball a bga chip which has been already heated up?
Well, this is a tricky question. It depends on several things. If the chip is broken within its inner structure, changing the balls underneath wont solve the problem for good. It could help for a while and thats because the heating itself could make the chip running not exactly the balls replacement. However this effect wont last long.
If you encounter a previously repaired motherboard take a good look and the chip. See if it has any bumps on the upper surface. If the chip is popcorned, it has been damaged due to previous heating. Amateurs blindly use standard hot air guns and often their work results in permanent damage of the chip. Naturally such chip can only be replaced to make the board run once again.
Also check if the motherboard itself is straight or is it bended. Heatguns make boards bend, and such deformed motherboard could make it impossible to solder back the chip correctly. Bending could also make some of the pads be torn off the board when you remove the chip.
Its best to at least know the history of previous repairs that have been taken in your case. It is highly risky to perform reballing in such cases. Even if the board would run after the process the risk of the chip failure is at hand. If your customer decides on reballing, make sure you explain the eventual consequences and do not guarantee that the board will run permanently.
Good luck.
