Advanced cooling techniques beat the heat for rugged embedded COTS systems

Considering the environmentally intense environments faced by modern military electronics, the ability to meet the widest range of ruggedization and operating temperature requirements is an absolute must. And, when traditional Direct Forced Air (DFA) and conduction-cooling techniques are insufficient, designers are turning to more advanced approaches to save the day: Airflow Through (AFT) cooling, spray cooling, and Liquid Flow Through (LFT) cooling.

Embedded COTS boards for deployed aerospace and defense applications have to be able to meet the widest range of operating temperature and ruggedization requirements. From air-cooled cards in relatively spacious racks aboard naval platforms, to conduction-cooled cards located in crowded corners of fighter jets, today’s COTS boards need to perform on the tarmac of the desert at noon as well as in the cold environs of high-altitude flight.

With the increasing popularity of highly integrated small form factor 3U cards to deploy compute power in space-, weight-, and power-constrained platforms, embedded system designers are faced with an array of cooling approaches to address heat dissipation in rugged deployed COTS systems. When traditional Direct Forced Air (DFA) and conduction-cooling techniques aren’t sufficient to handle cooling requirements (that is, -40 °C to +71 °C ambient), system designers must turn to advanced approaches such as Airflow Through (AFT) cooling, spray cooling, and Liquid Flow Through (LFT) cooling. Each of these cooling approaches has its particular advantages as well as design considerations to weigh before choosing which is the right one to adopt.

Airflow Through cooling

One recent entry to the realm of higher-power cooling approaches for military COTS circuit cards is AFT cooling. Though relatively new to the military COTS cooling arsenal, AFT has been used by systems integrators for many years, most often in the form of an air-cooled duct featuring some metal finning. These ducts are called “compact core heat exchangers.” In these applications, AFT is typically implemented by bonding two single-sided PWBs to either side of the heat exchanger. Unfortunately, that is not really viable for COTS because virtually all available military COTS cards are double-sided, with components resident on each side.

When applied to COTS cards, AFT can be implemented by placing the heat exchanger on the top of the primary side of the circuit card (see Figure 1). Mezzanine cards can then be placed on top of the heat exchanger. While similar to DFA cooling in that air is blown over the electronics to cool the card, there is no direct contact of the air with the electronics. This eliminates the risk of exposure to contaminants in the air that must be considered with DFA. For AFT to work properly, air seals must be provided at the heat exchanger inlet and outlet of AFT cards. These are typically applied on the chassis side and need to be able to withstand hundreds of insertions and extractions. Additional features of AFT cards are retainers and injectors/ejectors, all of which are being standardized in VITA 48.5, the AFT standard, which is currently in draft form.

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Figure 1: 6U AFT module with air heat exchanger mounted on primary side of military COTS circuit card. This module can cool >200 W with 55 °C inlet air.

With AFT cards using similar airflow rates to DFA, it is possible to cool boards rated at up to 200 W. While AFT should be considered more of a mid-power cooling approach, it is superior to forced air and conduction cooling. The limits of AFT are related to the use of air for cooling, as they are with DFA as well. An asymptotic curve develops as the amount of air flowing over the heat exchanger increases. Compared to DFA, the decline in effectiveness is not as sharp because AFT employs an enclosed duct that can be made quite smooth. This duct and the smooth surface result in less flow bypass and lower pressure drops compared to those created by the natural ducts formed between two DFA cards, in which the components on the card surfaces present a rough route for the air to travel over.

Spray cooling

A chassis-level implementation of spray cooling uses a mist of coolant directed at the circuit cards. When the coolant hits the circuit card hot spots, it vaporizes and then condenses within the chassis. Alternatively, the coolant can be sent to a remote heat exchanger where it is condensed further and pumped back into the chassis in a closed-loop system. Because spray cooling is able to cool very high power densities (such as processor die) to relatively low temperatures (such as -70 °C to +75 °C), it can enable the use of true commercial-grade circuit cards (0 to +70 °C) in rugged deployed applications. In comparison, rugged COTS cards are typically designed to -40 °C to +71°C or higher temperature ranges.

Spray cooling can also be used to mitigate cold startup issues. At the low end of the temperature range, the coolant can be heated to warm the electronics before they are turned on. One drawback to this approach, though, is that the commercial circuit cards need to be so-called “spray” ready. For example, any air cooled heat sinks on the cards need to be removed because the spray needs to be directed at the component or the die. Also, the circuit card must use materials, the thermal interface materials in particular, that are compatible with the type of coolant in the spray-cooled system.

Other drawbacks of spray cooling are the additional weight and volume that the apparatus adds to the chassis (roughly 50 percent) as well as the increased cost and complexity of the system. In addition, spray-cooling systems, unlike LFT cooling systems (to be discussed in the next section), are dependent on the physical orientation of the hardware because of unfavorable gravity force (and other body forces like acceleration) effects on the condensed liquid.

Liquid Flow Through cooling

LFT is another relatively recent entrant into the ranks of military COTS cooling options, though it has been used for many years (if not decades) by system integrators in costly custom configurations. While similar to AFT, instead of an air-cooled heat exchanger LFT employs a liquid cooled heat exchanger, which in military COTS systems is typically placed on the primary side of the circuit card. The design of the heat exchanger will vary greatly depending on the heat density and heat distribution of the card to be cooled. As the liquid flows through the cores, it picks up the heat and then flows out at an end point. Liquids have much higher heat capacities than air [that is, 1-10 J/(cm3°K) vs. ~10-3 J/(cm3°K)], allowing cooling of higher powers in narrower pitches. For example, LFT can efficiently cool boards with a 0.85" pitch for which AFT would need at least a 1.5" pitch. One design consideration for LFT, however, is that the liquid coolant requires an air heat exchanger at some point, which adds weight and volume to the system.

Two-level maintenance LFT systems have the additional requirement of Quick Disconnect (QD) liquid connectors that close the liquid loop between the chassis and the module. These QDs and other features of LFT for COTS systems are currently being standardized within VITA:

  • VITA 48.3: Defines an approach where the liquid manifold is located in the chassis below the backplane
  • VITA 48.4: Enables the liquid manifold to be placed above the backplane, which eases use of a standard ATR height chassis
  • VITA 48.6: For use with LFT modules larger than 6U

Parker Hannifin and Curtiss-Wright Controls designed, manufactured, and tested an LFT module with some specific attributes in the cover that was able to cool more than 650 W (see Figure 2). This impressive result was achieved at +55 °C with a liquid coolant called PAO, a synthetic oil which has a relatively poor heat capacity. This project highlighted a particular benefit of LFT: the ability to use spreadsheet calculations to accurately predict thermal performance. This is not the case with air-cooling approaches, which typically require the use of Computational Fluid Dynamics (CFD) tools for accurate thermal predictions.

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Figure 2: 6U LFT module with liquid heat exchanger mounted on primary side of military COTS circuit card. This module can cool >650 W with 55 °C inlet PAO coolant.

The design considerations for LFT include the risk of leakage from the QDs, as well as the resultant increase in weight, volume, and system complexity. LFT does, however, eliminate the orientation dependence of spray-cooling systems because it uses pumped, single-phase cooling.

Staying ahead of the heat curve

While the need for higher-performance processing in smaller environments continues its inexorable rise, the military COTS industry is meeting the challenge with advanced cooling methods to stay ahead of the heat curve. Going beyond air and conduction cooling requires more complexity and greater system challenges, such utilizing relatively new cooling methods of AFT, spray, and LFT cooling. The open standards community is tackling these challenges to ensure that today’s best computing technology is available to the warfighter in the most demanding and harshest environments.

Ivan Straznicky is a principal mechanical engineer for Curtiss-Wright Controls Embedded Computing, where his responsibilities include advanced thermal and packaging technologies. Ivan is currently the vice chair of the VITA Standards Organization and a key contributor to the following standards/specifications: ANSI/VITA 46, ANSI/VITA 47, VITA 48, and VITA 42. He has a degree in Mechanical Engineering from McGill University in Montreal, Canada. He can be contacted at ivan.straznicky@curtisswright.com.

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