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70 PCB007 MAGAZINE I NOVEMBER 2025 The Evolution of Interposers The definition of an interposer is evolving as semi- conductor manufacturing and advanced packaging technologies progress. Let's go step by step. The Classical (Still Valid) Definition Traditionally, an interposer: • Is a passive intermediate substrate that redis- tributes I/O signals between a semiconductor die and a larger substrate/PCB. • Is made of silicon, glass, or organic lami- nates. • Features fine-pitch wiring, SVs, and redistri- bution layers (RDLs). • Manages mechanical, electrical, and pitch- scaling challenges. With the rise of 2.5D, 3D, and heterogeneous integration, the role of interposers is shifting. Some new categories include: Passive Interposers (Traditional Role) These are still dominant in silicon photonics, GPUs, FPGAs, and HBM memory integration. They provide dense wiring and power distribution. Active Interposers These interposers include embedded transis- tors, power delivery circuits, and signal condition- ing logic. Examples include managing clock distri- bution, voltage regulation, and high-speed signal re-timing. They are increasingly discussed for use in chiplet ecosystems, where dies from multiple vendors need standardized interconnection. Organic / Hybrid Interposers Laminates + build-up films are now competing with silicon in cost-sensitive applications. Some use RDL fan-out interposers (no TSVs) for mid-perfor- mance, lower-cost solutions. Glass Interposers This group is emerging because of the low cost, low loss, dimensional stability, and ability to handle very high-density I/O. Expansion of the Term 'Interposer' The term "interposer" once meant mainly TSV- based silicon bridges. But today, the term can cover RDL interposers (fan-out redistribution layers that mimic interposers without TSVs), as well as embedded bridge technologies like Intel's EMIB (Embedded Multi-die Interconnect Bridge), some- times called "localized interposers." Even wafer- level packaging substrates with high-density inter- connects are blurring the line. In other words, the boundary between "interposer," "substrate," and "advanced packaging layer" is less rigid now. C o m p a re a n d C o nt ra st : I nte r p o s e rs vs . S u b st rate s ⊲