@aprepair not sure why the tech suggested that to you. The oven is not going to fix the issue at hand. 450F for 5 minutes without bottom heater and no ramping up and cool down period is not going to reflow this. A reflow is more than just blasting some heat on it. 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. 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 (455F)to ensure good wetting. The maximum reflow temperature is in the 245°C to 260°C range.
When using a reflow station the profile usually looks like this :
Average ramp-up rate (Tsmax to TP)
3°C/ second maximum
Minimum preheat temperature (Ts MIN)
Maximum preheat temperature (Ts MAX)
Preheat time (Ts MIN to TsMAX)
Time above liquidous temperature TL
Peak temperature (TP)
Time within 5°C of peak TP
Average ramp-down rate (TP to TsMAX)
6°C/ second maximum
Time 25°C to TP
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.
The bottom line here is the construction of the IC (if it even is the GPU). Here is a very "quick and dirty" explanation of what causes most of the RROD. It is not always a failure of the solder balls which connect the IC BGA package to the motherboard. It does happen and you can see why on here More commonly however is that the failure is due to the chip design itself.
As you can see the "bumps' are what actually connects the die to the substrate to make the chip complete. If these bumps fail the die does no longer make contact with the substrate and thus no contact with the circuit board. The chip has failed.
Here you can see the space where the bump has failed and no longer makes contact. We are talking microns of space here. So a bit of pressure on the top of the die potentially close the gap. Same with a “reflow”, it may allow some of material from the bump to reshape and starting to make contact again. The subsequent heating and cooling of the chip during use is what will eventually cause it to fail again.
If it is the GPU then the only proper way to fix it would be a complete reball with a new IC which is most likely not available