During reassembly:

Check the condition of the thermal pads—they'll be on the bottom of the logic board or the corresponding spot on the right side of the frame.

If you're reusing your logic board and either of the thermal pads are damaged, remove the old pad, clean the surface with highly-concentrated isopropyl alcohol (over 90%), and apply a new pad.

If you have a new logic board and it doesn't come with thermal pads pre-installed, apply the new pads now.