How to reflow a GPU from start to finish #2 reflow process and testing the repaired card
367In part #2 of this series the actual reflow process is presented for an nVidia gaming GPU card, where I show you the profile what I have developed for lead-free BGA rework. The series is a step-by-step video on reflowing the GPU in a defective Nvidia gaming graphic card using a semiprofessional Honton HT-R390 combined infrared hot-air BGA reworking station and Amtech NC-559 flux.
One important part of my profile is the stage, where the board is being heated up to about 90-95C by the infrared plates. At this stage the flux is being applied, instead of putting the flux on the board from the very beginning of the profile. This way the flux is being sucked right below the GPU chip, and during the activation process taking place at higher temperature, it removes the oxide from the lead-free solder balls below the BGA.
For the actual liquidous temperature I use 255C at the top hot air nozzle, and based on the temperature measurements with k-type thermocouples I have done on previous boards, I know that the chip hits around 225-230C actual temperature. This is necessary, because the melting point of the lead-free solder is in the range of 217-225C even under clean laboratory conditions. For comparison, the oxidized solder will melt at higher temperatures. Also, in contrast to the lead-containing eutectic Pb37-Sn63 alloy, the lead-free solder does not have a well defined melting point, but instead a temperature range.
Once the reflow profile is finished, one should cool down the BGA chips as fast as possible, because if you check a temperature curve suggested by the industry, you will see that after around 30 seconds of dwell time at liquidous temps, they are dropping very fast down. This is why on my infrared/hot-air rework station there is a cooling fan installed.
After BGA rework, the card is being cleaned, and fresh Gelid GC2 heat-sink compound applied.
Finally, the card is being stress-tested with FurMark, which is putting a lot of stress on the GPU and the RAM subsystem by calculating a 3D furry rotating donut.