Interposers, Substrates, and Advanced Manufacturing

Estimated reading time: 6 minutes

I attend a lot of industry trade shows and conferences. Lately, during conversations with technologists, I’ve noticed that there is some confusion about what exactly constitutes an interposer.

One question I hear every so often is, “Are all interposers substrates?” The short answer to that question is no. But some interposers are, in fact, full substrates.

The following is a brief overview of interposers—their development, composition, function in electronics, and relationship to substrates.

A Core Definition

In advanced electronics manufacturing, an interposer is a physical interface or intermediate substrate that provides electrical connections between two components that would otherwise be difficult or impossible to connect directly.

An interposer is typically a thin layer—often made of silicon, glass, or organic material—placed between a chip (die) and a package substrate, or between a package and a PCB. It acts as a bridge that redistributes signals, power, and ground connections using fine-pitch wiring or through-silicon vias (TSVs).

Interposers allow high-density interconnects and fine-pitch routing from the tiny pads on ICs to larger pads that are manufacturable on PCBs. In advanced packaging, interposers enable multiple chips (logic, memory, etc.) to be mounted side-by-side in one package. This is common in high-performance computing (e.g., GPUs, FPGAs).

They help manage differences in pitch and size between chips and substrates, reducing stress and improving assembly reliability. By shortening interconnect lengths and improving routing density, interposers enhance electrical performance (signal integrity, reduced latency) and can help with heat dissipation.

Interposers tend to fall into one of three camps:

• Silicon interposers feature TSVs for very fine-pitch, high-performance connections.
• Glass interposers offer low cost and good dimensional stability.
• Organic interposers are made from advanced laminate materials. They are lower in cost but support less dense routing than silicon interposers.

In a nutshell, an interposer is an enabling technology in advanced electronics packaging that serves as a high-density interconnection layer between chips and substrates, and it’s crucial for modern semiconductor devices.

Comparison and Contrast: Interposers vs. Substrates

Interposer:

• Sits between a die (chip) and the package substrate, or between multiple dies in 2.5D/3D packaging.
• Its primary role is to redistribute signals and manage fine-pitch connections that cannot be routed directly.
• Can be silicon, glass, or organic.
• Often includes TSVs for vertical interconnects.
• Has very fine pitch—they can handle micro-bumps in the range of a few microns.
• More complex and costly (especially silicon interposers). Used in high-performance computing (HPC, GPUs, FPGAs, HBM memory integration).

Substrate:

• Forms the base layer of the package that interfaces with the PCB.
• Primary role: provide mechanical support and electrical interconnects from the chip/package to the PCB.
• Typically made of organic laminates (BT resin, Ajinomoto build-up films, etc.), sometimes ceramic for high reliability.
• Has a much larger pitch—connects package to PCB using solder balls (BGAs).
• Acts more like a “mini PCB.”
• Less expensive, mass-produced.
• Standard in nearly all semiconductor packaging (CPUs, smartphones, memory).

The Evolution of Interposers

The definition of an interposer is evolving as semiconductor 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 redistributes I/O signals between a semiconductor die and a larger substrate/PCB.
• Is made of silicon, glass, or organic laminates.
• Features fine-pitch wiring, TSVs, and redistribution 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 transistors, power delivery circuits, and signal conditioning logic. Examples include managing clock distribution, 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-performance, 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), sometimes called “localized interposers.” Even wafer-level packaging substrates with high-density interconnects are blurring the line. In other words, the boundary between “interposer,” “substrate,” and “advanced packaging layer” is less rigid now.

The definition of the term “interposer” is broadening for a variety of technological reasons.

• Chiplet architectures: Companies want to connect dies from different process nodes, foundries, or vendors. Interposers are becoming more than “wiring boards”—they are integration platforms.
• Heterogeneous integration: Logic + memory + analog + photonics + RF can all sit on/through the same interposer.
• Power delivery & thermal constraints: Active interposers may help manage local power/heat at the package level.

Evolution of Interposers in Electronics Manufacturing

1. Early Generation (2000s – early 2010s): Classical Silicon Interposers

• Definition: Passive silicon plates with through-silicon vias (TSVs) and redistribution layers.
• Use case: 2.5D packaging, where multiple dies (e.g., logic + HBM memory) are mounted on an interposer and then onto a package substrate.
• Examples: Xilinx FPGAs, AMD GPUs with HBM.
• Industry View: Interposers were synonymous with TSV-based silicon.

2. Broadening (mid-2010s): Organic & Glass Interposers

• Organic Interposers:
  • Built from laminates (Ajinomoto build-up film, BT resin).
  • Lower cost, coarser pitch than silicon.
  • Used where TSV density wasn’t essential.
• Glass Interposers:
  • Entered research and early adoption.
  • Benefits: dimensional stability, low electrical loss, potential for very fine-pitch drilling.
• Industry Impact: “Interposer” now covered multiple material classes, not just silicon.

3. Redefinition (late 2010s – early 2020s): Bridges & RDL Fan-Out

• Fan-Out RDL Interposers:
  • Redistribution layers on reconstituted wafers or panels.
  • Function like interposers without TSVs.
  • Found in advanced fan-out wafer-level packaging (FOWLP).
• Embedded Bridges (localized interposers):
  • Intel’s EMIB is the flagship example.
  • Small interposer “slivers” embedded only where needed, reducing cost vs. full silicon interposers.
• Industry Impact: The word “interposer” broadened to include partial/localized connection platforms.

4. Next-Gen (2020s → future): Active Interposers

• Definition: Interposers that not only redistribute signals, but also integrate active circuits:
  • Power management (voltage regulators).
  • Clock distribution and signal re-timing.
  • Possible security or interconnect logic for chiplet ecosystems.
• Example: Research in Europe (CERN, CEA-Leti) and commercial development by foundries and OSATs.
• Industry Impact: Blurring of lines between “substrate,” “interposer,” and “system-on-interposer.”

The industry now uses “interposer” as a broad umbrella term for anything that sits between dies and substrates to enable high-density integration. The definition is no longer limited to passive silicon with TSVs.

Common Ground: HDI

Substrates in advanced semiconductor packaging are, in many ways, miniature HDI PCBs. They use laser-drilled microvias, fine-line routing, and multilayer stack-ups, all of which are technologies pioneered in HDI PCB manufacturing.

As line/space requirements shrink to single-digit microns (2–5 µm for UHDI), substrate manufacturing begins to look almost identical to the cutting edge of PCB manufacturing, just at smaller geometries and tighter tolerances.

The push toward UHDI in PCBs mirrors what has already been happening in advanced package substrates.

Substrates and HDI: Evolution by Necessity

Early substrates could be made with conventional PCB-like processes. As chip I/O counts rose, substrates had to evolve, with technologists adopting build-up layers, stacked vias, and laser drilling, which are hallmarks of HDI.

Today’s high-end substrates (for CPUs, GPUs, networking ASICs) are produced with UHDI-level features: <10 µm lines/spaces, microvias <50 µm, and highly controlled layer-to-layer registration. In many respects, substrate manufacturing has leapfrogged PCB HDI technologically, and the PCB industry is catching up.

Interposers and UHDI: The Overlap

Silicon interposers (with TSVs) are manufactured using semiconductor processes, not PCB processes. But the RDL fan-out interposers (like those used in FO-WLP) and glass/organic interposers use processes very similar to HDI PCB fabrication, but only at extreme resolutions.

Some in the industry even call fan-out panels “PCB-like interposers” because the toolsets and design principles (vias, laminates, redistribution layers) overlap with advanced PCB manufacturing. UHDI technology directly supports the density needed for chiplet interconnection, which is exactly where interposers operate.

Manufacturing Convergence

Historically, PCB fabs, substrate fabs, and semiconductor fabs were distinct. But with HDI/UHDI, those distinctions are blurring.

PCB fabricators are investing in UHDI capability to support next-gen substrates. Substrate makers (Ajinomoto, Ibiden, Unimicron, etc.) are adopting PCB-style panel processing at finer resolutions. Fan-out packaging is being done on large panels, using equipment that overlaps with PCB manufacturing.

This convergence means the skills and processes of HDI/UHDI PCB manufacturing are directly enabling the evolution of interposers and substrates.

Why Does This Matter?

As the boundary between chip package and PCB blurs, the technologies for substrates and UHDI PCBs are becoming interoperable. In some roadmaps, the “board” and the “package” will merge into a single integration platform.

PCB manufacturers with UHDI expertise may find opportunities to enter advanced packaging markets, while OSATs and substrate makers are exploring panel-scale manufacturing. UHDI capability is expected to be essential not just for PCBs, but for all interconnect platforms, including substrates, interposers, and possibly hybrid chip/board integration.

Substrates have long been a specialized branch of HDI PCB manufacturing, pushing finer features to keep pace with semiconductors. Interposers, especially organic and fan-out types, are deeply tied to UHDI processes and often use PCB-like manufacturing methods. As HDI evolves into UHDI, the distinction between PCB, substrate, and interposer manufacturing is narrowing, pointing toward a future where they form a continuum of high-density interconnect platforms.

Still Confused? A Handy Analogy

• Interposer = adapter/translator: It takes the ultra-fine signals from the die and disperses them.
• Substrate = base/carrier board: It takes those adapted signals and connects the whole package to the outside world (PCB).

Key Differences

• Interposers connect die-to-die or die-to-substrate with ultra-fine pitch routing.
• Substrates connect package-to-PCB with larger-scale routing and mechanical support.