Substrates and Interposers

Substrate and interposer

The Substrate Carries the Package; the Interposer Carries the Bandwidth

An HBM stack has more than 1,000 signals, so wiring four stacks to a logic die takes thousands of connections at a pitch no organic laminate can route.

In a simple flip-chip package the die sits directly on an organic substrate, which connects to the board. In a 2.5D module such as TSMC CoWoS (Chip-on-Wafer-on-Substrate), an interposer sits between the dies and the substrate and carries the dense die-to-die wiring; the substrate below handles power, ground, slower I/O and the mechanical mount.

• Organic substrates: flip-chip ball grid array (FCBGA) laminates built with Ajinomoto Build-up Film (ABF), the carrier under every server CPU, GPU and AI accelerator.
• Silicon interposers: silicon with through-silicon vias (TSVs), made captive by the foundry that makes the logic die (TSMC CoWoS-S, Samsung I-Cube).
• Silicon bridges: small silicon pieces embedded in the organic substrate where dense links are needed (Intel EMIB, TSMC CoWoS-L).
• Redistribution layer (RDL) interposers: thin-film copper routing, used in CoWoS-R and fan-out packages.
• Glass cores: the next carrier, in qualification at Absolics and sampling in Korea.
• Ceramic: hermetic carriers for aerospace, defense and high-reliability parts.

The routing gap

Silicon Routes 10 to 50 Times Finer Than an Organic Substrate

A silicon interposer routes thousands of signals across a few millimeters of silicon at a density close to the foundry's lower metal layers. AI interposers have outgrown the single lithography reticle field of about 858 mm², so TSMC now stitches interposers several reticles across; interposer sizes by generation are on CoWoS. Bridges and RDL trade some density for cost by putting silicon or fine copper only where it is needed.

Organic substrates

A Handful of Asian and European Makers Supply Advanced FCBGA Substrates

Organic substrates are laminated stacks: a glass-epoxy core (FR-4 family or BT resin) with ABF build-up layers carrying fine copper traces and vias. A new substrate line takes 18 to 24 months from announcement to first output, and qualifying an advanced substrate at a customer takes several more quarters, so capacity answers demand slowly. Substrate makers and analysts expect AI demand to keep high-end substrates tight into 2027.

• Made abroad

ABF and T-glass

Two Japanese Materials Set the Ceiling on Substrate Output

• ABF film: Ajinomoto, better known for food seasonings, holds above 95% of the build-up film used in CPU and GPU substrates. The substrate makers buy the film and laminate it; none makes the dielectric itself. Ajinomoto plans to raise ABF capacity by half by 2030, with at least JPY 25 billion committed.
• T-glass cloth: the low-expansion glass-fiber cloth inside advanced substrate cores comes mainly from Nittobo (Nitto Boseki). AI substrate demand roughly doubled the call on high-performance glass cloth from 2025 to 2026, pulling supply away from BT substrates for SSD controllers and phones. Taiwanese weavers are qualifying alternatives.

Either material can stall substrate output regardless of how many substrate lines exist. Both appear on the Bottleneck Atlas.

Interposers and bridges

Interposers Are Made by the Foundry; Bridges Are Embedded by the Substrate Maker

Silicon interposers, typically about 100 µm thick after thinning, carry TSVs and one or more BEOL copper layers. There is no merchant market for advanced interposers: each foundry sells the interposer as part of its packaging service.

• TSMC: CoWoS interposers in Taiwan; Arizona interposers follow the planned Arizona packaging plant, due before 2029.
• Samsung Foundry: I-Cube interposers, captive in Korea.
• Intel: EMIB bridges and Foveros base dies, made in the US and Malaysia; see EMIB.

Bridges are tens of square millimeters, use far less foundry wafer area than a full interposer, and leave most of the module on organic substrate; embedding them is a specialty step at the substrate maker. RDL interposers, used in CoWoS-R and in fan-out, sit between organic and silicon in density and cost. Which chips use which carrier is mapped on the chip supplier rings.

Glass substrates

Every Glass-Core Timeline Has Slipped; Absolics Is Closest

Glass cores warp less than organic laminate, route finer than 2 µm line and space, and expand with heat at close to the rate of silicon. They are brittle, and they need via-formation and handling equipment the organic supply chain does not have. No commercial glass-core product had shipped by October 2026.

• US planned
• Made abroad

Other packaging materials

The Materials That Hold a Package Together

Beyond the substrate, every package uses mold compound, underfill, solder and attach materials, most from Japanese and German chemical makers.

Ceramic

Ceramic Carriers for Space, Defense and Hermetic Parts

Ceramic substrates serve aerospace, space, military and high-temperature industrial markets, where hermetic sealing, thermal stability and radiation tolerance justify their cost and size. Kyocera, NGK Spark Plug (Niterra) and CoorsTek lead supply. Rad-hard parts and their fabs are on Rad-Hard Fabs.

Related coverage

Related Coverage

• CoWoS
• EMIB
• Fan-Out WLP
• Advanced Packaging Overview
• OSAT Landscape
• Chip Supplier Rings
• Bottleneck Atlas