Glass Core Substrate: Solving Warpage & I/O in Packaging

1. Introduction: The “Glass” Ceiling of Moore’s Law

As we navigate through 2025 and into 2026, the semiconductor industry is hitting a hard physical wall. While transistor scaling continues (albeit slowly), the real bottleneck has shifted from the die to the package. The era of the “System on Chip” (SoC) is evolving into the “System in Package” (SiP), driven by the insatiable demands of Artificial Intelligence (AI) and High-Performance Computing (HPC).

Leading AI accelerators, such as Nvidia’s Blackwell series and AMD’s MI300, have pushed organic substrates to their absolute limits. The industry is facing a trilemma: Warpage, I/O Density, and Thermal Management. The traditional organic core substrate, a staple for decades, is struggling to support the massive chiplet complexes that define modern computing.

Enter the Glass Core Substrate. Once a research novelty, it is now “heating up” as the definitive solution for the next decade of advanced packaging. This article explores why glass is not just an alternative, but a necessity for solving the warping issues of large packages, enabling higher I/O counts, and supporting the 2.5D and 3D packaging architectures (CoWoS, SoIC) of the future.

2. The Core Problem: Why Organic Substrates Are Failing

To understand the rise of glass, we must first understand the failure of organic materials. Standard FC-BGA (Flip Chip Ball Grid Array) substrates use an organic core (typically fiberglass-reinforced epoxy).

2.1 The Warpage Nightmare

As AI packages grow larger—exceeding 100mm x 100mm to accommodate more HBM (High Bandwidth Memory) stacks and larger logic dies—they become prone to warpage.

• CTE Mismatch: The Coefficient of Thermal Expansion (CTE) of organic substrates (12-17 ppm/°C) is vastly different from silicon die (2-3 ppm/°C). During the reflow process, this mismatch causes the package to curl like a potato chip.
• Yield Killer: Warpage leads to non-contact opens or bridged shorts in the thousands of micro-bumps connecting the die to the substrate. For a $30,000 AI chip, a 1% yield loss due to warpage is economically unacceptable.

2.2 The I/O Density Limit

Organic substrates rely on mechanical drilling or laser drilling for vias, which limits how close these connections can be.

• Rough Surfaces: Organic materials have inherent surface roughness, limiting the fineness of the Redistribution Layer (RDL) lines.
• Signal Loss: At high frequencies required for next-gen interconnects (like 224G SerDes), organic materials suffer from higher dielectric loss, degrading signal integrity.

3. Glass Core Substrate: The Technical Savior

Glass Core Substrates (GCS) replace the organic core with a slab of specialized glass. This simple material swap fundamentally alters the physics of packaging.

3.1 Unmatched Dimensional Stability

Glass is rigid. Its Young’s Modulus is significantly higher than organic materials.

• Tunable CTE: The CTE of glass can be chemically tuned to perfectly match silicon (~3 ppm/°C). This virtually eliminates warpage, even for massive substrate sizes needed for next-gen AI clusters.
• Flatness: Glass is manufactured to be atomically flat. This flatness allows for extremely fine lithography, enabling finer lines and spaces (L/S) in the RDL.

3.2 Through-Glass Vias (TGV): The Density Multiplier

The magic of glass lies in Through-Glass Vias (TGV).

• High Density: TGVs can be created with a pitch of less than 100 microns, far denser than organic vias.
• Electrical Performance: Glass is an excellent electrical insulator with low dielectric loss. This allows for higher speed signal transmission with less power consumption—critical for data centers where every watt counts.

3.3 Thermal Conductivity

While glass itself isn’t a metal-like heat conductor, its stability allows for thinner substrates and better integration of thermal solutions. Moreover, the ability to withstand higher processing temperatures enables high-performance copper interconnects that were previously impossible on organic cores.

4. Synergies with CoWoS, SoIC, and Chiplets

Glass substrates are not replacing the entire packaging stack immediately; they are integrating into the ecosystem of 2.5D and 3D Packaging.

4.1 Glass vs. Silicon Interposers (CoWoS)

TSMC’s CoWoS (Chip-on-Wafer-on-Substrate) currently uses a silicon interposer to connect logic and HBM.

• The Cost Factor: Silicon interposers are expensive and limited by reticle size (the maximum size a lithography machine can print).
• The Glass Advantage: Glass can be processed on large panels (Panel-Level Packaging), not just round wafers. A 510mm x 515mm glass panel can hold far more interposers than a 300mm wafer, potentially reducing cost while offering similar interconnect density.
• Interposer Replacement: In the long term, glass substrates with fine RDL could eliminate the need for the silicon interposer entirely, allowing chips to be mounted directly on the glass core.

4.2 3D Stacking and SoIC

For 3D stacking technologies like SoIC (System on Integrated Chips), the rigidity of glass provides a stable base for stacking multiple layers of active silicon. The thermal stability ensures that the precise alignment required for Hybrid Bonding is maintained throughout the package’s lifecycle.

5. Market Landscape: Who is Winning the “Glass War”?

The supply chain is racing to commercialize this technology.

5.1 Intel: The Pioneer

Intel is the most vocal champion of glass substrates, announcing a breakthrough in late 2023. They plan to introduce glass substrates for high-end packaging in the 2026-2030 timeframe. Intel claims glass will allow for 10x higher interconnect density and huge form factors for data center chips.

5.2 SKC (Absolics): The First Mover in Mass Production

While Intel researches, Absolics (an SKC subsidiary) is building. Their plant in Covington, Georgia, is one of the world’s first mass-production facilities for glass substrates. Backed by AMD and other key players, Absolics is positioning itself as the primary supplier for the AI merchant market.

5.3 Samsung & Rapidus

• Samsung Electro-Mechanics (SEMCO): Leveraging its massive MLCC and substrate experience, Samsung is fast-tracking glass substrate development, aiming for pilot lines in 2025/2026.
• Rapidus: The Japanese contender recently unveiled (Dec 2025) a glass interposer strategy to challenge TSMC, betting that glass will be the wedge they need to break into the advanced packaging market.

6. Challenges and Roadblocks

Despite the hype, “Glass Core” is not without cracks.

• Brittleness: Glass breaks. Handling ultra-thin glass cores during manufacturing without cracking them requires entirely new equipment and handling protocols.
• TGV Reliability: Creating reliable metal-filled vias in glass is difficult. Issues like copper migration or stress cracking around the vias are still being solved.
• Supply Chain Maturity: Unlike the organic substrate supply chain which is vast and mature, the glass supply chain (glass formulation, laser drilling tools, inspection) is still in its infancy.

7. Future Trends: Panel-Level Packaging (PLP)

The ultimate endgame for glass is Fan-Out Panel Level Packaging (FOPLP). By moving from wafers to large glass panels, the industry can achieve economies of scale similar to LCD manufacturing. Glass substrates are the enabler for this shift, providing the dimensional stability needed to process large panels without warping.

8. Conclusion

Glass Core Substrates are not just a material upgrade; they are a platform shift. They solve the critical Warpage problem that threatens to stall AI chip growth. They open the door to Higher I/O densities that silicon interposers struggle to scale to cost-effectively.

For the editor at Whychip.com, the message is clear: Glass is the new silicon for packaging. As we move toward 2027, expect to see the first wave of “Glass Inside” AI accelerators powering the data centers of the world. The question is no longer if glass will be adopted, but who will master the manufacturing yield first.

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9. Frequently Asked Questions (FAQ)

Q: Why is Glass Core Substrate better than Organic Substrate?

A: Glass offers superior flatness (less warpage), tunable Thermal Expansion (CTE), and supports much finer interconnects (higher I/O density) compared to organic materials.

Q: Will Glass Substrates replace Silicon Interposers?

A: Eventually, yes. Glass substrates offer similar interconnect densities to silicon interposers but can be manufactured on large panels, offering a significant cost advantage for large-area packages.

Q: Who are the key players in Glass Substrate technology?

A: Key players include Intel, Absolics (SKC), Samsung Electro-Mechanics, Dai Nippon Printing (DNP), and Rapidus.

Q: What is the main challenge for Glass Substrates?

A: The main challenges are brittleness (handling issues) and the maturity of the manufacturing ecosystem, particularly for Through-Glass Vias (TGV).