[sojuz151]

I don't understand why even put that HBM on top of the core.

From what I understand, in a typical gpu core you put logic and connectors on one side and innert silicon on the other. So unless you drill through silicon you don't get shorter routing.

Why not put GPU one one side and HBM on the other side of the PCB? This would fix the cooling problem?

[timschmidt]

Routing signals through PCB vias requires greater voltage and has lower available bandwidth than silicon-to-silicon bridges. AMD's first generation of cache dies bonded to the top of the CPU, but their second generation bonded to the CPU's bottom which improved cooling for the fast logic on top. Similarly, HBM under logic would be ideal.