After cleaning both sides of the PCB, identify the two plastic stays of the northbridge heatsink. On the bottom side of the board the stays can be seen to be fixed, each with two tiny delicate latches. It is these latches that are susceptible to failure.
Pinch the latches on the plastic stays with a pair of needle-nose pliers so that the stays fall out. They should pop out from the force of the springs on the other side. If they do not pop out, give them a gentle push from the back side of the PCB
Make sure not to lose the springs. You can throw away the plastic stays