AtlasPCBEngineering

Posted on May 25 • Originally published at atlaspcb.com

Glass Core Substrates for AI Packaging: How Intel's Clearwater Forest Changes the PCB Substrate Roadmap

#ai #hardware #electronics #semiconductor

The Organic Substrate Wall

For two decades, Ajinomoto Build-up Film (ABF) substrates have been the foundation of advanced semiconductor packaging. The laminate-based approach — layer upon layer of resin-coated copper foil, mechanically drilled and chemically etched — scaled elegantly from flip-chip packages to today's complex multi-die architectures.

But organic substrates have hit a physics wall. As AI accelerators grew larger and more power-dense, three fundamental limitations emerged:

Warpage: Organic materials (CTE 12-17 ppm/°C) expand and contract differently than silicon (CTE 3.1 ppm/°C). At package sizes above 70×70mm, this CTE mismatch creates warpage that prevents reliable flip-chip bonding.
Interconnect density: Standard ABF processes achieve minimum line/space of 8/8 µm in production. AI chiplet architectures require 2/2 µm for die-to-die communication — a 4× density gap.
Signal integrity: At 224G PAM4 signaling speeds emerging in next-generation AI networks, organic dielectric loss tangent (Df 0.003-0.008) becomes a bandwidth limiter. Glass offers Df <0.001 across relevant frequencies.

Intel's decision to bring glass substrates into high-volume manufacturing for Clearwater Forest wasn't incremental improvement — it was an acknowledgment that organic substrates cannot support the AI hardware roadmap beyond current generation.

Intel Clearwater Forest: First HVM Glass Substrate Product

Announced at CES 2026 and entering production in January 2026, Intel's Xeon 6+ "Clearwater Forest" processor represents the first commercial IC product fabricated on glass core substrates at high volume.

Technical Specifications

Die: Multiple chiplets on Intel 18A process
Package size: >80mm × 80mm (beyond organic warpage limit)
Substrate: Glass core with build-up redistribution layers
Interconnect: Through-Glass Vias (TGV) at 50µm pitch
Line/space: 2/2 µm (vs. 8/8 µm on latest organic ABF)
Layers: 10+ redistribution layers on glass core
CTE match: Glass core 3.2 ppm/°C vs. silicon 3.1 ppm/°C

Performance Benefits Demonstrated

Parameter	Organic ABF (Sapphire Rapids)	Glass Core (Clearwater Forest)
Maximum package size	70×70mm (warpage-limited)	>100×100mm (no practical limit)
I/O density	500 bump/mm²	2,500+ bump/mm²
Power delivery resistance	1.0 mΩ typical	0.4 mΩ typical
Signal loss at 56 GHz	-2.5 dB/cm	-0.8 dB/cm
Warpage at reflow	100-300 µm	<30 µm
Die-to-die bandwidth	16 Tbps (limited by pitch)	40+ Tbps

How Glass Substrates Are Made

Glass substrate manufacturing is fundamentally different from organic PCB fabrication:

Step 1: Glass Panel Preparation

Starting material: High-purity glass panels (Gen 3.5 display glass or specialty borosilicate)
Thickness: 300-500 µm (vs. 60-100 µm for ABF core)
CTE engineered via composition: adjustable from 3-8 ppm/°C
Surface preparation: Chemical mechanical polish (CMP) to <1nm roughness

Step 2: Through-Glass Via (TGV) Formation

Laser-induced deep etching: Ultrafast laser creates modification tracks
Chemical etch: HF-based wet etch selectively removes laser-modified glass
Result: Vias at 30-50 µm pitch (vs. 200-300 µm for organic PTH)
Aspect ratio: Up to 20:1 achievable

Step 3: Metallization

Seed layer: PVD sputtered TiW/Cu (vs. electroless Cu for organic)
Pattern plating: Semi-additive process (SAP) at 2/2 µm L/S
Via fill: Electroplated copper fills TGVs from bottom-up
Planarization: CMP produces atomically flat surface for next layer

Step 4: Build-Up Layers

Dielectric: Photo-definable polyimide or inorganic SiO₂ (vs. ABF resin)
Layer count: 6-12 redistribution layers built on each side of glass core
Registration: Lithographic alignment <±1 µm (vs. ±10 µm for organic lamination)

Step 5: Bumping and Singulation

UBM (Under-Bump Metallurgy): Sputtered multi-layer for flip-chip or micro-bump
Singulation: Laser or mechanical dicing of glass panel into individual substrates
Final test: Electrical probing of all interconnects

Why Glass Matters for AI Specifically

The connection between glass substrates and AI hardware is direct and structural:

Power Delivery

AI accelerators consume 500-1000W per package. Power must be delivered through the substrate with minimal resistance to avoid voltage droop during transient workloads. Glass substrates enable:

Thicker, wider redistribution layer traces (lower resistance)
More power/ground via columns (lower inductance)
Better planarity = more uniform bump contact = lower contact resistance

The cumulative effect: 40-60% reduction in package-level power delivery impedance, which translates directly to higher clock frequencies and wider voltage margins.

Chiplet Architecture Support

Modern AI processors (NVIDIA Blackwell, AMD MI400, Intel Clearwater Forest) use multi-chiplet designs where 4-12 compute dies communicate on a shared substrate. These die-to-die links require:

Extremely fine pitch (36-55µm bump pitch between chiplets)
Very flat substrate (warpage <30µm for reliable thermocompression bonding)
Low-loss interconnect (die-to-die links at 16-32 Gbps per lane)

Organic substrates cannot maintain flatness at the package sizes these multi-chiplet designs require. Glass solves this geometrically — matching silicon's thermal expansion eliminates the warpage that prevents scaling.

Bandwidth Density

AI training workloads are memory-bandwidth-limited. HBM4 memory stacks communicate with the processor through thousands of parallel lanes at 8-16 Gbps each. The substrate interconnect density directly limits how much memory bandwidth can reach the compute die:

Organic: ~500 connections/mm² → limited to 4-6 HBM stacks
Glass: ~2,500 connections/mm² → supports 8-12+ HBM stacks per package

More HBM stacks per package = more memory bandwidth = faster training. Glass substrates are literally a prerequisite for next-generation AI training hardware performance scaling.

The Competitive Landscape

Intel: First Mover Advantage

Intel's glass substrate program (announced 2023, HVM 2026) gives them a 2-3 year lead over competitors:

Proprietary TGV formation process
Integrated panel-level manufacturing in their Advanced Packaging facility
Clearwater Forest proves production viability — no longer a research project

Chinese Players Racing to Catch Up

As reported by ETNews and IC&PCB Union, Chinese companies are accelerating glass substrate investment:

Visionox: Display maker pivoting to glass substrates, leveraging existing glass handling and lithography expertise. RMB 5 billion investment program announced for 2026, targeting both display and semiconductor substrate applications.

AKM Meadville: Leading Chinese HDI substrate supplier has established a prototype glass substrate pilot line. Their existing dominance in organic IC substrates provides the customer relationships needed to transition designs to glass.

WG Tech / TGV Tech: Subsidiary focused exclusively on Through-Glass Via technology. Already delivering small-batch glass substrates for 1.6T optical modules — an early commercial application outside of compute.

Korean Push

South Korea is racing to close the packaging gap with Taiwan and China. Samsung's foundry division and Korean substrate makers (Samsung Electro-Mechanics, LG Innotek) are investing in glass substrate R&D to support Samsung's own AI accelerator packaging roadmap.

Impact on the PCB Industry

What Changes for PCB Fabricators

Glass substrates don't replace PCBs — they replace the IC substrate layer between the chip and the PCB. The implications:

Motherboard design changes: Glass substrate packages have different ball patterns, power delivery requirements, and thermal interfaces. PCB motherboards must be redesigned for each glass-substrate processor generation.
Signal integrity requirements increase: With the package-level interconnect no longer the bottleneck, the PCB motherboard traces become the limiting factor. Expect tighter impedance tolerances (±5% standard) and ultra-low-loss material adoption on PCB motherboards.
Power delivery copper weight: Glass substrates enable higher processor power (>600W TDP). PCB power planes must scale proportionally — 4-8 oz copper, embedded bus bars.
IC substrate market disruption: Traditional ABF substrate fabricators (Ibiden, Shinko, Unimicron) must either invest in glass capabilities or risk market share erosion to glass-native manufacturers.

What Stays the Same

PCBs still connect everything at the system level
Increasing PCB complexity supports glass substrate packages
High-layer-count, controlled-impedance PCB demand actually increases
Fabricators serving AI server markets see demand growth regardless of substrate technology choice

Timeline: Glass Substrate Adoption Roadmap

Timeframe	Application	Volume
2026 (now)	Intel Xeon 6+ server CPUs	Thousands/month
2027	NVIDIA AI accelerators (rumored)	Tens of thousands/month
2028	AMD data center CPUs + AI GPUs	Hundreds of thousands/month
2029	High-end networking ASICs (800G+)	Millions/year
2030+	5G/6G RF modules, automotive radar	Mass market penetration

The transition from organic to glass substrates will take a decade to fully play out — but for AI hardware designers, it's happening now.

What Hardware Engineers Should Do Today

Monitor package ball maps: Next-generation processors will have different substrate ball pitches — plan PCB footprint libraries accordingly
Specify ultra-low-loss PCB materials: Megtron 6/7, Tachyon, or equivalent for signals interfacing with glass-substrate processors
Design for higher power: Plan thermal solutions for >600W packages
Engage your PCB fabricator early: Discuss advanced material availability and controlled impedance capabilities for AI server boards

DEV Community