It is one of the biggest myths on Earth that lead free solder has ANYTHING to do with this. You can heat a dead chip to 120c and it will work again, this is 100c below what you need to melt lead free solder. The issue is with the eutectic bumps inside the chip, not the actual solder balls! EVERY LAPTOP for EIGHT YEARS has used lead free solder and the only ones having these issues to this level of insanity are Apple and the $@$* HPs, and also the few that got the dreaded Nvidia chipsets of the late 2000s.
The issue here is simple.
1) Slim design.
2) Sandy bridge quad core processor
3) Powerful GPU
4) Ventilation that sucks dust in without allowing it chance to escape.
Even if you do not use the GPU much, the quad core sandy bridge processor is heating it to !&&* and back everytime the CPU is used.
You tie a quad core, sandy bridge processor to a powerful GPU on the same, slim heatsink inside of a small case and you are begging for failure. This has nothing to do with lead vs. lead free. I have a Lenovo T520 with lead free solder from the same time period that I used and abused every day, and spent months mining litecoin on just to make the point that it is not about the balls used.
You don't get the most powerful CPU, most powerful GPU, and slimmest design without some compromise taking place. That compromise is no long term reliability. If the issue were the balls then reballing the chips would work, but it doesn't. It lasts for a week or two until they die again, because the chip itself is at fault.
Every single Apple laptop I open is riddled with dust. The entire design of the system just traps dust in without allowing it a method out. All fan based ventilation systems will allow a certain amount of dust in, but you look at a 3 year old Thinkpad and a 3 year old Macbook and it is obvious which manufacturer put thought into this and which didn't care.
Apple does fix these for free until February 2016, so I would suggest all of you kill your machine with GPUtest so you can get it replaced before they stop covering this issue.