I'm doing a little PC surgery, and part of the job is dumping the near-dead Northbridge/chipset fan on my ASUS A8N-E motherboard. I bought a new Zalman NB47J heatsink/cooler as a replacement.
I unclipped the old fan from the back of the mobo, gave it a little wiggle, and it came off.
On the back of the fan is some kind of black square, with a square hole where the chip would have been. (There's an NVIDIA imprint left on there.) It's diagonal to the fan. Beneath the black square is another square of light pink-ish material. Maybe some kind of adhesive. It's in line with the fan, diagonal to the black square.
Back on the motherboard, there's some plastic-like film left on top of the green circuit board surrounding the silver chip. Around the chip, there's some silver crud, and some off-white, pink-ish crud. Most of it seems to be under the film.
Looking at this site, in the pictures, there is some sort of silver square with a square hole in the middle, but I don't see a specific mention of what it's for. Thermal tape, maybe?