When do you get the black screen after the system has started or it never gives you an image?
I do have to say they did over do the white paste (which is the cheaper stuff) and you'll see the gaps within it shows the paste was dry so it did not flow evenly across. !#^& poor job to say the least.
As to cooking the card it really doesn't fix the root issue here as these cards had two distinct problems.
- The first is the solder Apple used to solder the chips was lead free which couldn't handle the heat breaking down over time. The tin would crystallize and once it does the solder needs to be completely redone to prevent re-crystallization. You see the reflow only masks the issue it does not fix it.
- The second problem is within the GPU chip it's self. The chips design required connections between the silicone dies internally these via's likewise breakdown from over heating and again the heating of the chip only masks the underlying failure.
So what to do? I would replace the graphics card if you want to save the system.
But, before you go down that path understand the root issue here is pushing the graphics too hard for what it was designed to handle. Often I see gamers with these systems running heavy graphical environments which these systems were not designed to handle.
Remember the time point when these systems were designed and what fantastic games we have today! The best at the time was Doom with simple polygon graphics. Now we have near real imaging. Which is just to much for these older systems.
If you do resuscitate this system use it for web browsing and light graphical stuff. I would recommend monitoring the temps not letting it cook.
At this point I think you have less than a 50-50 chance of recovering the graphics board if it is indeed a cold solder joint issue.
Are you sure the real issue is the graphics board? Did you review the diagnostics LED's? Did you try an external display? Or lastly try accessing the system in Target Mode from a second Mac?
Not saying the board doesn't need some attention here, just making sure we are going down the right path.
As for cleaning you'll need to wipe off as much of the old paste you can (save the pads as you'll need to reuse them). Use this: Arctic Silver ArctiClean to clean off the last bit of paste. I would recommend using this paste: Arctic Silver Thermal Paste on the GPU chip. Follow this guide: Technique: Applying Thermal Paste. For the RAM chips I like using pads Vs paste. If you look at your image the heat sink does not go low enough to touch the RAM chips which is why the white paste was used. As it is able to gap the difference. But its not as good in transferring the heat when it gets this thick. While I don't like pads often you are forced to use them. In this case this is where they are needed. Here is a source of pads. Note the different sizes. You'll need to dry fit the heat sink and measure the gap between the heat sink and the RAM chips, then get the correct pad. You want one that is just slightly bigger so it is compressed. Blue Thermal Pads