Interesting so, KnC is a 250/900 = 0.28 W/mm^2 chip and HashFast is a 350/324 = 1.08 W/mm^2 chip, requiring a cooling solution that can transfer 5.6x the heat per mm^2. I really hope your cooling solution holds up 24/7!
Out of comparison, an ATI 7970 is a 250/365 = 0.68 W/mm^2 chip and an nVidia GTX Titan is 250/561 = 0.45 W/mm^2 chip.
Hashfast's chip at nominal (400GH/s) is expected to consume about 250W of power, not 350W. i.e. 0.77W/mm^2. Overclocked Sandy Bridge E in shipping commercial products with the same cooling system runs 350W with a 425mm^2 die, 0.82W/mm^2.
Simon, regardless of what you think is doable, you are taking
unnecessarily high risks to develop such a high-TDP chip. It would have made a helluva lot more sense to go with, say, 4 chips of 63W each. The die would be 1/4th the size. 1/4th the cost too since you would have 4x more chips per wafer. A little more PCB space would be used (not a big deal). But this would have taken away a lot of the risk of cooling a single hot chip.
Bitcoin mining being an embarrassingly parallel algorithm, there is absolutely no point in trying to made a die as large / as hot as possible, when you can spread computations across chips with virtually no downside.
I see this as a very poor strategic decision made by Hashfast. (And Cointerra, and KnCMiner). Only BFL seems that they will get it right, since their 350W Monarch card should split the workload across 10-30 chips.
If I was an investor in Hashfast, I would have asked you "why do you take the risk of aiming at the highest TDP possible for your gen1 chip?". Please explain your choice.