Interposers，substrates，and advanced manufacturing

At many industry trade shows and technical seminars, when technical experts exchange ideas, there remains some confusion regarding the “true definition of an interposer.”

A frequently asked question is: “Are all interposers substrates?” The short answer is: No. However, it is worth noting that some interposers can indeed be considered complete substrates.

This article outlines the development, composition, and role of interposers (also known as interposers) in the electronics field, as well as their relationship with substrates.

Core Definition

In advanced electronics manufacturing, an interposer is a physical interface or intermediate substrate used to establish an electrical connection between two components when direct interconnection is difficult or even impossible.

Interposers are typically very thin (common materials include silicon, glass, or organic materials) and are placed between a chip and a package substrate, or between a package and a printed circuit board (PCB).

It acts as a bridge, redistributing signal, power, and ground connections via fine-pitch routing or through-silicon vias (TSVs).

• Functions and Advantages in Advanced Packaging

Interposers enable high-density interconnects and fine-pitch routing, thereby connecting the tiny pads on an integrated circuit (IC) to the larger, manufacturable pads on a PCB.

In advanced packaging, interposers also allow multiple chips (such as logic chips and memory chips) to be mounted side-by-side within a single package.

This type of packaging is very common in high-performance computing applications, such as graphics processing units (GPUs) and field-programmable gate arrays (FPGAs).

Interposers help address differences in trace spacing and dimensions between the chip and the substrate, thereby reducing stress and improving assembly reliability.

By shortening interconnect lengths and increasing routing density, interposers enhance electrical performance (such as improving signal integrity and reducing latency) and, to some extent, improve thermal management capabilities.

• Types of Interposers and Structural Comparison

Interposers can be broadly categorized into three types:

① Silicon interposers: utilize TSVs to achieve ultra-fine pitch, high-performance interconnects;

② Glass interposers: are relatively low-cost and offer good dimensional stability;

③ Organic interposers: made from advanced laminate materials, they are even lower in cost but have lower routing density than silicon interposers.

In summary, interposers provide a high-density interconnection layer between the chip and the substrate, making them crucial for the advanced packaging of modern semiconductor devices.

A comparison of interposers and substrates is shown in Table 1.

Table 1: Comparison Between Interposer and Substrate

Evolution of Interposers

With advancements in semiconductor manufacturing and advanced packaging technologies, the definition of an “interposer” has also continued to evolve.

This will be discussed in detail below. In the past, an interposer referred to:

① a passive intermediate substrate used to redistribute I/O signals between a semiconductor chip and a larger carrier board or PCB;

② a substrate made of silicon, glass, or organic laminate;

③ Featuring fine-pitch interconnects, through-silicon vias (TSVs), and redistribution layers (RDLs);

④ Designed to address mechanical, electrical, and scaling challenges. With the rise of 2.5D, 3D packaging, and heterogeneous integration, the role of interposers is changing. 

New types of interposers

Several new types of interposers have emerged, including:

(1) Passive interposers (traditional role).

These interposers remain widely used in fields such as silicon photonics, GPUs, FPGAs, and high-bandwidth memory (HBM) integration, all of which require high-density interconnects and power distribution.

(2) Active interposers.

These interposers integrate transistors, power delivery circuits, and signal conditioning logic. Their typical functions include managing clock distribution, voltage regulation, and high-speed signal retiming.

Active interposers are receiving increasing attention in the chiplet ecosystem, as chiplets from different vendors require standardized interconnects.

(3) Organic/hybrid interposers.

In cost-sensitive applications, laminate and stacked film technologies are emerging as strong competitors to silicon-based solutions.

To achieve moderate performance and lower costs, some solutions employ RDL fan-out interposers without silicon through-silicon vias (TSVs).

(4) Glass interposers. Glass interposers are gaining traction due to their low cost, low loss, excellent dimensional stability, and ability to handle ultra-high-density I/O.

Expansion of the Term “Interposer”

The term “interposer” originally referred primarily to silicon bridges based on TSV technology.

However, the scope of the term has since expanded to include RDL interposers (fan-out redistribution layers, which simulate interposers without TSVs) and embedded bridging technologies such as Intel’s Embedded Multi-Die Interconnect Bridge (EMIB), sometimes referred to as “local interposers.” Even wafer-level packaging substrates with high-density interconnects are blurring this line.

In other words, the boundaries between “interposers,” “substrates,” and “advanced packaging layers” are no longer as rigid as they once were.

For various technical reasons, the definition of the term “interposer” is continually evolving.

(1) Chiplet Architecture:

Companies aim to connect chips from different process nodes, foundries, or suppliers. Interposers are no longer merely “wiring boards”—they are becoming integrated platforms.

(2) Heterogeneous integration:

Logic, memory, analog, optoelectronic, and radio frequency (RF) modules can all be integrated onto the same interposer or interconnected via the same interposer.

(3) Power delivery and thermal management constraints:

Active interposers can help manage local power and thermal conditions at the packaging level.

Evolution of Interposers in Electronics Manufacturing

• Early Stage (2000s–Early 2010s): Classic Silicon Interposers

Definition: Passive silicon boards featuring TSVs and RDL.

Applications: 2.5D packaging in which multiple chips (e.g., logic chips and HBM memory) are mounted on an interposer, which is then mounted onto a package substrate.

Examples: Xilinx FPGAs, AMD GPUs, and HBM memory.

Industry Perspective: The term “interposer” is virtually synonymous with “TSV-based silicon.”

• Development Stage (Mid-2010s): Organic and Glass Interposers

Organic interposers:

① Made from laminates (ABF film, BT resin);

② Lower cost, with wider pitch than silicon;

③ Suitable for applications with low TSV density requirements.

Glass Interposers:

① Currently in the research and early adoption phase;

② Excellent dimensional stability, low electrical loss, and potential for ultra-fine-pitch drilling.

Industry Impact: Today, the term “interposer” encompasses a variety of material types and is no longer limited to silicon.

• Redefinition Phase (Late 2010s–Early 2020s): Bridging and RDL Fan-Out Technologies

Fan-Out RDL Interposers:

① A redistribution layer on a restructured wafer or panel;

② Functions similarly to an interposer without TSVs;

③ Primarily used in advanced fan-out wafer-level packaging (FOWLP).

Embedded Bridging (Partial Interposers):

① Intel’s EMIB is a typical example;

② Small interposers “slices” are embedded only where needed, reducing costs compared to full-size silicon interposers.

Industry Impact: The term “interposer” has been expanded to include partial/local interconnection platforms.

• Next Generation (2020s–Future): Active Interconnect Boards

Active interconnect boards are not only capable of redistributing signals but also integrate active circuits:

① power management (e.g., voltage regulators);

② clock distribution and signal retiming;

③ security or interconnect logic for use in chip-on-chip ecosystems.

Examples:

Research in Europe [such as the European Organization for Nuclear Research (CERN) and the French Atomic Energy Commission’s Electronics and Information Technology Laboratory (CEA-Leti)] as well as commercial development by wafer fabs and outsourced semiconductor assembly and test (OSAT) companies.

Industry Impact:

The boundaries between “substrates,” “interposers,” and “system-level interposers” have become blurred.

Today, the industry uses “interposer” as a broad term to refer to any component located between the chip and the substrate that enables high-density integration.

This definition is no longer limited to passive silicon interposers with TSVs. Table 2 summarizes these four developmental phases.

表2

A Common Foundation: High-Density Interconnect (HDI)

In advanced semiconductor packaging, substrate boards resemble miniature high-density interconnect (HDI) PCBs in many ways.

For example, the laser-drilled microvias, fine-line traces, and multilayer stacking technologies used in substrate boards were originally applied in HDI PCB manufacturing.

As line width and spacing requirements shrink to single-digit micrometers, carrier substrate manufacturing begins to resemble state-of-the-art PCB manufacturing technology, albeit with smaller geometries and stricter tolerances.

The evolution of PCBs toward ultra-high-density interconnect (UHDI) parallels the changes occurring in advanced packaging carrier substrates.

(1) Substrates and HDI Boards:

Evolution Driven by Demand. Early substrates could be manufactured using traditional PCB-like processes.

As the number of chip I/Os increased, substrates had to evolve; engineers adopted techniques such as lamination, stacked vias, and laser drilling, all of which became hallmark features of HDI.

Currently, high-end substrate boards used for CPUs, GPUs, and application-specific integrated circuits (ASICs) feature UHDI-level technical characteristics: line widths/spacing of less than 10 µm, microvias of less than 50 µm, and highly controlled interlayer alignment.

In many respects, carrier board manufacturing has already surpassed HDI PCB technology, and the PCB industry is striving to catch up.

(2) Interposers and UHDI Boards:

Areas of Overlap. Silicon interposers with TSVs are manufactured using semiconductor processes rather than PCB processes.

However, RDL fan-out interposers (such as those used for FOWLP) and glass/organic interposers employ processes very similar to HDI PCB manufacturing, albeit with extremely high resolution requirements.

Some in the industry even refer to fan-out panels as “PCB-like interposers” because their tool sets and design principles (such as through-holes, laminates, and redistribution layers) overlap significantly with advanced PCB manufacturing processes.

UHDI technology directly supports the density required for chiplet interconnects, which is precisely where interposers come into play.

• Manufacturing convergence.

In the past, PCB manufacturers, substrate manufacturers, and semiconductor manufacturers operated independently.

However, with the development of HDI/UHDI technology, these distinctions are gradually blurring.

PCB manufacturers are investing in UHDI capabilities to support the production of next-generation substrates.

Substrate manufacturers (such as Ajinomoto, Ibiden, Unimicron, etc.) are adopting PCB-style panel processing with higher resolution.

Large panels are being used for fan-out packaging, and the equipment employed overlaps with that used in PCB manufacturing.

This convergence means that the skills and processes involved in HDI/UHDI PCB manufacturing are directly driving the development of interposers and substrates.

Relationship Between Adapter Boards, Substrates, and PCBs

• Why This Matters

As the boundary between chip packaging and PCBs continues to blur, substrate and UHDI PCB technologies are becoming increasingly interoperable.

In some technology roadmaps, “circuit boards” and “packages” will merge into a single integrated platform.

PCB manufacturers with UHDI expertise may find opportunities to enter the advanced packaging market, while OSAT (Outsourced Semiconductor Assembly and Test) companies and substrate manufacturers are exploring panel-scale manufacturing.

UHDI technology is expected to play a critical role not only for PCBs but also for all interconnect platforms, including carrier boards, interposers, and even potential chip-on-board (CoB) integration.

Carrier boards have long been a specialized branch of HDI PCB manufacturing, continuously driving finer features to keep pace with semiconductor advancements.

Interposers, particularly organic interposers and fan-out interposers, are closely related to UHDI processes and typically employ PCB-like manufacturing methods.

As HDI evolves into UHDI, the distinctions between PCB, substrate, and interposer manufacturing are narrowing, foreshadowing a future where they will form a continuum of high-density interconnect platforms.

• Still confused? Here’s a helpful analogy:

Interposer = Adapter/Converter: It distributes the ultra-fine signals coming from the chip.

Carrier = Base/Support Board: It connects these adapted signals to the entire package and links it to the outside world.

• Key Differences

Interposer: Connects chips to other chips or to a carrier board via ultra-fine-pitch wiring.

Carrier Board: Connects the package to the PCB via larger-scale wiring and provides mechanical support.

• Impact on PCBs

As technology continues to advance, traditional definitions—and even the functions of carrier boards and adapter boards—are becoming increasingly broad. Where exactly will this leave PCBs?

Based on the information presented here, carrier boards and adapter boards—once merely connectors between PCBs and chips—now serve as more complex interconnection carriers, paralleling the traditional role of PCBs in many respects.

So, will both continue to coexist, or will carrier board and/or adapter board technology render PCBs obsolete once certain technological boundaries are crossed?

For many technologies, it will still be the most economical option for interconnection.

But as HDI gradually transitions to UHDI, and military/aerospace/defense suppliers face pressure to provide state-of-the-art high-tech manufacturing capabilities—beyond what domestic suppliers can currently achieve—might the manufacturing of carrier boards and adapter boards, despite requiring different equipment and materials, become the next logical evolution for traditional PCB factories?

Conclusion

Looking at the PCB manufacturing industries in the United States and Europe, it is currently unrealistic to repatriate advanced PCB manufacturing from China to the U.S. because private companies cannot bear the high costs.

Therefore, is the natural solution to move beyond advanced PCB technology and directly enter the manufacturing of substrate and interposer boards, thereby ensuring that the manufacturing sector can master key technologies in the future and secure its pivotal role in future technologies?

Indeed, there are currently more questions than answers.

However, the blurring of boundaries and expanding definitions in traditional substrate and interposer technologies have far-reaching implications for the PCB market and its downstream processes.