OK, so two questions:
1)
https://hashfast.com/shop/babyjet/ states 350W power draw (+/- 20%), but the chip only consumes 250W? 100W for cooling/misc?
2) An overclocked Sandy Bridge-E doesn't "run" at 350W, for example check
this (pic below). The 349W here is
System Peak Power, a very different metric (
system vs CPU and
peak vs sustained).
Subtracting the system idle power (
85W@4.7GHz), even at peak usage it would run at 264W (and it would
die from electromigration if run like this 24/7), so 264/425 =
0.621W/mm^2, still 20% below 0.77W/mm^2.
Edit: Let's not forget that a Sandy Bridge-E uses a high quality heatspreader (IHS) with fluxless solder, so the actual contact area with the cooler is much bigger than the die size, reducing the W/mm^2 requirements by a large amount.
350W is a rounded up number for the whole system, including the power lost in the 2 stages of power supply, and fans etc. The chip itself draws 250W @ nominal.
Depends on how hard you cool and overclock your Sandy Bridge E. The company that is assembling our systems specializes in overclocking. They run Sandy Bridge Es overclocked to 350W (CPU power alone, not whole system), using the same cooling system we are using. We are also using a heatspreader.
Metal migration is a well understood phenomenon. We have followed all the fab's rulesets for electromigration so that the current levels we're going to see will not be a problem (even current distribution, and thicker metal layers). Currently in the simulator for EM our chip passes the test for a 5 year lifetime, but fails the 11 year test - and that is running somewhat overclocked, at about 540GH/s.