In part 1of this two-part article discussed primary thermal technologies and their specific features and benefits. In part 2, we will review examples, illustrating how thermal design choices meet the computing challenges of specific military program applications.
Introduction The modernized battlefield is characterized by a range of extreme applications featuring ever-increasing computational performance and communication bandwidths, resulting in greater than ever thermal challenges. For designers and packaging engineers, ensuring high availability and high reliability performance calls for effectively addressing the significant power densities generated at the board, chassis and system levels for each of these designs. Recognizing that thermal management is a critical element of mil/aero design, Kontron has developed a two-part white paper series as a resource to support designers. This series was developed to provide an increased understanding of thermal management issues, design options and technologies, and how they are implemented in real-world, rugged computing scenarios.
Kontron’s thermal design resource materials outline essential thermal management methodologies including the range of conduction and convection design options. Part One describes primary thermal technologies and their specific features and benefits in great detail. This paper is Part Two of the series, and follows with additional useful information through specific application examples, illustrating how thermal design choices meet the computing challenges of specific military program applications.
Part one summary To set the stage for the information presented in Part Two of the series on thermal management, the following is a brief summary of the content provided in the first paper. Higher performing processors, smaller system footprints and the evolution of extremely rugged environments continue to challenge military designers. Application evolution is constant, as troops and command centers are effectively armed with a flexible mix of mobile networked technologies and systems focused on sharing real-time information and transferring risk from soldiers to machines. UAVs, for example, have evolved considerably since the singular days of large surveillance drones such as Predator and Global Hawk; today soldiers also rely on a range of small UAVs, challenging packaging engineers to further address systems significantly constrained by SWaP.
For designers of these systems, using COTS-based solutions is essential to meeting military needs quickly and with a sustainable, long-term approach. Standards-based solutions have become even more critical as the SWaP protocol has transitioned into SWaP-C (Size, Weight, Power and Cooling). Standards-based solutions are providing the building blocks for semi- and full-custom systems, yet designers must further understand the related role of various thermal management methodologies in order to maximize system performance and the benefits of COTS design.
Proven cooling methodologies include a range of conduction and convection options, each with specific advantages and limitations that are further defined by the application environment itself. Designers must evaluate key environmental parameters such as altitude, ambient temperature and required power dissipation in order to make optimal thermal design choices early in the development process. Further, real-world expertise is essential to solving these complex thermal challenges, allowing a technical approach based on proven and validated legacy enclosures.
As temperature increase reliability decreases. For example, the failure rate, ?p, of a resistor is: ?p = ?bpTpPpSpQpE where pT is the temperature factor multiplier for the overall failure rate. From this example, we can see that increasing the temperature from 20°C to 40°C results in a failure rate increase of about 1.7. Mechanical failure can occur from differential thermal expansion and fatigue from thermal cycling.
Applying thermal methodologies to real-world designs
The modern military’s ongoing transition from Future Combat Systems to Brigade Combat Team (BCT) modernization is fueling the need for highly integrated embedded designs that leverage mobility, security, bandwidth and reliability in networked systems.
As a result, military computing represents a range of diverse applications, each with significantly different physical demands yet sharing a common requirement for rugged reliability. Man wearable computers, vetronics, shipboard, UAV and other airborne settings, command and control, imaging, and more challenge designers to understand how environmental conditions impact advantages and limitations of various platforms. For example, thermal management schemes for aircraft and satellites must be as light as possible, and meet aggressive parameters for SWaP. Thermal management of aviation systems must ensure extreme reliability, while accommodating limited weight and physical space – particularly since mission capability is often the key objective for military aircraft. Naval applications must effectively manage corrosive aspects of ocean environments while dissipating potentially excessive heat loads. Dust or other airborne contaminants may limit a system’s ability to blow ambient air across its electronics.
The cooling effectiveness of four essential methodologies is listed in increasing cooling effectiveness below, with natural convection being the lowest and liquid through sidewalls the highest. Empirical test results show the following temperature change associated with 75 Watts total power dissipation and a board edge temperature of 75°C:
The thermal design approach that has proven most effective over time is to implement all of the required system functionality in a chassis that has already been certified for ruggedized operation and is not simply listed as “designed to meet.” For example, selecting a chassis that is manufactured to meet the requirements of MIL-E-5400 Class 1 thermal performance, MIL-901D shock, MIL-STD-810F vibration, etc., assures the designer that it can withstand specified extremes of temperature, vibration, shock, salt spray, sand and chemical exposure, while maintaining a sealed and temperature-controlled environment for the computing elements and electronics inside.
The following application examples will highlight specific implementations of cooling solution, chosen based on key data points such as altitude, ambient temperature and power dissipation requirements – and also considered within the framework of application-specific requirements and physical constraints such as SWaP, form factor, shock and vibration and exposure to airborne contaminants.