OpenSFF Compute Node Specification

6. Airflow and Cooling

The compute node features front-to-back airflow, from intake fans that are part of the enclosure, through a plastic shroud that optimizes airflow, and out the perforations on the I/O shield. The node’s major components are passively cooled through the node’s thermal solution that MUST function as part of the active cooling setup provided by the enclosure.

6.1 Cooling Requirements

The cooling system must effectively manage thermal output across a range of inlet air temperatures, specifically from 10°C to 35°C. This includes ensuring that the CPU/APU die and memory module temperatures remain below a specified maximum junction temperature (Tj​), to avoid thermal throttling and/or component damage.

• Maximum Ambient Temperature / Intake Temperature: 35°C
• Maximum Junction Temperature (CPU/APU Core): 85°C
• Maximum Junction Temperature (Memory Modules): 85°C
• Maximum VRM Mosfet Casing Temperature: 120°C

To ensure reliable thermal operation of compute nodes within the OpenSFF enclosure environment, the cooling system must deliver sufficient volumetric airflow across critical thermal components. The following airflow thresholds are defined:

• Minimum airflow requirement: The system must maintain a minimum of 68 m³/h of effective airflow across the primary thermal interface zones., measured at the shroud inlet.
• Required static pressure: The system must sustain a minimum static pressure of 11.943 mmH₂O at 68 m³/h to ensure that airflow is preserved through the node’s thermal soulution (e.g, heatsink fins), ducting, filters, and any restrictive chassis features.

These figures are based on theoretical airflow calculations, but adjusted to 200% based on empirical data from comparable systems to ensure broader system compatibility.

6.2 Thermal Solution Design

Effective thermal management is critical to ensuring reliable operation and performance of the compute node. This section defines the requirements and guidelines for designing thermal solutions that keep all critical components within safe operating limits, while allowing flexibility in methods of implementation.

6.2.1 Mandatory Requirements

• The compute node MUST incorporate a thermal solution capable of keeping components within the temperature limits defined in Section 6.1, including but not limited to the CPU/APU, memory modules, and VRMs
• In designs where component power dissipation is sufficiently low (e.g., below 10 W TDP), a dedicated node-level thermal assembly MAY be omitted if the enclosure-provided airflow system maintains compliance within the limits defined in Section 6.1
• Where thermal conduction to a cooling assembly is required, high-quality Thermal Interface Material (TIM) or an equivalent solution MUST be applied
• The thermal solution MUST operate effectively in conjunction with the enclosure-provided airflow system as specified in Section 6.1.

6.2.2 Design Guidelines

• The thermal solution SHOULD be optimized for the enclosure’s airflow path and pressure characteristics
• Acceptable methods include, but are not limited to heatsinks, vapor chambers, heatpipes, and other thermal transfer assemblies
• Configurations utilizing high-TDP components MAY require a thermal solution design that extends over surface-mounted memory (if applicable) and Voltage Regulator Modules (VRMs)
• Cutouts, pedestals, or equivalent design features MAY be used to minimize TIM bond-line thickness over designated components

6.2.3 Solution Specifications

• Maximum TDP Supported: 120W
• The solution’s physical dimensions MUST comply with top-component height limits (see Section 3.2).

6.3 The Shroud

A shroud MUST be used to direct enclosure-provided airflow across the node’s thermal solution and other heat-sensitive components. The shroud helps reduce airflow losses inside the enclosure and can protect components during handling.

• Maximum External Dimensions: 215mm (Length) x 144mm (Width) x 52.4mm (Height)
  • The top and side sections of the shroud’s inlet MUST extend beyond PCB length by 7mm (same as the length of the plug) forming the inlet section, in order to create a seal against the enclosure’s intake fan array.
  • The width and height of the shroud must be tapered at the top corners to ensure sufficient clearance for the screw holes of the I/O shield, allowing space for the thumb screw mounting posts of the enclosure (as shown in Figure 3.4.2).
• Shroud Inlet: Minimum inlet area of 40mm (H) x 96mm (W).
• Shroud Outlet: I/O shield perforations SHALL cover the exhaust requirements of the shroud’s cooling system. Manufacturers MUST decide the most optimal perforation design and coverage, with optional I/O ports and the I/O shield structural integrity kept in consideration.
• Shroud Design: Baffles or channels MAY be used to optimize airflow over other heat-generating components such as system memory and VRMs
• Aerodynamic Shaping: Curves and fillets MUST be incorporated in the shroud design, whenever possible, to reduce turbulence and flow separation.

Note: A dedicated shroud MAY be omitted for low-power designs where the thermal solution requirements in Section 6.2 can be met without additional airflow guidance. In such cases, the compute node MUST still comply with all operating temperature limits in Section 6.1 when installed in a compatible enclosure.