Not sure how you can over-tighten a heatsink. I certainly have not worked with every one out there but all that I have just won't let you over-tighten them. You can crush the motherboard mounting the back plate, but mounting the cooler bracket to that typically uses screws with built-in stops just to prevent crushing the CPU or its socket - again that's with the coolers I've seen. This assumes the CPU is sitting and cradled completely and properly in its socket before clamping it in and mounting the cooler's bracket. It also assumes the screws are tightened a little bit at a time (if not the 1/4 turn type) using the cross-diagonal method and not just in a circle.
Am I missing something here?
The use of conductive (or capacitive) TIM (thermal interface material) is not a problem AS LONG AS none of the TIM gets on the pins or pads. That simply requires a steady hand during application and, of course, using as little TIM as possible - so it is not squished out over the edges when pressure is applied - while still ensuring full coverage over the die.
The back plate touching something should not happen either - if properly mounted - unless you have some oddball proprietary case. But you said you were "replacing" the TIM so I am assuming that is the same cooler assembly as before and its back plate had already been installed before. So I don't see how it could have touched anything - again if properly mounted. I have had cases where access to the back of the motherboard was tight so mounting the back plate with the motherboard outside the case was necessary. But once mounted, the clearances around the plate were fine and nothing could touch anything else once the board was re-mounted in the case.
So I am confused about that too.
Did you unplug the computer from the wall before starting this to ensure the +5Vsb standby voltage was removed too? Did you observe proper ESD precautions when handling or even coming near the CPU and its socket with your fingers?
It seems to me if you were just replacing the TIM, there would have been no need to remove the motherboard or loosen the back plate assembly - again, unless I'm missing something and desperately need another cup of coffee.
Problem solved. The Op just ordered new mobo.
Not sure I would have that sort of confidence to declare the problem solved. There clearly was some excessive current flowing where it shouldn't have been. How do we know the CPU was not zapped? How do we know a zapped CPU won't zap the new motherboard?
I'm seeing too many unanswered questions.
I would be making backups of my data while waiting for the new board to arrive.
