TL;DR
The problem, per SK, is that HBM cubes are capped at a total thickness of 775 microns, the standard thickness of a 300mm logic wafer.
Hot Chips 2026: SK hynix pushes hybrid bonding to HBM5 as AI memory hits 775-micron ceiling — firm extends MR-MUF through Nvidia Rubin
The problem, per SK, is that HBM cubes are capped at a total thickness of 775 microns, the standard thickness of a 300mm logic wafer.
Source: TomsHardware