Sophisticated thermal management solutions cool hi-rel systems - Part 1

In part 1 of this two-part article, we will discuss 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.

Today’s demanding military applications require components and systems to push the limits of bandwidth and performance while enduring intense environmental conditions. Survivability is critical, as sophisticated features are only as good as their ability to operate without fail. The rigors of battle in remote locations continue to present an evolving range of unique challenges to military system designers. Everything from severe temperatures and shock and vibration, to explosive decompression, immersion, and exposure to sand and dust are variables that must be considered when building rugged, high performance systems for the armed forces. Innovative thermal management techniques have become essential to meeting these requirements for fault-free performance, and military designers are now making it a priority to solve cooling challenges early in the design phase.

By integrating cooling capabilities into the size, weight, and power (SWaP) protocol, packaging engineers have created SWaP-C (size, weight, power, and cooling) as a focus for next-generation solutions. Using commercial office-the-shelf (COTS) solutions as the foundation for semi- and full-custom thermal management, designers can increase the capabilities of subsystems and enable new designs that meet or exceed current mil/aero thermal requirements. Thermal options will vary accordingly depending on application, expertise, cost, and development time. As a result, engineers need to have a thorough understanding of how designs generate heat and how design choices reliably dissipate that heat. Knowledge of the primary thermal management methods illustrated in this paper will help designers determine the most appropriate path for their specific design.

Thermal demands
The modern military’s more extreme applications and increased performance needs mandate rugged design. The most critical and unique design requirements encompass a range of characteristics grounded in ever-increasing computational power and communication bandwidths. Many airborne and ground mobile applications, for instance, operate high-definition vision systems with real-time processing. These systems drive the need for significant power densities at the board, chassis, and platform levels. The growing deployment of unmanned aerial vehicles (UAVs) combined with current initiatives such as Brigade Combat Team (BCT) modernization has designers dealing with system aggregate power needs of tens of kilowatts. At the same time, system developers are required to deliver smaller, lighter, faster solutions with effective thermal management ensuring extreme reliability.
 
Higher temperature decreases overall reliability, and more transistors mean more heat is generated within these systems. Faster, higher-density chips equate to higher power densities overall and increased thermal cycling can lead to fatigue failures. Improved cooling has become a priority, as illustrated by designers embracing the concept of SWaP-C.

Thermal management options
Cooling challenges have increased in step with higher performing processors, smaller system footprints, and evolving rugged environments. The overall goal for cooling electronic military equipment is to maximize the flow of thermal energy from heat-generating equipment to a local heat sink. The heat sink could be ambient air, a cold plate, or a liquid exchange system. However, for all the demands of military applications—bandwidth, performance, form factor, and more—designers are always bound by the laws of classical physics.

Newton’s three laws of motion form the foundation of classical physics, which through the application of statistical methods are used to derive the basic laws governing thermodynamics. These laws establish that heat will always move from warmer areas to cooler areas through the action of one or more of the following principal modes of heat transfer (see figure 1):

1. Convection: Energy transfer by mixing action of fluids (gas or liquid)
2. Conduction: Energy transfer from one molecule to another
3. Radiation: Energy transfer by electromagnetic waves

Over time, real-world requirements have shaped these basic tenets into several principal and proven cooling methodologies:

• Forced convection
• Conduction paired with forced air or liquid
• Conduction paired with passive convection
• Conduction paired with a cold plate

The various cooling methods dissipate heat to varying degrees (see table). It’s important to note that there may be considerable variation in these values depending on environmental conditions.
All of these methods benefit from the ever-present effects of cooling by radiation; however, the contribution from radiative cooling is usually ignored unless the overall power dissipation is low. In these low-power scenarios, the fractional contribution of radiation can be significant and should not be ignored.

Military applications can vary greatly in their mission objectives. For example the thermal design goals for short- and long-range UAVs, man-wearable computers, and environments sealed to avoid any number of airborne contaminants may be quite different. A given thermal solution might need to tolerate the corrosive aspects of marine environments, or airborne dust may limit the design’s ability to force ambient air across electronic components. To determine an optimal cooling solution, packaging engineers must fully understand the boundary conditions of the system, its form factors, and its component-level attributes. By analyzing thermal demands of the end-use application, as well as understanding the unique requirements of individual devices designed into the system, designers can determine an optimal cooling scenario.