I've heard the easily replaceable hash board before (including from Bitmain) and it never quite works out the way you think it will. Have a look between the generations and try and work out how many Bitmain generations are a. board compatible and b. heatsink compatible and that'll give you an answer as to the sustainability.
Yes. This has by far been the most challenging aspect of the design.
Varying sizes of chips and thickness.
Surrounding components.
Heat dissipation through the top of the chip or through the PCB.
Different PCB layout where chips are now in new locations.
It's no surprise where Bitmain is now: Individual Heat sinks for each ASIC.
Our solution may not be elegant, but should work.
Not ready to go into detail yet and the topic deserves its own thread.
The surrounding components is a pretty important point -- mainly the decoupling caps around the chips and the bypass used for each node when powering a string. It may look like, "well just put some (random value) caps here and..." BUT -- the actual values and more importantly,
where they are located and what type they are (material used) is of vital importance. You use different ASIC's, you will need to find a new set of optimum values for the caps or at a minimum, verify how the different ASIC's work with a compromise set of values.
In many ways the power planes feeding ASIC's are much like RF circutry and must be treated as such or else you get some mighty weird and very localized dips/spikes in the 'DC' power feed to them.
ref:
http://powerelectronics.com/power-electronics-systems/five-things-every-engineer-should-know-about-pdn