Stacking optics into the chip in 3D wins electrically but pays across the board in thermal management. imec's paper quantifies 2.5D vs 3D CPO thermal crosstalk one number at a time: 8 K vs 60 K, a 13x spatial gradient, a 60x temporal gradient. All 18 figures explained.

imec pushed 3D-stacking hybrid bonding to a 200 nm pitch, but the real message is that the deciding factor has moved from bonding to overlay, CMP and pad design. Why is 200 nm a yield cliff, why are overlay accuracy and electrical yield locked in a linear relationship, and where is the next bottleneck for AI 3D integration?