I have fixed many of my Xenon 360's with the "heat itself up trick." (This is basically a "Ghetto Relfow" i like to call it." (Spelled purposely like that

Most people get bridged joints after 0102, because they are not tightening the heat sinks down correctly.
The motherboard of any computer system is delicate, and your fighting with fire. I can only understand this if you have a JTAG system and can't get a new one.
Here is what you need to do:
Always make sure the heatsink and chip are clean from any left over gunk, as this will cause the heatsink to sit on an angle on the chip. Causing one side of the chip to be pushed down and the other to hover. This will BREAK a solder joint.
Always use even amount of washers, on both sinks, if you use two on the top on the GPU, use two on the top of the CPU. I usually use one on the bottom. (On each screw obviously)
Use the CORRECT size MM screws. If you get ones too long for the GPU it's going to push against the heat sink, therefore causing again uneven pressure. If you get ones too long, go back to the store and get ones smaller, DO not be lazy about this.
Your goal here is to cause EVEN pressure. Look at the board eye level, you can see if your heat sink is on an angle, or your board is being bent up too much. Believe me you can see. take a flash light, or high intensity light and look underneath the heat sink to see if the sink it actually TOUCHING the chip. If it's not, your just going to overheat the chip and nothing will happen, plus your cooling will be shit.
The heat up trick will LITERALLY cause the solder to become malleable, as non-leaded solder will start becoming soft at around 170 degrees Celsius. This is ALL you need for the chip to reconnect. (If you have worked with solder, which I have it's amazing what just getting it hot can do.) Too hot and the solder will melt causing a bridge. This means to of the solder balls underneath the chip have COMBINED making a BIG solder ball. This also means on the chip there is now a solder ball MISSING, meaning a missing connection.<--really REALLY bad. Always impossible to fix without a reball/maybe TRUE reflowing will work.
At this point, the Xbox should throw an overheat code and turn itself off.
This is why it's important NOT to keep the CPU cooling while this is on, or you will end up MELTING the solder underneath the GPU chip and destroying your joints. The CPU is the breaking point of the turn off code. The Xbox 360 does not turn off if the GPU overheats, only the CPU.
So DO NOT cool the CPU. This is a LOT of where people make there mistakes.
The most important thing here is EVEN PRESSURE ON THE CHIP. If you don't have even pressure, you will need a reball/true reflow.
happy fixing!