As AI accelerators and HPC devices grow larger and more complex, advanced chip design continues to move away from the transistor and toward the package. Over the past year, packaging engineers and foundries have converged on the same conclusion — that 2.5D integration is now the primary tech responsible for power delivery, signal integrity at extreme bandwidths, mechanical stability across large areas, and in some cases, even active functionality.
That is apparent across the industry, from thicker silicon interposers designed to support HBM4, to renewed interest in silicon bridges and longer-term interest in organic and glass alternatives. The technology itself is not new, but it’s facing all-new constraints as AI has pushed 2.5D packaging into territory where its original cost and yield assumptions no longer hold, and the next phase is about deciding which problems are worth solving in silicon and which are not.
Interposers are becoming power platforms
One of the more immediate trends in 2.5D packaging is the steady thickening and layering of silicon interposers. Early interposers were largely routing fabrics, optimized to move signals between logic dies and the adjacent memory stack, but that role has since grown. As memory interfaces widen and chiplet counts increase, the interposer is expected to carry far more current and maintain signal integrity across much denser wiring.
Vendors now routinely talk about moving beyond four metal layers. Designs targeting post-HBM3 systems are already planning for eight or nine layers — or sixteen — driven by a combination of routing density and power distribution needs. Each additional layer increases fabrication cost, but the more pressing issue is mechanical. Thick, multilayer interposers are prone to warpage, and controlling flatness across a larger area has become a concern.
(Image credit: TSMC)
This has given rise to something of a balancing act. Thinner interposers reduce signal path lengths and can improve electrical performance, but they are mechanically fragile. Thicker interposers offer more routing headroom and stiffness, but increase stress and thermal complexity. Engineers are leaning on workarounds like thin-film stress control and tighter process windows to keep large interposers flat enough for assembly, but margins are narrowing as sizes increase.
The logical extension of this trend is the active interposer. Once the interposer carries significant power and routing infrastructure, the temptation to integrate logic grows. However, active interposers remain rare because they fundamentally change the yield model. A passive interposer can be treated as a mechanical and wiring component, but an active one must be tested as a functional die, introducing new failure modes and validation steps.
This means active interposers are largely confined to high-value AI and HPC designs where the system-level benefits outweigh potential yield penalties.
Some vendors are already blurring the line. Certain advanced packages use base dies that look like interposers, but include active circuitry for data movement and memory control. These designs shift complexity downward in the stack, reducing the burden on the top-level compute dies while concentrating risk in fewer large silicon components. The approach works, but it reinforces the idea that the interposer is becoming part of the overarching architecture.
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