Advanced Thermal Management Strategies for High-Power Pluggable Optical Modules

Bandwidth for chip-to-chip and chip-to-memory communication is becoming a bottleneck in modern computing systems. As a result, critical emphasis has been placed on increasing the throughput between system components. Despite many efforts to improve the efficiency of interconnect systems and develop more sophisticated communication protocols, the demand for higher throughput comes with an inherent thermal cost with an increase in power consumption of these modules. Recent advances in artificial intelligence (AI) are driving these rapid changes, including the transition from 112 Gbps-PAM4 to 224 Gbps-PAM4 and adoption of next-generation 1.6T modules.

Thermal management within electronic systems in data centers aims to maintain component temperatures within safe operational limits under specified loads and conditions. These temperature limits are determined based on their temperature-versus-lifetime relationship and their target lifetime in the field. Other operational aspects, such as voltage, and environmental factors like humidity or ambient temperature fluctuations, will also affect the lifetime of parts in the data center environment. 

Effective thermal management strategies should consider several factors, including power dissipation, power density and its spatial distribution, as well as the temporal and transient characteristics of the load and operating conditions of the target systems. 

Maintaining lower operating temperatures enhances component reliability and extends their lifetime in the field. Lower operating temperatures will also reduce the overall power consumption of the system. Balancing the increased power requirements of cooling solutions against the decrease in the overall power consumption of the electronics is necessary for determining the system’s optimal operating point and maintaining power efficiency.

For years, air has been the coolant of choice for electronic systems. Air cooling is preferred for its dielectric nature at low voltage operations, mostly inert nature, ease of application and lower implementation cost compared to liquid cooling. The infrastructure supporting the delivery of cold air to electronic systems and collection of hot air from the racks has been well optimized in recent decades.

In air-cooled systems, airflow directly above the optical modules and strategic thermal optimization of the module heatsink — whether it is a riding heatsink on top of a flat top module (QSFP-DD) or an integrated heatsink (OSFP) — ensures efficient heat dissipation.  In such cases where a riding heatsink is used, it is important to ensure good thermal contact between the heatsink and module case to create a low-resistance path for the heat. This is achieved by first optimizing the riding heatsink. In the past, the industry was focused on changing the heatsink from aluminum extrusion to a denser zipper fin/stacked heatsink. However, in future higher power modules, the contact resistance between the pluggable module and the riding heatsink is an emerging bottleneck. Here, special attention must be paid to improve this contact resistance — through using a thermal interface material (TIM) for that contact interface, for example.

The design of these heatsinks involves several considerations, including mechanical system requirements and thermal performance relative to system airflow and pressure dynamics. Modern heatsinks must be optimized for these customer-specific boundary conditions and system environments — gone are the days of standard heatsink options that cover all applications. 

In addition to optimizing the heatsink, it is important to minimize the impedance of the air path downstream from the heatsink and module. This includes thermal optimization of cages and connectors where vent holes are added to the cage while keeping Electromagnetic interference (EMI) shielding requirements in mind. Connectors are mechanically designed so that they lower the airflow impedance by minimizing the airflow blockage.

For stacked cage configurations, a co-design approach is required to have an optimized heatsink design for the modules that will be placed in the rack. In co-design, coolant flow is simulated, considering all components on the blade. A full system-level analysis is necessary to ensure all modules will receive adequate airflow and to minimize the temperature gradient between the modules.

Despite the effectiveness of air cooling, its capacity is inherently limited. ASHRAE’s Emergence and Expansion of Liquid Cooling in Mainstream Data Centers (2021) suggests a limit of approximately 400W per chip for air-cooled systems, while the Open Compute Project (OCP) Open Accelerator Module (OAM) Design Specification Rev 2.0 (2023) mentions a limit of approximately 600W for air-cooled systems. The recent trend of high-end processors, however, exceeds these limits. Such high-power values for the processor demand liquid cooling, which offers a more efficient and compact solution for the main processor. 

This trend comes with an interesting dilemma for cooling other parts of the system, such as pluggable optical modules, which typically have relatively lower power than the main processor. Such components will still require some sort of active cooling. With power levels expected to be as high as 35W in 1.6T optics, liquid cooling is a growing area of interest and discussion for next-generation pluggable optics. In air-cooled systems, those peripheral components will benefit from the airflow supplied to the system for the cooling, meaning the main system fans could provide adequate airflow. In some liquid-cooled systems, the system is designed based on a hybrid approach, where liquid-cooled methods are used for the high-power components (ASIC/GPUs) and air cooling is applied for other parts of the system. These systems will require fans either at the rack or blade level to provide sufficient airflow.

Another approach for cooling pluggable optical modules involves employing a cold plate system to efficiently manage the temperature of multiple optical modules. These systems utilize individually floating pedestals on the cold plate to ensure adequate thermal contact with each one of the modules (plugged into ports that may have different tolerance stack up). Deploying systems using this method comes with major challenges in design and manufacturing, including:

• Ensuring uniform cooling among modules and flow distribution in the cold plate
• Balancing the pressure drop between different system components  
• Managing manufacturing complexity and increased assembly cost 
• Conducting more intricate tests in the manufacturing stage to ensure optimal performance and reliability 

While these challenges exist, they are not insurmountable. In fact, Molex has solved these challenges in real-world applications.

For the next generation of optical modules, a key priority is the end-to-end optimization of the heat flow pathway, minimizing the resistance from the components’ junction to the cooling fluid, whether air or liquid. This will include: 

• Optimizing electronics packaging for individual components 
• Ensuring thermal-aware placement of components on the PCB and inside the module 
• Creating a low-resistance thermal path from the components to the surface of the module (Example: Use of high thermal conductivity pads, use TIMs to improve contact resistance, use of higher thermal conductivity module housing)
• Improving the thermal spreading of the module cover to avoid localized hotspots, which can result in inefficient cooling (Example: Using Copper slug and heat pipes in the module)

Equally important is a change in how these modules are thermally characterized. The traditional way of characterizing modules (using generic case temperature limits) leaves margin on the table — and with higher power modules the margins are slim. 

The need for better cooling of high-powered optical transceivers in data centers has never been more crucial. With networks struggling to keep up with skyrocketing bandwidth demands, designers can’t afford to let these indispensable components overheat. We have arrived at a make-or-break point for scaling up system cooling capabilities that is driving the requirement for performance-driven thermal innovations. As data centers grapple with ever-growing thermal challenges, Molex is at the forefront of innovation. As an active participant in OCP and its Cooling Environments project, Molex is actively developing next-gen cooling technologies that meet the rising thermal management needs of data centers. Trust Molex to deliver robust, dynamic solutions for data center architectures that are both resilient and future-ready.