Seventy-one degrees C is the temperature of a steak done medium-well. It is also the temperature of an oven used to test thermal characteristics of military electronics. Electronic components in the COTS industry often have temperature limits of 85°C, leaving 14°C of thermal potential to move the heat generated by the components away. Among various cooling approaches, the correct solution for your application depends on meeting your requirements at the lowest possible cost and complexity.

Approaches to Thermal Management

Table 1. Performance characteristics for different types of enclosures.
This article will identify five cooling approaches. In every case, the design differs by the type of enclosure used to house the electronics. Table 1 starts with the simplest possible vented enclosure in free air, progresses to a sealed forced-air system, and ends with a cold plate approach. Each approach is evaluated in terms of power, weight, ruggedness, and complexity. A computer simulation of the circuit card assembly (CCA)is shown in Figure 1.

The enclosure type identifies the cooling approach used. A vented approach encloses the CCA with a sheet metal box with holes provided in the top and bottom to allow air to directly cool the CCA. An example of a vented enclosure is a television. Fan-driven venting adds forced air to the enclosure, as used in laptop computers. Sealed with exposed fins means that the CCA is separated from external environments by the enclosure. Fins are added to the exterior of the box. Computers in armored fighting vehicles often use this approach. Sealed with fan-cooled fins adds forced air to the exterior of the enclosure. This approach shares popularity with fandriven open venting in aircraft electronics. Cold-plate presumes that cooling happens away from the enclosure, with heat moving through the material of the enclosure to a cooler temperature. The possible sources of the cool temperature are not included here.

Other factors to be considered in the evaluation of appropriate technologies for military electronics include thermal performance, enclosure weight, weight-specific performance, environmental susceptibility, and design and manufacture. The higher the thermal performance rating, the better. Thermal performance is the amount of heat in watts dissipated for each degree C that the CCA increases in temperature. Enclosure weight, measured in kilograms, depends on the substances used to create the enclosure. Simple sheet metal is the lightest component. A sealed box with large external fins can become impractically heavy. Weight-specific performance divides the performance number by the weight added to the CCA simulation by the cooling enclosure. Environmental susceptibility includes factors such as mechanical shock, electro-magnetic disturbances, and airborne contaminants. Design and manufacture takes into consideration the complexity of the design including a parts count and ease of procurement/ assembly.

Simulation

Figure 1. Simulation Model
Using a 3U form factor CCA with dimensions shown in Figure 1, we predict the temperature rise of a PCB and can calculate the thermal performance of the system. The PCB material property is COTS 8-layer, a laminate of fiberglass and copper that allows heat to travel thru the volume shown in green. 16 components are attached to simulate the presence of individual electronic components. Power is applied to the CCA as a uniform volume-based source inside the printed circuit board (PCB). The components are passive, but will transfer heat from the PCB to the surroundings to aid in cooling.

The vented design shown in Figure 2 is as simple to design and build as possible. The air in direct contact with the CCA heats up, creating air movement. After factoring in weight, a free-air vented system provides superior performance per pound of enclosure when compared to the sealed free-air system. However, at an ambient temperature of 71°C, most 3U CCAís dissipate more heat than this design can handle.

Figure 2. Vented Enclosure
In the fan-driven vented design, forced air and increased power dissipation are added to the system. Industry-wide “air-cooled” circuit card assemblies use this approach, with 10 cfm of air flow per CCA as a common specification. The fan-driven vented design offers good thermal performance and mediocre shock and vibration ratings. With weight factored in, the performance per pound of enclosure is excellent. The addition of a fan to an array of these CCA’s adds little to the overall system weight, although it negatively affects the parts count, complexity, and assembly. If air pressure drop across the CCA is sufficient, not many 3U CCA’s dissipate more power than this design can handle.

A sealed enclosure with exposed fins is shown in Figure 3. It was constructed from the previous enclosure by sealing the vents and adding external finning. Minimal environmental susceptibility is expected from this design, as the CCA is protected from exposure to environmental contamination and the lack of a cooling fan eliminates moving parts. The trade-offs are weight, parts count, and parts complexity. Although the highest power CCA can be kept cool with this design, the amount of cooling surface needed becomes impractically large.

Figure 3. Sealed wth exposed fins
Any sealed enclosure requires an additional conductive frame on the CCA to make up for the loss of direct cooling air on the electronic components , as shown in the Detail View of Figure 3. Further, the thermal performance of this frame may depend on a high quality surface contact to precision mounting rails added to the enclosure. The frame increases resistance to shock and vibration, making this design even less susceptible to environmental conditions.

The fan-cooled version of the sealed enclosure is shown in Figure 4. It shares all aspects of the previous design regarding parts complexity and adds a fan to the parts count and to environmental susceptibility. The payoff is in thermal performance. This finning approach weighs one tenth of the previous version, has only one sixth of the cooling area, but still performs twice as well. It performs 4 times better than the previous version if weight is factored in.

Figure 4. Sealed with fan cooled fins
The cold plate enclosure eliminates the cooling fins and the cooling air as well. It retains the environmental and shock toughness of all the sealed designs. Cooling is now merely the act of holding the conductive frame at a constant temperature. This design is the best thermal performer of all, even at 71°C. The sink can be so effective that the power dissipation of the CCA can’t raise the temperature of the contact surface.

The trade-off is dependency. Implied in this approach is a separate heat sink, whether it be a vehicle, the hull of a ship, or a refrigeration system.

No matter what the application in military electronics, available thermal technologies can accommodate the heat generated and dissipated. If the next breakthrough in electronic hardware technology is significant, every enclosure would ideally be a vented box. Until the creation of such revolutionary technology, thermal engineers need to evaluate their options in selecting a cooling method on cost, complexity and weight to keep todayís electronics from damaging applications.

This article was written by Mark Dimick, Mechanical Engineer, Curtiss-Wright Electronic Systems (Santa Clarita, CA). For more information, visit http://info.hotims. com/34459-502.