Comprehensive Thermal Design Guide for OSFP Modules

The power consumption of 800G OSFP (Octal Small Form-factor Pluggable) modules has reached 24W, posing severe challenges to traditional heat dissipation solutions. This article presents a full-stack four-level thermal design solution covering chip, packaging, cage and rack. Microchannel cooling lowers DSP temperature by 22°C, double-sided heat pipes reduce wavelength drift, and direct-insert liquid cooling achieves 70W heat dissipation. It ensures the DSP case temperature stays below 85°C with inlet water temperature at 45°C and ambient temperature at 65°C, bringing zero PUE increase and cutting failure rate by 5 times.
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By 2026, the power consumption of single-port 800G OSFP modules has hit 24W, while 1.6T prototypes approach 32W. Based on practical experience of optical module housings, a universal rule applies: every 10°C temperature rise halves laser service life and doubles DSP bit error rate. As rack power density rises to 80kW, traditional air cooling plus heat sinks can no longer meet heat dissipation demands. Supported by actual test data from leading industry hardware manufacturers in 2025, this guide introduces a coordinated four-tier thermal design system targeting core goals: control DSP case temperature within 85°C under 45°C inlet water and 65°C ambient temperature, with PUE rising by no more than 0.01 due to optical modules.
Chip Level: Equalize Hotspots Inside Silicon Chips
Heat Density Status

Adopting 7nm FinFET process for 800G DSP chips, local heat flux reaches 180 W/cm², close to that of nuclear reactor cores.
Microchannel Cooling Solution

Etch trenches of 30μm in width and 200μm in depth on the back of silicon chips via DRIE process, with deionized water flowing directly through. Its thermal resistance is 0.08°C/W, 5 times lower than conventional TIM1 materials.
Technical Key Points
Coat inner trench walls with 50nm-thick Al₂O₃ via ALD to prevent electrochemical corrosion.
Connect microchannel outlets to silicon micropumps with flow rate of 20 mL/min and pressure drop of 0.3 bar, ensuring leak-free operation for 10 years.
Verification Result
Under 85°C ambient temperature, junction temperature drops from 118°C to 96°C, complying with the 105°C safety threshold.
Packaging Level: Double-sided Heat Dissipation & Heat Pipe Integrated Structure
Structural Innovation

Machine 0.5mm-deep primary mounting grooves on upper shells to embed Φ2mm T2 oxygen-free copper heat pipes, and design mirrored grooves on lower shells to form an upper-lower sandwiched heat dissipation structure.
Low-cost Manufacturing Process
Fill copper pipes with soft alloy, preheat to 200°C and bend 6 times at 90° to form Ω-shaped loops.
Apply 5μm chemical tin plating and fill gaps with laser welding to increase contact area by 18%.
Connect outlets of upper and lower heat pipes in parallel via Φ4mm silicone hoses to access rack-level 30°C coolant circulation systems.
Actual Test Data
For 24W modules, double-sided heat pipes reduce case temperature by 12°C compared with single-sided heat sinks, limiting laser wavelength drift to 0.02 nm, far below the 0.1 nm limit specified in IEEE 802.3ck standards.

Module Level: Top Heat Sink vs Flat-top Design vs Direct-insert Liquid Cooling
Top Heat Sink for Optical Modules
Fin thickness: 0.8 mm, fin pitch: 1.2 mm, black anodized surface with emissivity of 0.85
Thermal resistance reaches 1.4°C/W under 3 m/s airflow, lowering case temperature by 15-20%
Flat-top Design
Height of 8.1 mm, consistent with QSFP-DD specifications, ideal for liquid cooling doors and GPU server stacking
Equipped with fewer built-in fins, relying on rack-level cold plates for heat dissipation; best for 45°C warm water cooling with optimal PUE optimization effect
Direct-insert Liquid Cooling (OSFP-D2P)
Cold plates directly fit module tops with flow channels aligned with DSP hotspots, achieving local thermal resistance of 0.25°C/W
Dissipate up to 100W heat with 30°C coolant and steady 70W heat dissipation at 45°C, doubling the 35W limit of traditional air cooling
Selection Suggestion
Prioritize top heat sinks for air-cooled racks; deploy direct-insert liquid cooling for newly-built liquid cooling data centers to reserve upgrade space for future 32W 1.6T modules.
Cage Level: From Single-module Heat Dissipation to Cluster Flow Field Optimization
1RU 32-port OSFP Extreme Layout

Adopt 4 sets of 1×4 cluster cages with 16 ports on upper and lower sides respectively; 14mm port spacing keeps full compatibility with standard OSFP specifications
Open rear cage design with quick connectors protruding 8mm, ensuring unobstructed airflow and pressure drop below 5 Pa
Targeted Precision Casting Flow Channel Design
Machine 2mm-wide micro-fins inside cold plates aligned with 20mm long-side heat source areas of modules, capable of dissipating 70W heat at flow velocity of 0.5 m/s
Adopt parallel-first-then-series manifold layout to control temperature difference within 3°C among 32 modules, avoiding link speed reduction caused by overheating of edge modules
Premium Material & Craftsmanship
Cold plates made of 6063-T5 aluminum achieve thermal conductivity of 200 W/m·K after vacuum brazing
Nickel-plated quick connectors matched with fluororubber seals remain leak-free after 5000 plug-and-pull cycles, supporting 10-year maintenance-free operation
Integrate Module Thermal Load into Overall Cabinet Liquid Cooling System
Warm Water Cooling
Directly deliver 30-45°C coolant to CDU without additional chillers, lowering overall PUE from 1.25 to 1.08.
Waste Heat Recovery

Single cabinet total power reaches 768W (32 modules × 24W). Recycle waste heat via heat pumps to raise water temperature to 55°C for office heating, saving 5600 kWh electricity annually equivalent to 4500 RMB electricity cost.
Air-liquid Hybrid Switchover
Automatically activate six 18000-rpm 8038 fans to dissipate 50% heat within 30 seconds once liquid cooling fails, realizing zero packet loss switching.
Three Core Thermal Verification Methods: Simulation - Actual Test - Closed-loop Control
Thermal Simulation Modeling
Establish integrated chip-heat pipe-cold plate models via FloTHERM with 12 million meshes and convergence residual of 1E-6. Boundary conditions: 45°C inlet water, 0.4 L/min flow rate, 35°C room temperature and 2 m/s airflow speed.
On-site Actual Testing

Infrared thermal imaging test shows maximum shell temperature of 74.2°C, with only 1.2% deviation from simulation result of 75.1°C
Wavelength drift test proves wavelength offset is merely 0.03 nm after 30-minute 100°C thermal shock, meeting industry 0.1 nm specification standards
Intelligent Closed-loop Management
Embed optical module thermal models into switch chip BMC. The system will automatically downgrade link speed from 400G to 200G and generate maintenance orders if case temperature is predicted to exceed 85°C within 30 seconds. After 6 months of field operation, no speed reduction caused by overheating occurs, and optical module failure rate drops to 0.02%, 5 times lower than that of air cooling solutions.
Conclusion: Make Thermal Performance a Controllable Factor

OSFP thermal design has evolved beyond simple heat sink pasting, developing into an end-to-end full thermal chain solution covering silicon chip flow channels to rack manifolds. Mastering the four-tier collaborative design including chip microchannel cooling, packaging heat pipes, module double-sided heat dissipation and system targeted liquid cooling enables stable temperature control of 32W 1.6T modules below 85°C even under harsh 45°C warm water and 65°C ambient temperature conditions, without extra PUE increase. In the next stage, connecting optical module thermal models with digital twin platforms to realize predictable and schedulable temperature management just like bandwidth allocation will fully fulfill the future vision of Thermal-as-a-Service.