Its interesting what you say, I wish I knew more about the whole affair. I am aware that the process window is much tighter, eg the soldering melt temp is a lot closer to the top temp of the IC's.
Interestingly enough, it only happens to the boards using AMD/Nvidia/Nvidea IC's, not the Intel/Intel/Nvidea ones. Also why does it start to show only after about 18 months use ?.
Ken
Typically thats a joint enbritlement issue or even a standard device failure.....we have had issues with device failure due to contamination of the glue used to seal the packages! Also dont rule out PCB de-lamination due to the material not standing the higher temperature of the reflow process, they might have cut corners and used up old board stock!
The BGA's clearly idealy have to have lead free solder balls but, during the change over, you could use up old stocks of leaded device (they were already made so silly to scrap, not even the EU is that stupid) on a hybrid process which can cause issues.
It sounds more like a quality control or process issue or possibly even a non-solder related failure!
Bottom line is that leaded solder does require a slightly higher peek reflow temperatur (about 15-20deg higher) but, this is quite manageable and the jonts produced are equally as good as leaded items.
Probably got some sectioned parts we did 3 or so years ago during our lead free process trials
There may also be oher issues related to package