The push for increased capabilities and performance in manned and unmanned military vehicles is driving a need for reduced Size, Weight, Power and Cost (SWaP-C)—to be able to add new capabilities without placing an undue burden on the vehicle. As the demand for more processing capability on these platforms continues to increase, thermal management has become more of a challenge.

The embedded computing industry has developed new technologies such as OpenVPX in order to address the increased performance requirements of leading-edge multiprocessing and DSP applications. The 3U OpenVPX form factor is targeting small form factor applications, including unmanned vehicles. Unfortunately, the high performance of OpenVPX often comes with a price: increased power dissipation as compared with older architectures such as VME or CompactPCI. The ability to thermally manage higher power payloads is critical in these leading-edge multiprocessing and DSP applications while decreasing size and weight.

Alternative to Aluminum

Traditional electronic system packaging for military embedded systems has typically used aluminum. A different approach is to use a mixture of advanced metal composites to achieve enhanced thermal performance along with the structural characteristics that are needed for rugged applications. A thermal metallic composite core is captured within a structural metallic composite shell. The metallic composite shell has structural characteristics that are similar to aluminum, so this composite shell approach provides superior thermal performance without compromising ruggedization.

The metallic composite materials that are used in this implementation are field proven for different applications and have been qualified for severe environments including spacecraft. A full suite of environmental testing is planned for the next generation of Curtiss-Wright’s SFF-6 enclosure, which is in process now. Called CoolWall, this patent-pending technology has been developed to address needs for increased power and reduced weight by providing dramatically higher thermal conductivity at a weight that is lighter than aluminum. CoolWall can be used both to improve thermal performance and to reduce weight.

Improved Thermal Conductivity

Test results for the new base plate cooled SFF-6 chassis, shown in Figure 1 with CoolWall Technology, show a 2.4x increase in thermal conductivity at the chassis level (2.4x decrease in sidewall temperature rise) along with a 10 percent weight decrease as compared to aluminum construction. This is better performance than copper—which is commonly used as a heat spreader—but weighs 3x less than copper. Further refinements are underway, and we expect to achieve a 3x increase in thermal conductivity for CoolWall as compared to aluminum.

The composite shell technology has been initially applied in baseplate conduction-cooled small form factor chassis, targeting high power 3U VPX applications. However, the technology is not limited to chassis applications, nor is it limited to baseplate style applications. With high performance payload in baseplate conduction-cooled applications, the technology supports cooling of 2x the payload power vs. aluminum construction with a 71°C baseplate. 

Thermal profile test results show approximately a 9°C temperature rise of the chassis rails at 67W/slot power dissipation. This alone is a significant breakthrough for embedded computing applications. Beyond this level, CoolWall technology continues to be refined; over time, it is expected to achieve a 3x improvement in thermal conductivity as compared to aluminum (at lower weight).

Chassis Applications

The composite shell technology is patent pending for conduction-cooled chassis applications of various types. For example, the technology will be used to enhance thermal performance of natural convection-cooled, forced air conduction-cooled, or liquid conduction-cooled chassis, as well as baseplate conduction-cooled chassis such as the SFF-6. This will allow hotter payloads to be cooled without increasing weight compared to conventional aluminum construction, and/or weight can be reduced by switching to the technology.

Applications that are highly dependent on the thermal conductivity of the chassis walls benefit most from composite shell technology, providing the largest potential gains in thermal performance or weight reduction. Preliminary assessments of CoolWall technology have been performed for several different types of chassis configurations.

Forced Air Conduction Cooled 

A forced air conduction-cooled style of chassis forces air over heat sinks in chassis walls to remove heat. Preliminary thermal analysis of a forced air conduction-cooled ½-ATR enclosure with four slots of 6U cards arranged longitudinally according to Figure 2 predicts an approximately 38 percent reduction in the chassis rail temperature rise at the same power dissipation. There is meanwhile about a 37 percent increase in chassis power handling capability at 6 percent less weight. This chassis configuration is highly dependent on the thermal conductivity of the chassis walls, which is why significant thermal benefit occurs.

This type of chassis can be flexibly optimized for the application. Based on the requirements of the application, this opens up a lot of options. It can be optimized to increase the thermal performance for a higher power payload as described above. Or it can be optimized to reduce weight, to reduce fan power consumption or to improve reliability based on the lower temperatures. Some forced air conduction style chassis have air-cooled heat sinks located directly on conduction rails. These topologies do not benefit as dramatically with CoolWall, but improvements are still significant.

Preliminary thermal analysis of a forced air conduction-cooled 3/4-ATR tall side-load chassis with seven slots of 6U cards arranged transversely predicts an around 8 percent reduction in the chassis rail temperature rise at the same power dissipation. This improves to approximately 15 percent with uneven heat load due to improved heat spreading. There’s a 6 percent increase in chassis power handling capability. And this improves to about 10% with uneven heat load due to improved heat spreading, at 6 percent less weight.

Another variation on conduction cooling is baseplate conduction. These chassis styles conduct heat from the chassis conduction rails to a baseplate. Analysis was also done on the baseplate conduction-cooled SFF-4 (4 payload slots plus power supply) with thinner conduction wall using the composite shell technology. This application of the technology showed a 30 percent chassis weight reduction. It also exhibited the same thermal performance as aluminum construction SFF-4.

Natural Convection Cooling

Natural convection-cooled chassis styles conduct heat to the finned outer surfaces where natural convection air currents over exterior fins and radiation cooling remove heat. This type of chassis has relatively low power handling capability due to the high thermal resistance of a natural convection heat sink as compared to conduction or forced air convection heat sink.

Tests were done using the composite shell technology in a natural convection-cooled enclosure with two slots of 6U plus a one slot 3U power supply. The analysis predicted about a 17 percent reduction in the chassis rail temperature rise at the same power dissipation. Meanwhile, there’s a 17 percent increase in chassis power handling capability at 6 percent less weight. Here again this chassis configuration is highly dependent on the thermal conductivity of the chassis walls, which is why significant thermal benefit occurs. That said, it can be flexibly optimized for the application—optimizing for thermal performance, weight or lower temp suitability—in a similar way to the previous configurations.

Module Implementation

The composite shell technology can also be applied to conduction-cooled module conduction frames. (Curtiss-Wright has patents pending on these as well.) Conduction-cooled modules typically use aluminum conduction frames, but higher power modules have used copper or heat pipes because aluminum could not provide the required thermal performance. Because of its dramatically higher thermal conductivity at lighter weight than aluminum, the composite shell approach can provide significant thermal benefits for module applications as well.

Conduction-cooled module thermal frames can be flexibly optimized for the application. Based on the requirements of the application, one option is to optimize to increase the thermal performance for a higher power payload. Designers can also optimize to reduce weight or to allow operation at a higher card edge temperature. A third option, here again is the choice to optimize to improve reliability based on the lower temperatures.

Putting it All Together

The use of a new alternative approach like composite shell technology offers dramatic improvements in SWaP-C reduction for rugged conduction-cooled applications. The ability to handle higher power payloads is critical in many leading-edge multiprocessing and DSP applications using new products based on switch fabric architectures such as VPX, and CoolWall technology can be used to improve thermal performance, while reducing weight. Many lower power applications can also benefit from reduced weight, and CoolWall technology can be used to reduce weight while maintaining thermal performance.

The amount of thermal benefit is dependent on the chassis configuration and the application’s thermal constraints, but in any case CoolWall still offers a weight reduction on the order of 6 percent to 30 percent in many forced air conduction applications. Figure 3 shows a summary of the predicted power handling and weight improvements for different types of chassis.  

Curtiss-Wright Controls Electronic Systems
Littleton, MA.
(978) 952-2017.
[www.cwcelectronicsystems.com].