OSFP Thermal Form Factors Explained: Finned Top, Closed Top, and Flat Top (RHS)

As data center networks evolve from 400G to 800G and 1.6T, optical module power consumption is rising rapidly, making thermal design a primary system-level constraint rather than a secondary consideration. In modern high-speed platforms, inadequate cooling can directly limit port density, throttle performance, or even prevent certain network architectures from being deployed.

The OSFP (Octal Small Form Factor Pluggable) form factor has emerged as a leading choice for next-generation 800G and 1.6T optical transceivers, not only because of its electrical and mechanical scalability, but also because it was designed from the outset to support higher power envelopes through improved airflow interaction and closer integration with host cooling systems.

This article explains why multiple OSFP thermal form factors exist, compares the structural differences between OSFP-IHS and OSFP-RHS, and provides guidance on selecting Finned Top, Closed Top, and Flat Top (RHS) designs for 800G and 1.6T OSFP transceivers across switches, NICs, and AI computing platforms.

Why OSFP Thermal Designs Have Diverged

At 400G, most optical modules could rely primarily on ambient airflow generated by system fans. As data rates increase to 800G and beyond, however, several factors converge to dramatically increase thermal stress:

• More complex DSPs operating at higher symbol rates.
• Higher laser output power and driver losses.
• Increased integration density within the optical module.

Simply adding larger heat sinks or taller fins is no longer sufficient. Instead, OSFP thermal performance increasingly depends on how effectively heat is transferred from the module into the host platform's overall cooling architecture.

At the same time, host platforms have diversified. Traditional air-cooled switches, high-density top-of-rack systems, AI NICs, and GPU servers all impose different constraints on module height, airflow direction, and thermal responsibility. These diverging system requirements have driven OSFP into two clearly defined thermal design paths:

• OSFP-IHS (Integrated Heat Sink), where the module includes its own heat sink.
• OSFP-RHS (Riding Heat Sink), where thermal dissipation is handled primarily by the host platform.

OSFP-IHS vs. OSFP-RHS: Core Structural Differences

What Is OSFP-IHS (Integrated Heat Sink)?
OSFP-IHS modules integrate a metal heat sink directly onto the top of the optical module. The total module height is typically around 13 mm, and thermal performance depends on both the heat sink geometry and the system airflow provided by the host device.

Because cooling is largely self-contained, OSFP-IHS modules are well suited to platforms where switch or router airflow is predictable and well characterized. This approach also simplifies deployment by reducing dependence on cage-level or chassis-specific thermal components.

OSFP-IHS designs are commonly used in traditional Ethernet and InfiniBand switches where airflow management is handled primarily at the system fan level.

What Is OSFP-RHS (Riding Heat Sink, Flat Top)?
OSFP-RHS modules, commonly referred to as Flat Top OSFP, remove the integrated heat sink entirely. The module top surface is flat, and overall height is reduced to approximately 9.5 mm.

Figure 1: Side View of a typical OSFP (top) and a typical OSFP-RHS (bottom)

In this design, thermal responsibility is transferred to the host platform. A riding heat sink, typically spring-loaded and mounted above the cage, makes direct contact with the module surface when inserted. Heat is conducted upward into this external heat sink and then dissipated through airflow or liquid cooling, depending on the platform design.

Figure 2: OSFP-RHS cage only (left) and OSFP-RHS cage with module and riding heat sink (right)

Important: OSFP-IHS and OSFP-RHS are mechanically incompatible. Differences in module height, cage structure, and cutouts mean they cannot be used interchangeably within the same host system.

Finned Top and Closed Top: Two OSFP-IHS Thermal Architectures

Within the OSFP-IHS category, two distinct top structures are commonly used: Finned Top and Closed Top. While both share the same overall height, their interaction with airflow differs significantly.

What Is Finned Top (Open Top)?
Finned Top OSFP modules feature exposed metal fins that extend upward from the module, directly interacting with ambient airflow.

This design maximizes surface area and can deliver excellent cooling performance when airflow is strong and uniformly directed. However, thermal performance is highly dependent on system airflow conditions. In environments with uneven or turbulent airflow, cooling efficiency may vary from port to port.

Finned Top designs are commonly used in general-purpose air-cooled switches where airflow is abundant and mechanical simplicity is preferred.

Figure 3: Open Top Heat Sink (isometric view), top edge

What Is Closed Top?
Closed Top OSFP modules enclose internal fins beneath a flat metal lid. Although this reduces direct exposure to ambient airflow, it enables tighter control over how air moves through the heat sink.

By forcing airflow to enter from the front of the module and pass through internal fin channels, Closed Top designs improve pressure utilization and reduce airflow bypass. The enclosed structure also provides enhanced mechanical protection and additional EMI shielding.

As 800G and 1.6T OSFP modules push toward higher power levels, Closed Top designs often deliver more predictable and consistent thermal performance, particularly in dense switch environments.

Figure 4: Closed Top Heatsink Details, top trailing edge

Flat Top (OSFP-RHS): Optimized for AI and High-Density Platforms

Flat Top OSFP-RHS modules represent a fundamentally different approach to OSFP thermal management. Rather than optimizing the module itself for airflow, they enable the host platform to manage cooling at a system level.

This approach offers several advantages:

• Reduced module height, enabling installation in space-constrained NICs and server backplanes.
• Flexible cooling strategies, including large air-cooled heat sinks or direct liquid cooling.
• Strong alignment with AI system design, where thermal management is already centralized at the chassis level.

This design choice shifts additional thermal and mechanical complexity to the host platform, but in return enables higher power scalability. As a result, OSFP-RHS is widely adopted in AI NICs, DPU cards, GPU servers, and advanced switch platforms used in AI fabrics.

Selecting the Right OSFP Thermal Form Factor

Choosing between Finned Top, Closed Top, and Flat Top OSFP modules is not a matter of thermal performance alone. It requires evaluating how the module interacts with the host platform's cooling architecture.

In general:

• Finned Top OSFP-IHS works well in traditional air-cooled switches with strong, uniform airflow.
• Closed Top OSFP-IHS is preferred in high-density switches where airflow control and predictability are critical.
• Flat Top OSFP-RHS is ideal for AI NICs, GPU servers, and platforms that rely on host-level or liquid cooling.

Once a platform is designed for OSFP-IHS or OSFP-RHS, the two approaches are not interchangeable. Understanding these constraints early in the design process helps avoid costly compatibility issues as port speeds and power levels continue to rise.

Conclusion

The evolution of OSFP thermal form factors reflects a broader trend in data center design: thermal management has become a system-level co-design challenge between optical modules and host platforms.

As networks move toward 800G and 1.6T optical transceivers, selecting the appropriate OSFP thermal architecture is essential for achieving long-term performance, reliability, and scalability. By understanding the differences between OSFP-IHS and OSFP-RHS, and by carefully matching Finned Top, Closed Top, or Flat Top OSFP modules to the target platform, network architects can build infrastructures prepared for the next generation of high-speed optical connectivity.

Recommended Reading:
OSFP-IHS vs. OSFP-RHS: How to Choose the Right Thermal Solution for 800G and 1.6T Optical Modules

Article Source: OSFP Thermal Form Factors Explained: Finned Top, Closed Top, and Flat Top (RHS)