There is no shortage of challenges facing thermal engineers in today’s military and aerospace applications, from the inherent problems caused by harsh environments to the challenges created by advances in electronics and other technologies. And military specifications are continually upping the ante when it comes to performance, footprint and weight.

Figure 1. The encapsulated APG material has three times the conductivity of copper with a mass density less than that of aluminum. The APG insert provides a high k path while the encapsulating shell sets the CTE and structural properties. Thermal vias may be used to increase the conductance into the APG.
Thermal devices for today’s military system must be small and compact, due to stringent weight and space/volume constraints, as in fighter aircraft, where every ounce and each cubic inch can impact performance. Thermal solutions must also be able to reject heat efficiently, even where packaging volumes are limited and cannot be expanded, as in satellite applications. Maintenance needs must be kept to a minimum because repairs may be difficult in the field or at sea (e.g., submarine electronics cooling systems), or even impossible in certain aerospace applications such as satellites. Military systems must operate under conditions far more demanding than most civilian applications, from radar transmit-receive modules functioning in oppressive desert or tropical heat, to satellite and aerospace systems that deal with both electronicsgenerated heat and intense ambient cold. Yet mission-critical reliability is a must in all cases, whether conditions allow traditional maintenance or not. Finally, thermal engineers are being asked to help electronics designers meet the strict size, weight and power (SWaP) requirements set forth in today’s military specifications.

Thermal engineers working with military systems must also deal with the same challenge that all thermal engineers have faced in the past few decades, namely increased heat generated by advanced and increasingly miniaturized electronics in spaces that are continually becoming smaller. Thermal engineers have developed a number of creative approaches to deal with these challenges, but all have their drawbacks. Traditionally, designers have favored passive heat transfer devices like heat sinks or heat pipes, generally made from aluminum or copper. These thermal devices offer the advantage of having no moving parts to fail, reducing maintenance needs to a minimum. But aluminum has limited conductivity, which has a greater impact as electronics become more powerful, and copper is relatively heavy (three times as heavy as aluminum).

Table 1. Common electronic packaging materials. The relatively high specific conductivity of aluminum, combined with its affordability, explains its wide use as a heat sinnk material for space and airborne applications.
However, there is one promising thermal management approach that avoids these problems. It involves the use of annealed pyrolytic graphite, or APG, encapsulated within a structural shell made from traditional materials such as aluminum, copper, beryllium, ceramics or composites. Encapsulated APG was first used operationally in high-flying DoD aircraft, where its lightweight characteristics earned it early acceptance in applications where each pound saved could be transformed into another pound of fuel or additional avionics. The low mass of encapsulated APGbased solutions is still a key factor in reliable cooling solutions for remote electronics and navigational avionics.

But encapsulated APG offers additional thermal advantages that go beyond light weight, and apply to many military systems. The most fundamental advantage is high conductivity at low mass. Encapsulated APG material offers three times the conductivity (k) of copper with a mass less than aluminum. This results in a significant improvement in conductivity for any encapsulant paired with APG. Encapsulated APG’s high conductivity, combined with its low mass density, results in a material system with outstanding performance per pound, or specific conductivity (W/m·K/g/cm3). The specific conductivity of encapsulated APG materials range from 4- to 10- times that of traditional thermal management materials. For example a copper encapsulated APG heatsink with an 80% APG volume fraction would have approximately eight times the specific conductivity of copper alone.

Table 2. Encapsulated APG components with common electronic packaging materials as the encapsulating shell. The calculated values are for the in-plane heat flow with a 60 percent volume fraction of the APG insert.
Another benefit of encapsulated APG is that the coefficient of thermal expansion (CTE) offered by this solution can be tailored to specific application needs by altering the choice or configuration of the encapsulant. CTE can also be matched to a specific application, allowing dissipation of dramatically increased heat fluxes by permitting direct attachment, thereby minimizing thermal resistance. By combining the high thermal conductivity of APG with an easily-tailored CTE encapsulation material, engineers can create solutions for high-powered military electronics while keeping weight and footprint under control. Designers can choose the encapsulant that most closely matches the CTE of electronic materials such as silicon and gallium arsenide, allowing the direct attachment of these devices and providing the thermal benefits of both APG and the encapsulation material.

Encapsulated APG also offers simple integration into current and planned systems. Because the APG is hermetically sealed within the encapsulating material, it is compatible with standard finishing and processing manufacturing steps as well as with the encapsulation materials themselves. These encapsulated APG solutions, with no moving parts, give thermal engineers greater design flexibility, more durability, and less maintenance concerns.

Figure 2. The placement of a thermal via will improve the through-the-thickness conductivity to that of the encapsulation material.
All of these advantages make encapsulated APG an ideal material for military applications, enabling the technology that satisfies today’s needs for high-density packing requirements in a limited space. Thermal technologies based on encapsulated APG have proven to perform well under demanding temperature, stress, load, vibration and other conditions as protection for avionics, target acquisition, imaging and other systems in mission-critical applications onboard fighter aircraft (such as the F-16, F-22 and F-35 Joint Strike Fighter) and helicopters. Encapsulated APG-based solutions help sensitive electronics continue to function in temperatures down to -70° C and in 9g load conditions.

Another critical and popular use of encapsulated APG is in outer space. Here, where maintenance is obviously impossible, encapsulated APG-based radiator panels, thermal doublers, brackets, and circuit card heat sinks provide lightweight, high-performance heat management in the harshest of environments, keeping irreplaceable electronic components online. The reliable conductivity of encapsulated APG is particularly valuable in these applications because the thermal area for rejecting heat cannot be increased.

Figure 3. Aluminum encapsulated APG power supply chassis for an airborne electronics application. The conductance of this k-Core chassis was nearly four times higher and had an 11% lower mass than the baseline aluminum part of the same geometry.
In addition, encapsulated APG-based technology has proven successful in meeting one of the strictest requirements in military systems design – reduce weight and footprint to a minimum so more weight and space can be devoted to other system elements. At the same time, APG delivers the performance that cools even the most powerful electronic components and next-generation components, allowing designers to focus less on thermal management and more on core technology. For example, encapsulated APG lets system designers pack in more powerful electronics and more components in tighter spaces. Electronic components can also be made much smaller, both because APG-based solutions can handle the resulting increased heat, and because the APG-based solutions are smaller in themselves.

The properties of encapsulated APG, at work in so many military applications today, are also opening up new possibilities for the future. One example is flexible thermal links for aircraft, integrating APG with a flexible heat pipe to cool target acquisition sensors while isolating them from the aircraft’s vibration. The flexibility of APG and the strength of the encapsulation material combine to provide a thermal solution with mission-critical reliability.

Figure 4. These thermal scans of two power supplies dissipating the same power and having the same geometry illustrate the benefit of high conductance. The k-Core supply chassis’ high conductance results in a significant temperature reduction over the baseline aluminum design.
In conclusion, encapsulated APG offers significant application-based ad vantages for thermal engineers that outweigh the higher initial cost. APG is relatively easy to encapsulate, manipulate and finish. It is compatible with a wide range of encapsulation materials and standard manufacturing steps, and offers the opportunity to CTE-match encapsulants with electronic materials. The resulting design flexibility can lead to more effective, reliable military electronic systems, with a greater range of applications. Finally, thermal engineers are still exploring the potential of this thermal technology, and are developing new solutions that will make encapsulated APG even more valuable and widely used in future military systems.

This article was written by Mark J. Montesano, VP of Engineering and Technology, Thermacore, k Technology Division (Langhorne, PA). For more information, Click Here