2.5D + 3D = “3.5D”!

By Reela Samuel

May 5, 2026

Architecting the Next Generation of AI Silicon

The semiconductor industry is no longer defined solely by transistor scaling. As Moore's law decelerates, advanced packaging has become the primary lever for achieving system-level performance gains. Within this landscape, the equation 2.5D + 3D = 3.5D—defying the instincts of basic math and physics—captures a pivotal architectural evolution: one that balances performance, manufacturability, cost, and thermal efficiency in ways neither traditional planar designs nor purely vertical stacks can.

At its core, "3.5D" integration represents a new class of heterogeneous system architecture that directly addresses the technological and economic pressures driving AI and high-performance computing (HPC) silicon today.

From Planar Modularity to Vertical Bandwidth

Initially, the industry transitioned from conventional 2D monolithic systems and multi-chip module (MCM) designs to 2.5D integration to meet growing system bandwidth demands. In this model, multiple dies, logic, high-bandwidth memory (HBM), and various accelerators are placed side by side on an advanced package, which includes a high-density silicon interposer. The interposer, acting as a high-speed interconnect plane, provides significantly denser die-to-die connections than organic substrates and enables heterogeneous integration across process nodes. This approach has matured over the past decade and has become the backbone of many leading AI accelerators and HPC processors.

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