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.