Steven, right now my concern would be that you need to know that it is not necessary to "to fully melt the solder" for a reflow. I suggest that you get an old board first and practice a reflow. I assume you are going to use a hot air gun. There is a lot more to a reflow than blasting some hot air onto the processor. You could also tell us why you "believe to have a faulty connection between the GPU and the logic board due to over heating." Yes, it is lead free solder. Here are some basic temps that you will need to know "The lead-free alloy used for BGA solder balls has a melting point of 217°C. This alloy requires a minimum reflow temperature of 235°C to ensure good wetting. The

maximum reflow temperature is in the 245°C to 260°C range.

When using a reflow station, for a processor like your NVidia, the profile looks usually like this :

Profile Feature

Average ramp-up rate (Tsmax to TP)

3°C/ second maximum

Minimum preheat temperature (Ts MIN)

150°C

Maximum preheat temperature (Ts MAX)

200°C

Preheat time (Ts MIN to TsMAX)

60–120 seconds

Temperature (TL)

217°C

Time above liquidous temperature TL

60–150 seconds

Peak temperature (TP)

260°C

Time within 5°C of peak TP

30sec

Average ramp-down rate (TP to TsMAX)

6°C/ second maximum

Time 25°C to TP

8-minutes maximum

All temperatures refer to the topside of the processor, measured on the processor body surface. Of course this is for a reflow station based on a lead free BGA IC. Best places to check out first, would be the Xbox360 and PS3 forums. After all, those are the birth places for the home reflows. I attached an image which also shows a common reflow profile. Check this video for a GPU reflow, different computer, same idea. Hope this helps, good luck