Finned-Top vs. Flat-Top in 400G and 800G: How Thermal Design Reshapes the Future of Optical Modules

As the computing power race in data centers intensifies, the thermal design of high-speed optical modules has moved from the background to the forefront, becoming a critical factor in determining network performance.

In the computing era driven by AI and cloud computing, 400G and 800G optical modules have become the core hub of data center networks. However, within these tiny devices, a quiet revolution in thermal design is quietly taking place.

Traditional thermal management solutions are no longer able to cope with the staggering heat generated by high-speed data transmission—a single 800G optical module can consume up to 16.5 watts, equivalent to the energy consumption of a low-power server.

Architectural Differences in Thermal Design

Optical module thermal design primarily falls into two approaches: finned-top and flat-top designs. These two designs share the same internal structure, but differ in their appearance and cooling strategies.

Finned-top optical modules improve heat dissipation efficiency by increasing the surface area for heat dissipation. The built-in fins transfer heat directly to the surrounding environment. This design is particularly suitable for high-density rack environments with limited airflow. The flat-top design, on the other hand, utilizes a flat design to conserve vertical space and relies on an external cooling system for heat dissipation. This type of module offers greater installation flexibility and is particularly suitable for stacking scenarios.

The Critical Role of Thermal Management

As transmission rates evolve from 400G to 800G, the power density of optical modules has increased exponentially. Thermal management is no longer an afterthought but directly impacts module performance and reliability.

Excessive heat can cause laser wavelength drift, decrease receiver sensitivity, and even cause permanent damage to components. Studies have shown that for every 10°C increase in electronic component temperature, reliability decreases by 50%.

In the 800G OSFP series, the SR8, DR8, and 2FR4 optical modules all utilize a heatsink-top design to ensure stable operation during high-speed data transmission. The heatsink is integrated into the module housing, providing a larger heat dissipation surface area than QSFP-DD optical modules and optimizing thermal contact between the heatsink and the components.

Differentiation of Application Scenarios

The different characteristics of the two heat dissipation designs determine their optimal application scenarios.

Optical modules with top-mounted heat sinks are primarily suitable for specific switch models, such as the NVIDIA Quantum-2 NDR InfiniBand and Spectrum-4 SN5600 400GbE air-cooled Ethernet switches. These switches require effective cooling solutions in high-load scenarios.

Flat-top optical modules are primarily used in network adapters and GPU servers, such as the ConnectX-7 network adapter, the DGX H100 Cedar7 GPU server, and liquid-cooled systems. These applications often face space constraints but require additional cooling support.

System-Level Thermal Design Challenges

Optical module thermal design cannot be considered in isolation; it must be optimized at the system level. The thermal interface design between the module and the switch port is crucial.

Heat sinks enhance airflow management but occupy more vertical space. This can pose challenges in certain high-density deployments.

Flat-top designs save space and increase deployment flexibility, but may require additional cooling solutions in large-scale data centers. System designers must strike a balance between space efficiency and cooling capacity.

The rise of liquid cooling systems offers new approaches to optical module cooling. Some advanced data centers are beginning to adopt direct liquid cooling technology, directing coolant directly to the components generating the most heat.

Future Development Trends

Future optical module thermal design will develop diverse paths. LPO (Linear Pluggable Optics) technology significantly reduces power consumption by removing the DSP chip. The power consumption of an 800G LPO module can be reduced to below 8.5 watts, over 50% lower than traditional designs.

Co-packaged optics (CPO) technology places the optical engine and ASIC chip in close proximity, shortening the thermal management path. This technology is expected to further optimize energy efficiency and address the thermal challenges associated with high-speed transmission.

Advances in materials science will also promote innovative thermal solutions. High-performance thermal interface materials such as carbon nanotubes and diamond films are beginning to be used in high-end optical modules, significantly improving heat transfer efficiency.

Final Words

Choosing between a heatsink and a flat-top design is no longer just a technical issue; it’s a strategic decision.

The heatsink design provides better thermal management, ensuring stability in high-performance applications; the flat-top design offers greater deployment flexibility and is suitable for space-constrained environments.

In the future, as new technologies like CPO and LPO mature, optical module heat dissipation design will become more refined and diversified. It will no longer be a one-size-fits-all solution, but will be tailored to specific workloads and environmental conditions.

Heat dissipation, once a neglected engineering aspect, is becoming a key innovation area driving breakthroughs in data center performance.