I spent most of the afternoon taking apart, measuring and reassembling my iMac 21.5" mid 2011.
Reinstalled the Radeon HD 6750M, and it booted fine with perfect video. (of course I didn't plug in the HDD... but I'm going to take it apart again soon).
So everything is working.
I did make some measurements. I'll assume y'all have seen the basic layout, here it is connected to the motherboard from the backside.
After taking it apart and getting the calipers on it, the two clearances are:
the camera isn't exactly in plane with the board, the measurements are off the elements on the motherboard for the lower one, the post on the left, and the 4 bolts that the X-bracket screws into, they have a spring-loaded washer and a defined standoff with a
maximum spacing of 0.102", the springs pull the heat sink onto the GPU (the GPU is held firmly in place by the connector on the mother board and the posts that accept the two screws that clamp the GPU into place).
The two MXM-A cards I have are the nvidia Quadro K1100M which didn't work, and the AMD Radeon HD 6750M which does. Here they are side-by-side, nvidia on the left, AMD the right (obviously)...
The GPU chips are in the same spot with respect to the X-bracket holes, the memory is pretty much in the same place, but the ICs on the right edge of the boards is very different, and especially their height off the board.
The backside has a surprise too:
The nvidia chip has more memory, and it is located on the backside. There is no thermal contact on this side (the mother board side), there is an insulating layer of plastic, as well as various hardware elements (see the cross section shot above), and well out of any cooling fan air. It is worrisome that these memory chips will operate in what will be a pretty harsh temperature environment. The mechanical clearances are more difficult to determine, but these clearances may be important for other cards with the backside loaded with ICs.
The side of the heat sink that contacts the memory and GPU chip looks like:
and if you look hard, there is a feature on the top-right corner that is insulating the chips on the board from the heat sink, a piece of clear tape that is obvious engineered for just that spot. It took me some time to realize this... I thought it was a very subtle machined step. Taking this image and flipping it around the horizontal axis, then overlaying it on the boards gives you this picture:
The pink shaded area is the insulating tape. This protects a high chip on the AMD card. The upper right square is a thin step, the central square, and the two rectangles on top of the memory are thicker steps. The GPU chip makes direct contact with the copper plate, the memory has to have some sort of thermal compound or tape to contact the heat sink (I used the K5 compound).
When I slid a heavy sheet of paper (thickness: 0.016", 0.41mm) into the gap before taking the K1100M out it was very tight with the chips between the two large features "R33", the one closer to the memory was very very close. The chip below the bottom "R33" was also very close.
Assembling the HD6750M there were no tight gaps.
I had put some very thin mylar tape on the regions where the K1100M had a tight fit anticipating that I would try to reinstall the card again if everything worked with the AMD.
In addition, in spite of the admonitions to modify the K1100M X-bracket, I used the AMD X-bracket, after having looked very carefully for clearance, which visually checked. Having failed in the first install attempt, I've modified the K1100M X-bracket and checked that it would work. This will be used in the next attempt.
The chore of taking the iMac apart and putting it back together is getting less burdensome.
Maybe someone can take a picture of the 27" iMac heat sink plate and we can make a mechanical drawing of it, as well as a line drawing that can be overlaid on a card image to help anticipate the differences in various card installations.
I'll work on the 21.5" iMac drawing, but I'm pretty much done with this today.
