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Solutions and Standards Advance for Box-Level Cooling

In order to use the fastest, cutting edge processing, box-level systems need ways to eliminate the resulting heat. A variety of technologies and products help smooth the way.


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In part because military system developers are risk-adverse by nature, it's taken years for acceptance of any kind of cooling technique more exotic than clever heatsink configurations. But as demand for more computing power continues, methods like liquid cooling, spray cooling and even fan-based cooling are being designed in and deployed by some, and at least considered by others.

Driven by Moore's Law, electronic systems can always expect ever faster processors, ever larger memories and ever speedier interconnects to feed even the most hungry computing appetites. However there's a down side when these cutting edge processors are clocking at high speeds: the silicon becomes thermally excited and heats up. It's a constant battle to remove that heat before it gets to the threshold where that heat can't be removed fast enough. Moreover, achieving that same leading edge of computer performance in an embedded application is a real challenge-particularly in harsh or rugged environments like military aircraft or ground vehicles.

JLTV Cooling Design Win

In a vehicle example of cooling, in May Aspen Systems announced they would be supplying Environmental Control Units (ECU's) for critical communications electronics cooling for the Joint Light Tactical Vehicles (JLTV) being produced by Oshkosh Corporation (Figure 1). The initial JLTV production contract called for a delivery total of nearly 17,000 vehicles, while the total program is expected to result in over 50,000 JLTVs delivered to the US Military.

Figure 1
Aspen Systems’ Environmental Control Units (ECU’s) were selected for electronics cooling for the Joint Light Tactical Vehicles (JLTV) being produced by Oshkosh.

Aspen's miniature vapor compression environmental control units have been successfully deployed on the SOCOM MRAP's. These ECU's are used to cool electronics mounted outside the passenger cabin in the rear of the vehicle.  In addition to providing 550 W of cooling, the system also provides up to 300 W of heating for temperature and humidity control for critical communications systems. These ECU's are 5 times lighter, 4 times more energy efficient, and half the size of thermoelectric based systems.

Many Cooling Options

As military system developers have begun to embrace cooling methods beyond basic conduction-cooling, a number of vendor-created cooling solutions have emerged. At the same time, VITA has churned at a set of Working Groups and VITA Standards addresses different types of cooling. The table in Figure 2 lists several types of cooling options and their associated VITA standards (where applicable). For its part Mercury Systems offers solutions across all those types of cooling.

Figure 2
Table lists several types of cooling options and their associated VITA standards (where applicable). (Source: Mercury Systems whitepaper “Agnostic, scalable OSA packaging and cooling”).

According to Mercury they focus is on lowest risk adoption and scalability and on keeping cooling solutions modular and open system agnostic enabling them to span across platforms and form factors. The companies cooling technologies cool the most powerful processors and RF/M devices for greater processing density across embedded open systems, including 3/6U OpenVPX and AdvancedTCA. Figure 3 shows Mercury's Air Flow By solution which is it considers the most efficient OpenVPX air-cooling technology available. Modules are sealed removing the need for filtration

Figure 3
This Air Flow By solution provides an efficient OpenVPX air-cooling technology. Modules are sealed removing the need for filtration.

Earlier this year, Mercury rolled out its newest cooling technology Liquid Flow-By. Liquid Flow-By enables open system architecture (OSA) modules to operate unrestricted and reliably, regardless of the presence of a cooling air supply. Liquid Flow-By integrates liquid cooling capability into Mercury Systems' Air Flow-By technology. Both Air and Liquid Flow-By have a technology readiness level of nine (TRL-9), uniquely deliver double-sided cooling and are compliant to the rugged OpenVPX and ATCA OSAs for technology insertions. The cooling liquid may be the platform's own fuel supply which enters each module through non-drip, quick disconnects to complement or take over the native air cooling capability of each module.

New Air Flow Open Standard

In January VITA announced its most recent cooling Working Group focused on the air flow through (AFT) cooling standard, VITA 48.8, for use in size, weight, power and cost (SWAP-C) constrained 3U and 6U VPX module-based systems. VITA 48.8 seeks to achieve weight and cost reduction for AFT cooling by eliminating the use of wedgelocks and ejector/injector handles.

The resulting open standard for the VITA 48.8 will bring AFT cooling to COTS 3U form factor VPX modules. It will also help reduce weight and cost for cooling high density, high power dissipation 3U and 6U module based systems. Because VITA 48.8 does not use module-to-chassis conduction cooling, it also promises to help drive innovative use of new lightweight plastic or composite material based chassis. The VITA 48.8 Working Group is chaired by Curtiss-Wright Defense Solutions, with Lockheed Martin serving as the standard's editor. The working group has set a goal of submitting the finalized VITA 48.8 draft for ANSI ratification later this year.

Cooling in High Shock Environments

One of the reasons the defense industry was so reluctant to embrace exotic cooling technologies is that that often the solutions are at odds with harsh environmental requirements of platforms like military vehicles. General Micro Systems addressed that problem by crafting a cooling solution "from the ground up." Its cooling approach is that each electronics module in the system is designed from the ground up to be conduction cooled and to meet rugged MIL standards, such as MIL-STD-810G, MIL-S-901D, MIL-STD-1275E, MIL-STD-461F and DO-160D, to mention a few. GMS's products are equipped with GMS' patent pending RuggedCool.

The company claims that it allows systems using Intel-based CPUs with a TjMax of 105 degrees C to operate in an industrial temperature environment (-40 to +85 degrees C) at full operational load without throttling the CPU. At the heart of Rugged Cool is the choice to employ a corrugated alloy slug with an extremely low thermal resistance to act as a heat spreader at the processor die. That's in contrast with other approaches that use thermal gap pads to conduct heat from the CPU to the system's interface to the cold plate.  Once the heat is spread over a much larger area, a liquid silver compound in a sealed chamber is used to transfer the heat from the spreader to the systems' enclosure.

An advantage of RuggedCool technology is its effect on shock and vibration. With this technology, the CPU die does not make direct contact with the system enclosure, but rather connects via a liquid silver chamber which acts as a shock absorber. This shock absorber prevents shock from being transferred from the enclosure to the flip chip ball grid array (FCBGA), thus saving the CPU from micro-fractures.

Cooling System Specialists

While embedded computing vendors offer some of the most innovative system cooling solutions, there's a number of cooling product specialists that continue to advance their offerings. One example is Advanced Cooling Technologies (ACT). This month ACT announced it has increased its design and manufacturing capabilities to meet increasing demand for its Phase Change Materials (PCM) heat sink products. According to the company, these temporary thermal storage solutions are becoming a critical element in military applications involving short mission durations, pulse mode operation, and directed energy.

In these heatsinks, the PCM absorbs heat during device "on" times, without increasing temperature, by undergoing a solid to liquid phase change. The heat is dissipated during device "off" times through the reverse liquid to solid phase transition. This method allows for simple, lightweight thermal solutions. In many cases, complex liquid or air cooled systems can be replaced or coupled with a thermal storage component to reduce SWaP. Specifically ACT has developed the several tools to enable faster and more accurate response to customer needs.  These include Internal Geometry Optimization Tools; Cross Channel Communication, Pressure Management; Repeatable and precise processing and sealing; Transient Modeling and Testing Validation; Multiple Enclosure Manufacturing Options (3D printing, laser welding, TIG welding, brazing).

Multi-Sectional Fansinks

Another cooling specialist product is Advanced Thermal Solutions QuadFLOW fansink, It uses directed airflow in multiple fin fields to cool electronic components up to 30 percent more effectively than the liquid cooling methods currently deployed in 1 and 2U commercial servers. Conventional heat sinks provide large amounts of cooling surface relative to their volumes. They work well in many applications where space is not an issue, including multi-U servers and workstations. But where space is highly constrained, cooling methods that rely on increased surface areas and airflow rate can't adequately dissipate heat from high power electronic components.

The patented QuadFLOW fansinks are engineered for cooling high power electronic devices, such as CPUs and GPUs, using air as the coolant (Figure 4). They are designed to maximize extraction of heat from their integral heat sink fins. ATS uses its own Multiple Flow Entrance Technology to engineer several fin fields on the same heat sink base area. These fin fields are not parallel to each other, but instead are designed to allow airflow to enter from multiple directions. The patented QuadFLOW internal wall design prevents any coupling of airflows between adjacent fin fields.

Figure 4
QuadFLOW fansinks maximize extraction of heat from their integral heat sink fins. These fin fields are not parallel but instead are designed to allow airflow to enter from multiple directions.

Advanced Cooling Technologies
Lancaster, PA
(717) 295-6021

Advanced Thermal Solutions
Norwood, MA
(781) 769-2800

Aspen Systems
Marlborough, MA
(508) 281-5322

Curtiss-Wright Defense Solutions
Ashburn, VA
(703) 779-7800

General Micro Systems
Rancho Cucamonga, CA
(909) 980-4863

Mercury Systems
Chelmsford, MA
(978) 967-1401