When reinstalling the heatsink, for example after a memory upgrade, be sure that the ferrite bead on the display cable is not under the heatsink. If the ferrite bead is under the heatsink, the EMI gasket and ferrite bead will stack up possibly causing a slight bow in the keyboard after reinstallation.
Before screwing the heatsink down, gently push the ferrite bead towards the rear of the PowerBook creating the necessary clearance for the heatsink.
The following images show incorrect and correct alignment of the ferrite bead with regards to the processor heatsink.
Figure 1 Incorrect placement of ferrite bead.
Figure 2 Correct placement of ferrite bead.