90% yield? 
That is WAAAAAY off base. That yield is common for higher nodes like 28nm on up but currently 16/14nm production yields are around 40% good dies and lower. They only began to hit 40% late last year...
Now the foundries are of course trying to get better but the processes are still under development. Biggest issue is the EUV light source used for the photo lithography. That monstrosity is still pretty hairy to run and is in no way capable of running 24x7. Is more like 8-20hrs followed by around 6 hrs to a full day of cleaning/realignment/process verification before starting another run of chips.
90% is a lowball estimate. The actual yield will be higher. The mining chips are so repetitive that they are commercially valuable even if less than 10% of it is operating correctly. It is the same story as with NAND flash memories. That is WAAAAAY off base. That yield is common for higher nodes like 28nm on up but currently 16/14nm production yields are around 40% good dies and lower. They only began to hit 40% late last year...
Now the foundries are of course trying to get better but the processes are still under development. Biggest issue is the EUV light source used for the photo lithography. That monstrosity is still pretty hairy to run and is in no way capable of running 24x7. Is more like 8-20hrs followed by around 6 hrs to a full day of cleaning/realignment/process verification before starting another run of chips.
The 40% yield refers to number of good chips per wafer - not cores per chip. As to what is a good chip, it one that meets spec. Preferably with 100% cores active @ stated speed. From there they will start stepping down the grade hopefully mainly by speed capability instead of by dead cores.