How is the Thermal Structure of OSFP Optical Modules Designed?

The power consumption of ultra-high-speed optical modules with 400G OSFP and higher rates has significantly increased, making thermal management a critical challenge.

For OSFP package type optical modules, the protocol explicitly specifies the impedance range of the heat sink fins. Specifically, when the cooling gas wind pressure does not exceed the threshold TH1 and the airflow does not exceed the threshold TH2, the module temperature must be at or below the specified value, and the airflow impedance of the heat sink fins must remain within the safe zone between the upper and lower limit curves.

If the heat sink structure changes, causing its characteristic curve to approach the upper limit, the module temperature will rise under constant wind pressure. To maintain a constant temperature, the airflow pressure must be increased.

Let’s first examine the existing OSFP packaging solution, as shown in the diagram below, which illustrates the internal structure of an optical module with a heat sink when the heat-generating components are relatively large.

In this scheme, when the height of the heat-generating components is significant, their high thermal output compresses the height of the heat sink fins at the corresponding position, creating a thermal dissipation defect.

In traditional schemes, the height of the heat sink fins decreases progressively, reducing the cross-sectional area of the airflow channels between the fins and pushing the impedance close to the protocol’s upper limit (as shown in the wind pressure and airflow diagram above). This makes it difficult for cooling gas to penetrate to the rear of the module, posing a risk of localized overheating. To address this, FiberMall proposes an innovative dynamic thermal dissipation structure: the height of the heat sink fins is inversely adjusted based on the height of the corresponding heat-generating components. The higher the heat-generating component, the lower the heat sink fin height; the lower the heat-generating component, the higher the heat sink fin height. In some cases, the fins directly above the highest heat-generating component may even be entirely removed. The schematic diagram of FiberMall’s optical module with this thermal structure is as follows:

The schematic diagram of FiberMall’s optical module with this thermal structure is as follows:

In this internal thermal management scheme, very low or no heat sink fins are placed at the air inlet and above the first heat-generating component, allowing more cooling gas to reach the subsequent heat-generating component areas.

This approach expands the inlet cross-sectional area by 40%, and by removing or lowering the fins in the high-heat component area, the airflow channel is increased by 30%-50%. Progressively increasing fin heights are set above the second and third heat-generating components.

FiberMall’s scheme features progressively increasing fin heights, primarily addressing airflow channel blockages in high-heat component areas (by removing or lowering fins), thereby reducing the overall impedance of the heat sink.

The base scheme described above lacks a heat sink cover, resulting in a thermal bottleneck at the rear.

To enhance thermal performance, a detachable aluminum alloy heat sink cover, with a thickness of 0.5 to 1 mm, is added to the top of the module and precisely fixed via a support system.

The higher fin heights at the rear of the heat sink create larger cross-sectional gaps between the fins. The heat sink cover, combined with the gap above the third heat-generating component’s fins, forms a cavity that promotes internal and external circulation of cooling gas. Additionally, the rear of the heat sink fins remains open, allowing some cooling gas to exit through the gap between the heat sink cover and the cage.

The open-ended rear fins also enable cooling gas to flow out from this gap.

The entire structure leverages thermodynamic principles to guide airflow: cooling gas is injected from the inlet, preferentially flowing through the low-resistance channels at the front, then redirected downward toward the substrate in the high-fin rear area.

As the heated gas rises, it exits through the gap between the heat sink cover and the cage. The sealed cavity formed by the heat sink cover and support components is a critical part of this airflow path.

Experimental data verify that, at a 40W power consumption, compared to the traditional uniform fin scheme:

The rear area temperature is reduced by 12-15°C;

Airflow impedance is reduced by 35%;

The wind pressure-airflow curve remains stably within the middle of the protocol’s safe zone.