A silicon interposer, or silicon bridges in an organic layer, carries thousands of fine wires between the logic dies and the HBM. Building it and assembling the dies onto it is the advanced-packaging step.
What does 2.5D packaging do?
A GPU and its HBM need thousands of connections that a circuit board cannot route, so they sit side by side on an interposer with far finer wiring, which in turn sits on a larger substrate. TSMC’s CoWoS comes in three forms: a silicon interposer (CoWoS-S, up to 3.3 reticles), a polymer-and-copper redistribution layer (CoWoS-R) and local silicon bridges (CoWoS-L, in volume since 2024). Each Blackwell GPU joins two reticle-limited dies at 10 TB/s, and TSMC says it produces 5.5-reticle CoWoS and plans a 14-reticle version with about 10 compute dies and 20 HBM stacks for 2028. That Blackwell uses CoWoS-L is widely reported but not confirmed in our sources.
Who packages the accelerator?
NVIDIA’s FY2026 10-K says it uses TSMC wafers and CoWoS packaging. In April 2025 NVIDIA named Amkor and SPIL, a wholly owned subsidiary of ASE Technology, as its packaging and testing partners in Arizona. ASE’s 2025 Form 20-F lists 2.5D, CoWoS, CoPoS, CoWoP, CPO and 3D IC among its solutions, and Amkor’s 2025 10-K describes packages that use 2.5D silicon interposers to join HBM and graphics processors. Neither says which steps it performs for a given GPU, and Amkor’s Arizona facility was still under construction at its 10-K date. Computing was 20% of Amkor’s 2025 net sales, so AI packaging is one business among several.
Where does the ABF substrate fit, and who makes it?
The substrate is the multilayer organic base that fans the package out to board pitch, built with Ajinomoto Build-up Film as the insulating layer. Ibiden reported GPU, AI-server switching IC and high-end CPU substrate demand above plan, and industry demand exceeding supply, in its quarter to June 30, 2026; Samsung Electro-Mechanics reported more FCBGA supply for AI accelerators and server CPUs in Q2 2026. Unimicron markets FCBGA up to 110 × 110 mm for AI servers and switches, Nan Ya PCB is developing substrates larger than 100 mm square, and ASE’s 20-F names Kinsus, Nan Ya PCB and Unimicron among its substrate suppliers. None names a GPU customer, and Ajinomoto sells the film, not substrates.
How to research a packaging or substrate company
- Split the chain into steps: wafer fabrication, chip-on-wafer or interposer assembly, substrate attach and final test. Different companies can do each step for the same GPU.
- Separate a named partner from a capability: NVIDIA’s Arizona list names Amkor and SPIL, while a filing that lists 2.5D packaging shows capability without a customer.
- Record the variant and whether it is confirmed. CoWoS-S, CoWoS-R and CoWoS-L are different structures, and press reports of which one a GPU uses set no evidence level.
- Keep substrate makers and material suppliers apart: Ibiden builds substrates, Ajinomoto sells the film they are built with, and stock3d lists the film maker as an upstream enabler.
- Check the group: SPIL sits inside ASE, whose US line is the NYSE ADR ASX, and TSMC holds 34.84% of GUC, which is listed separately and is not a stand-in for TSMC.
- Use the list as a starting point: it is curated, not exhaustive, and the model is illustrative, not a bill of materials.
Sources
- TSMC: CoWoS
CoWoS-S (silicon interposer, up to 3.3 reticles), CoWoS-R (RDL interposer) and CoWoS-L (local silicon bridges, in volume since 2024).
- TSMC: 2026 North America Technology Symposium
5.5-reticle CoWoS in production; a 14-reticle CoWoS with about 10 compute dies and 20 HBM stacks slated for 2028.
- NVIDIA: Blackwell architecture
Two reticle-limited dies joined at 10 TB/s on a custom TSMC 4NP process, 208 billion transistors per GPU.
- NVIDIA: FY2026 Form 10-K
NVIDIA uses TSMC wafers and CoWoS packaging.
- NVIDIA: American-made AI supercomputers, Apr 14, 2025
Amkor and SPIL named as NVIDIA’s packaging and testing partners in Arizona.
- ASE Technology: 2025 Form 20-F
2.5D, CoWoS, CoPoS, CoWoP, CPO and 3D IC solutions; SPIL wholly owned; Kinsus, Nan Ya PCB and Unimicron among its IC substrate suppliers; ABF substrate supply risk from AI demand.
- Amkor: FY2025 Form 10-K
2.5D interposer packages for HBM and graphics processors, an Arizona facility under construction, and computing at 20% of 2025 net sales.
- Ibiden: Q1 FY2026 Q&A, Aug 2026
Higher-than-planned demand for GPUs, switching ICs for AI servers and high-end CPUs; EMIB-T development.
- Samsung Electro-Mechanics: Q2 2026 business results
Increased FCBGA supply for AI accelerators and server CPUs to unnamed big-tech customers.
- Unimicron: FCBGA
FCBGA substrates up to 110 × 110 mm for HPC and AI server and switch applications, including 2.5D packaging.
- Nan Ya PCB: IC substrate development
High-layer-count (>22L), large-form-factor (>100 mm square) substrates in development for AI and HPC.
- Ajinomoto: Build-up Film (ABF)
ABF is the insulating film of multilayer package substrates, bridging a chip’s fine terminals to board-scale parts.
Engineering reference: GPU module example. Company-specific evidence is linked from the profiles below.
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