So for christmas, instead of asking for parts for my gaming PC and stuff, I asked for a good soldering iron/reflow station. So I ended up with a unit of crummier quality to this one
http://www.ebay.com/itm/853D-Rework-Soldering-Station-SMD-Hot-Air-Iron-Gun-DC-Power-Supply-6-Gifts-/252027806219?hash=item3aae07220b:g:uRQAAOSwu4BVphKXBut I mean it works great, much better than guess and check with the 20 year old two temp heat gun and my old weller 40w plug-in iron.
So first, I determine if the GPU needs a reflow in the first place. I run it, if it artifacts, almost certainly a reflow is needed. If it simply shows no video or crashes under load, it might be a bad BIOS image, faulty voltage regulators, it could actually need a reflow, or is might just be broken beyond economical repair.
The PROPER way to fix stuff like this is a reball, but reballs are a HUGE pain.
So if it needs a reflow or I simply cannot figure out any other fix to try, I usually just dissasemble the card, remove anything that can melt (like plastic parts near the GPU die) clean it well, prop it up on my table with little tinfoil balls and slowly warm it up over the course of 3-10 minutes, depending on the board. I set my hot air gun to 270 degrees. I know this is a little high, but I think the calibration is a bit off on my unit.
Removable mobile videocards (imac/gaming laptops) are usually a 3 minute preheat. Desktop cards I usually give longer, so maybe 5-10 minutes, and large laptop boards might even need more. The goal here is even heating to minimize thermal expansion strain across the board.
After preheating, I "go in for the kill." This is where I slowly move the nozzle closer to the GPU chip, trying to keep it evenly heated and the surrounding area warm as well. Once I work my way about 3/4in away from the chip, I keep applying heat for a couple minutes, again, more time for the larger boards as they contain lots of copper and will sink the heat, making it harder for the solder to melt and reflow.
After this, I slowly back out in the same procedure as before. After slowly cooling it down by moving the hot air gun farther away, once I get about 2 feet away and the board is under 100 degrees, away I usually put a cardboard box or something over it and let it cool down on its own for another 10 minutes. The key is NOT to rush.
After this, it is time to put it back together and test it! Apply new thermal paste, thermal pads, and make sure everything is fastened properly. Put it in the machine and boot it up!
So far I have fixed:
R9 270x
Three iMac video cards
HD4850x2
And nothing has yet to fail after a reflow so far. I know there are plenty of things I should do differently, but I have learned all of this through experimentation. Feel free to drop a comment or suggestion below, I look forward to hearing your opinions!