Originally Posted by Banjo

It's the heatsink clamp design. All of the pressure holding down the heatsink is in one small point. On the centre of the X clamp, there is a plastic spacer button. It sits hard against the bottom of the mainboard. What happens is that it causes all of the stress to push upwards, which is towards the bottom centre of both GPU and the CPU. As a result, the corners of the mainboard flexes into the opposite direction. The only problem with that is the mainboard isn't screwed down properly in the area where the CPU and GPU are. What should be happening is to have the board flex downwards slightly due to the pressure in the middle pushing upwards.

So, when the console heats up, the solder balls will become soft. Then what will happen is the mainboard will be allowed to flex further resulting in cracking. So, there is too much tension for the corners of the CPU and GPU due to the poor design of the heatsink clamps.