CoWoS, SoIC & Direct Liquid Cooling: Breaking Thermal Walls

AI Accelerator Thermal Crisis

AI workloads drive GPU power beyond 1000W, creating a “thermal wall” where traditional cooling fails for advanced packaging.This challenge limits CoWoS and SoIC adoption. Direct liquid cooling integration marks a critical turning point.

Advanced Packaging: CoWoS and SoIC

CoWoS for High-Performance Computing

TSMC’s CoWoS integrates chiplets on silicon interposers mounted on organic substrates, offering:

• High-bandwidth interconnects: Dense routing with superior electrical performance
• Heterogeneous integration: Multiple process nodes in one package
• Reduced latency: Shorter chiplet distances
• Power efficiency: Lower interconnect power consumption

CoWoS concentrates heat beyond 2000W/cm², challenging traditional cooling methods.

SoIC: 3D Integration Evolution

TSMC’s SoIC enables vertical chip integration via hybrid bonding, delivering:

• Ultra-high density interconnects: Sub-10μm bonding pitches
• Superior electrical performance: Lower resistance vs. TSV
• Improved thermal management: Direct heat paths between dies
• Reduced form factor: Compact mobile/edge packaging

Despite vertical heat spreading advantages, 3D power density demands advanced cooling.

Advanced Packaging Capacity Constraints

CoWoS Capacity Limitations

CoWoS demand exceeds supply due to:

• Complex manufacturing: Specialized equipment, difficult scaling
• Capital intensity: $1B+ per production line
• Long lead times: 18-24 month setup
• Yield challenges: Large interposers increase defects

Capacity expansions are underway, but fierce competition persists.

Supply Chain Impact

Capacity expansion drives demand for:

• Large silicon interposers: Specialized wafer processing
• Ultra-fine RDL materials: Advanced polymers and dielectrics
• Hybrid bonding equipment: Precision SoIC tools
• Thermal interface materials: High-performance TIMs

Component delays bottleneck production, affecting AI system launches.

Thermal Resistance Challenge

Thermal Resistance in Advanced Packages

Thermal resistance (°C/W) from junction to ambient includes:

• Junction-to-case (Rjc): Silicon to heat spreader
• Thermal interface (RTIM): Package to cooling contact
• Heat spreader: Lateral/vertical conduction
• Coolant interface: Surface to fluid transfer

Traditional TIMs (0.1-0.3 °C/W) cause 100-300°C rise at 1000W—unacceptable for reliability.

Traditional Cooling Limitations

Air cooling faces fundamental barriers:

• Low heat transfer: 50-200 W/m²K insufficient for concentrated loads
• Large form factors: Bulky heat sinks required
• Noise generation: Acoustic challenges
• Energy consumption: 10-15% system power for fans

Indirect liquid cooling improves performance but retains significant thermal resistance.

Direct Liquid Cooling Solutions

Direct Cooling Operation

Direct liquid cooling eliminates thermal interfaces by bringing coolant near/to chip surfaces, drastically cutting resistance.

Two primary architectures:

• Microfluidic cooling: Micro-channels route coolant near heat sources
• Direct impingement: Jets target hot spots

Both achieve 10,000+ W/m²K—100× better than air cooling.

Integration with CoWoS and SoIC Packages

Direct liquid cooling integration with advanced packaging requires co-design of:

• Fluidic interconnects: Fluid paths without compromising electrical connections
• Leak prevention: Sealing to protect electrical components
• Thermal expansion management: Accommodating material CTE differences
• Pressure management: Optimizing flow without mechanical stress

CoWoS packages integrate cooling channels into substrates or manifolds, cutting thermal resistance 40-60% versus cold plates.

SoIC stacking enables top and bottom heat removal. Advanced designs add cooling layers between dies, increasing bonding complexity.

Material Innovation for Advanced Thermal Management

Next-Generation Thermal Interface Materials

Remaining interfaces require advanced materials:

• Liquid metal TIMs: Gallium alloys with 70+ W/mK conductivity
• Carbon nanotube composites: Aligned CNTs for thermal paths
• Phase-change materials: Optimal consistency across temperatures
• Graphene-enhanced polymers: Processability with thermal performance

These TIMs connect packages to cooling manifolds where every degree matters.

Coolant Selection and Chemistry

Direct liquid cooling requires coolants balancing thermal performance, compatibility, and operational needs:

• Dielectric fluids: Non-conductive fluorocarbons/hydrocarbons for direct component contact
• Water-glycol mixtures: High heat capacity, requiring robust sealing
• Nanofluids: Nanoparticle suspensions enhancing thermal conductivity

Coolant choice affects pump power, heat exchanger size, and reliability—a critical system-level decision.

Impact on Semiconductor Supply Chain and Production

Shifting Investment Priorities

Advanced packaging and direct liquid cooling convergence reshapes semiconductor investment:

• Packaging foundries: Expanding CoWoS/SoIC capacity with thermal-aware designs
• Cooling solution providers: Developing package-integrated technologies
• Material suppliers: Scaling TIM, coolant, and sealing production
• Equipment manufacturers: Creating thermally-enhanced package assembly tools

This evolution requires cross-segment collaboration, making thermal management a first-order design consideration.

Implications for AI System Architecture

Advanced packaging with effective thermal management enables:

• Higher power chips: Designs otherwise thermally limited
• Denser rack configurations: More computing per square meter
• Reduced cooling infrastructure: Lower facility-level requirements
• Improved reliability: Extended component lifetimes

These benefits reduce total cost of ownership, driving adoption despite higher initial costs.

Industry Adoption and Future Outlook

Current Deployment Status

Leading AI providers deploy direct liquid cooling with advanced packaging in:

• Hyperscale data centers: Highest-performance AI training clusters
• Supercomputing facilities: Next-generation exascale systems
• Edge AI applications: Compact cooling for powerful edge servers

Adoption accelerates, but standardization, maintenance, and cost modeling challenges remain.

Technology Roadmap: What’s Next?

Future thermal management will likely include:

• On-chip cooling integration: Microfluidic channels in silicon during wafer processing
• Two-phase cooling: Evaporation and condensation for enhanced heat transfer
• Thermoelectric cooling integration: Active cooling in packages
• AI-optimized thermal management: Dynamic cooling based on workload

These innovations push power density boundaries, enabling next-generation AI capabilities.

Conclusion: A Holistic Approach to Thermal Challenges

Breaking the thermal wall requires integrating package design, cooling technology, materials, and system architecture. CoWoS and SoIC expansion with direct liquid cooling enables continued AI performance scaling.

This convergence demands new collaborations, investments, and capabilities. Companies successfully navigating this transition will lead in AI, while those treating thermal management as secondary risk falling behind.

The thermal wall drives innovation across the semiconductor ecosystem, delivering computing power for transformative AI applications.