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Posted on 01 February 2019

Thermal Management Takes Another Look at Liquid Cooling

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Micro-structure heat exchanger should be optimally designed

Recent technical advances are driving the commercial acceptance and use of liquid cooling systems for high heat flux processors in computers and workstations

By Girish Upadhya, Ph.D., Director, Thermal Design & Applications Development Cooligy, Inc., Mountain View, CA

 

The thermal characteristics of high-powerdensity CPUs in today’s high-end computing applications are rapidly outpacing the cooling capabilities of most commercially available strategies. The problem lies in three compounding trends: higher total chip power, higher local heat flux in chip hotspots, and smaller system enclosures. Meeting these thermal needs with conventional cooling systems poses a number of challenges, including:

• Eliminating heat with high average heat density, above 100W/cm2
• Maintaining consistent die temperature in the presence of local hot spot zones of 1-2mm2, with power densities of 500W/cm2 or above
• Increased system noise due to high-volume air flow
• Reduced system reliability due to increased numbers of high-speed fans

Typical solutions include multiple heat pipes, vapor chambers attached to fan heat sinks and optimized fan heat sinks with new designs. Of these, however, none scales for heat flux higher than 100W/cm2, revealing the need for an alternative cooling solution.

Recent pumped-liquid cooling system (LCS) technology advances represent a promising alternative, available today, for cooling highpower- density processors. However, successful implementation requires innovation and careful attention to design details to optimally cool high heat flux chips within a targeted system volume. Key system elements must include (1) a micro-structure heat exchanger capable of high heat flux removal, (2) a reliable mechanical pump for delivering fluid with the required flow rate and pressure, and (3) an efficient liquid-air radiator heat exchanger.

Cooling elements

Optimizing the physical dimensions of the liquid cooling system’s heat-exchanger channels, liquid flow rate through the channels, radiator fin surface area, and airflow available for heat rejection can result in very high thermal performance from low airflow volume. This in turn enables system fans to run at lower speeds and more quietly, and where fan noise is not of concern, higher airflow results in even better performance.

A closed-loop LCS for a typical dual-CPU cooling application is shown in Figure 1. In operation, cold liquid enters the micro-structure heat collector at a specific volumetric flow rate, driven by the mechanical pump. The liquid absorbs heat from the CPU, exits the heat exchanger, flows into a fan-cooled radiator, then repeats the process. The pressure drop of the liquid as it flows through the system is managed by an appropriate fluiddelivery mechanism built into the design of the individual components. This concept is readily adaptable to single-CPU configurations, racks, servers, graphics chips, and high-output LEDs, as well as voltage regulators, isolated gate bipolar transistors (IGBTs), power semiconductors and field effect transistors (FETs).

Main system elements of the Cooligy closed-loop Active Micro-Structure Cooling System from Emerson Network Power

Performance considerations Micro-structure heat exchanger

Micro-structure heat exchangers should be optimally designed to accommodate the high heat flux of a high-performance microprocessor. Figure 2 shows the relationship of heat transfer efficiency and pressure drop on channel width. High performance is achievable with fine channel dimensions. However, to reduce the resultant high pressure drop, the fluid-delivery mechanisms must be able to provide very low thermal resistance and high flow rates.

Close up of micro-structure heat collector attachment to CPU

Charts show dependence of thermal performance and pressure drop on channel width

Heat sink attachment

Conventional heatsinks mated to the processor package by clips or screws can harm thermal performance due to resulting variations in thermal interface material (TIM) thickness, which requires a novel attachment to achieve device-wide uniformity and optimal performance.

Radiator design

Liquid flow rate, fin surface area and tube attributes impact radiator performance. Radiator designs must be optimized through sophisticated thermal tests and various numerical simulation techniques and analytical models that are also used to validate design parameters.

Working fluid

Most liquid cooling systems use a 30 percent propylene glycol and water mixture. Other systems use a proprietary water-based fluid that provides much higher thermal conductivity and almost half the viscosity of water-glycol mixtures, resulting in markedly better thermal performance versus anti-freeze type coolants.

Mechanical pump

Recent advances in compact mechanical pumps have resulted in high reliability and greater flow rates than earlier pump systems that were reliable but produced inadequate flow pressure and rate for the latest high heat flux processors.

Low-noise acoustics

Liquid cooling systems allow system fans to run much slower, and therefore more reliably, while producing equivalent heat rejection at given performance levels. In multi-processor systems with high heat loads, the multiple fans typically required by conventional cooling solutions can be reduced considerably by using an LCS.

Reliability design issues Particle control

Particle control plays a crucial role in ensuring the reliable long-term performance of the LCS. The material/fluid combination must be optimized by careful analysis, testing and characterization. Material selection, along with refined assembly processes during manufacturing, significantly impact the reliability of the finished system.

Water loss control

Closed-loop cooling systems completely eliminate fluid loss by means of robust, completely sealed tubing joints that prevent leaks during shipping, storage and use.

Freeze-protection

Especially important during shipping and storage, the system’s cooling loop should include adequate freezemanagement techniques that allow the system’s water-based working fluid to expand without damaging the system or harming thermal performance.

Material science control

Extensive performance and reliability characterizations must be performed in the selection of all materials used in the fabrication of the cooling system in order to eliminate corrosion and optimize the service life of major system components.

Cost

One of the main hurdles facing adoption of the liquid cooling system is higher cost versus conventional air-cooled solutions. One solution now in practice has been to collaborate with strategic manufacturing partners in Asia.

Application

One real-world example required cooling a high-performance workstation to remove heat from a bare die with high heat flux, while keeping the junction temperature below 85C. The average heat flux was approximately 150W/cm2, total power was nearly 220W, and available system airflow was in the range of 30-35cfm. To solve this problem, detailed simulations of the CPU power map were performed to optimize the design of the micro-structure heat exchanger and radiator. Table 1 shows the general operating specifications of the high-performance liquid cooling system developed for this application.

Thermal Performance - Mechanical Characteristics

Bottom line

Recent technical advances are driving the commercial acceptance and use of liquid cooling systems for high heat flux processors in computers and workstations, and new applications like high power LEDs for projectors and displays are also emerging. System designers are encouraged to explore liquid cooling for their own designs, being mindful of the trade-offs that define the optimized liquid cooling solution in terms of performance, reliability and cost versus conventional technologies.

 

 

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