Effective cooling of high-powered CPUs on dense server boards

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

The main goal of electronics thermal management is to efficiently remove enough heat from a device’s active region so that it stays within its rated temperature. Providing effective cooling presents different design challenges, not all of which involve the chip itself. Some thermal challenges are related to the system in which the chip resides. A common example is cooling a device positioned on a crowded printed circuit board (PCB). The congestion of components restricts airflow and space, which makes the use of many conventional cooling devices difficult.

Dense Server boards

Figure 1. A dense motherboard from Gigabyte Technologies featuring an Intel P55 chipset. [1]

Optimizing PCB for thermal management has been shown to ensure reliability, speed time to market and reduce overall costs. With proper design, all semiconductor devices on a PCB will be maintained at or below their maximum rated temperature. Applying thermal management can sometimes be problematic for dense boards employing fine pitch devices. (Pitch is the space between the center of one BGA ball to the center of the next one.)

But if certain layout guidelines are not followed and considerations are not given to a PCB’s thermal performance, the device and the overall system can suffer from sub-par performance and reliability in the field. [2]

Today’s circuit boards are often assembled with increasing density with the goal of making smaller, lighter systems, or to provide more processing power in demanding applications such as data centers and IoT (Internet of Things) applications. PCB designers must use proven layout techniques to ensure effective thermal performance for the board and its components.

Figure 2. Crowded boards have limited space from where chip cooling air can be drawn. (Wikimedia Commons)

Part of the trend toward higher density boards is related to the industry’s adoption of increased server density. This means increasing the power of the chips, putting more chips per rack unit, and filling up the racks as much as possible. Rack power has transitioned from a normal of about 4 kilowatts to 70 kilowatts per rack.

High current electronic components like microcontrollers can generate a significant amount of heat. To keep the board temperature lower, it is usually best to mount these components near the center of the board. Heat can diffuse throughout the board and the temperature of the board will be lower.

Many components in this situation, such as GPU, will require a dedicated cooling system, such as a fan sink. But simply installing a fan sink on top may not provide the needed level of cooling. It is good practice to quantify system flow bypass on the fan sink, and to also consider the proximity of components neighboring the fan sink. The mass airflow rate is the true measure of available coolant, along with the air velocity.

Obstructions in the intake or exhaust of the fan (e.g. neighboring components) must be carefully considered as their presence will impact the performance of the fan sink. The size and position of adjacent components can impact the fan’s performance. [3]

Figure 3. The QuadFlow CPU cooler draws air from four sides, passing it through cooling fins and expelling warm air. (Advanced Thermal Solutions, Inc.)

One new and effective solution for cooling hot components on congested PCBs is the QuadFlow CPU cooler from Advanced Thermal Solutions, Inc. (ATS). The liquid-free cooler features a high-power blower that draws in air from four different directions. So, while proximate components may block local air in a couple of directions, the QuadFlow fin fields will pull in air from the other directions to make sure that the component is being cooled.

QuadFlow coolers are just 29 mm tall, so they will fit into standard 1-U racks and there are several options for base material (aluminum, copper, or vapor chamber) depending on performance, weight, or cost requirements. [4]

Before applying any thermal management hardware, the smartest engineering activity may be investing is various PCB design services. These include CFD studies on boards at the CAD stage to wind-tunnel testing of actual or dummy boards in conditions that simulate air distribution in real-world applications. Services are available for characterizing boards using research-quality instruments, heat and air velocity sensors, and PCs.

Figure 4. FloTHERM image reveals hotter and cooler regions on a PCB. (Advanced Thermal Solutions, Inc.)

Dummy or working PCBs can be tested in isolation or installed in their own packaging domain. Computational simulations can be made of engineered designs using computational fluid design packages such as 6SigmaET, FloTHERM and CFDesign.

These services are available from ATS, whose engineers can design board layouts to improve cooling airflow in dense systems. Natural airflow can be enhanced to individual hot components and to active cooling systems that rely on airflow for effective performance. Often, these studies head off more expensive cooling solutions by showing that minor changes to component layouts or to the volume of airflow will resolve thermal problems. [5]

References
1. https://www.techpowerup.com/103375/gigabyte-unwraps-latest-p55-series-motherboards
2. https://www.embedded.com/design/configurable-systems/4395845/Ultra-fine-pitch-devices-pose-new-PCB-design-issues
3. https://www.hpe.com/us/en/insights/articles/why-youll-be-using-liquid-cooling-in-five-years-1710.html
4. https://www.qats.com/cms/2013/06/21/how-system-flow-affects-fan-sink-performance/
5. https://www.qats.com/Consulting/PCB-Board-Layout

Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. On Thursday, Jan. 29 from 2-3 p.m. ET the webinar will cover “Methodologies for Fan Characterization and Deployment within a System.” Learn more and register at https://qats.com/Training/Webinars.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/Consulting/Custom-Cooling-Solutions or contact ATS at 781.769.2800 or ats-hq@qats.com.

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