Thats not entirely fair. You measure the size of a heatspreader, the actual diesize will determine the thermal density and for KnC's sake, I hope thats no where near 1000mm˛.
True I was simplifying to avoid turning it into a page long post. However heat spreaders are larger than the die for a reason. The thermal conductivity between silicon and the IHS is generally higher than the IHS and heatsink (or waterblock or air). That is one reason the IHS is generally larger than the die. If one the die size and die heat flux mattered there would be no reason (well no thermal reason) to use a larger IHS then the die.
Moreover, the entire surface of these asics will generate about the same amount of heat per mm˛ very uniformly . On a modern cpu, something like 90% of the power is used by the cpu core which in most cases occupies only a small fraction of the die (the rest is cache, memory controller, etc). Im too lazy too look up numbers, but Im pretty sure a highend cpu will have a far higher thermal density at its core than any of these asics. Silicon has fairly good thermal conductivity, mitigating the problem for cpu's to some extend, but its no were as good as copper or even aluminum.
That is a good point on the "even" heat flux however GPU unlike CPU tend to be mostly core engine and thus are more like mining ASICs in that respect. One ASIC vendor (HF) uses 4 9mm x 9mm dies with 5mm space between them in a single package to lower the heat flux.
So thermal density isnt going to be problem with these chips. Of course you still need to get rid of 100's of watts of power, no doubt that requires a serious cooler, but the size of the chips wont make a lot of difference.
Well no. Thermal density does matter, it may not be a problem but it does matter. A Bitfury rig uses ~400W yet needs no heat sink. 200 USB miners use ~400W yet don't even need a fan. If you have a large lawn it probably has a thermal output (from IR exposure to the sun) of a couple KW (but at a very low heat flux) yet I doubt you run out there on a hot day and crank up a serious cooler to keep the lawn from bursting into flames.
All things being the same the higher the heat flux the more complex the cooling to remove the same amount of energy (watts) from a smaller space. Baring some exotic tech that means larger heat sinks, more efficient (heat pipes) heat sinks, and more airflow. With the constraint of a dual slot cooler that puts some boundaries on what is possible in the airflow and size parameters.
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