Conduction paired with cold plates or liquid
Some applications cannot afford access to a replenishable air supply for either forced convection or passive convection cooling. In these cases, a conduction chassis paired with a cold plate may be the optimal thermal configuration (see figure 6). As with any conduction-cooled chassis, heat is conducted to the chassis side walls, but in this case the heat is directed to a bottom- or side-mounted cold plate. The cold plate itself can be either actively or passively cooled.
Figure 6: A cold plate design (top) provides a way to dissipate heat (bottom) when a design cannot access a replenishable air supply.
When power dissipation demands exceed the thermal capabilities of forced-air or passive convection systems, designers can consider liquid-cooled conduction systems (see figure 7). The liquid typically used to move heat is a water/glycol mixture or non-conducting fluid such as Fluorinert, an inert fluorocarbon fluid. Design options include liquid flowing through hollow side walls of the chassis or direct impingement of a dielectric (non-conducting) fluid on the system components known as spray cooling. In both methods, the liquid requires a separate heat exchanger, which can be within the enclosure itself or external to it. The need for a heat exchanger is a disadvantage to liquid cooling, adding weight, cost and complexity to the system that ultimately decreases overall MTBF. For enclosures generating power in excess of 1 kW, liquid cooling may be the only option.
Figure 7: This liquid-cooled enclosure uses a high performance, blind-mate-style connector to enable rear I/O, including copper and fiber-optic signaling. Fluid is introduced through quick-disconnect-style connectors that are designed with automatic shut-off poppet valves.
It is a common misconception that liquid cooling is required for systems dissipating power in the range of 500 W to 1 kW. These systems can be cooled using conduction paired with forced air; however, they do require multiple high-performance fans and careful design modeling. The advantage to using conduction with forced air in these applications is the absence of the heat exchanger, which requires a costly and heavy infrastructure for support. This is an especially important consideration for airborne or ground mobile combat applications in which reliability and reduced weight is essential.
Another example of liquid cooling is the use of liquid flow-through modules. These designs provide superior thermal performance by delivering coolant closer to the sources of heat within the system, in turn enabling higher power densities. In these designs, liquid, in addition to flowing through the chassis side walls, also flows through special channels on the system boards themselves (see figure 8).
Figure 8: 6U VPX liquid flow-through modules route cooling liquid directly through the system boards themselves.
Semi- and Full-custom thermal solutions
To adequately cover the range of functionality in today’s military mobile device applications, designs must protect systems against heat, dust, and other airborne contaminants. Some of these added requirements can further constrict system airflow, and demand customized attention to thermal management.
Due to the diversity of requirements, optimal designs address performance, cost, reliability needs, and development timelines. Thermal management, in particular, must be dealt with head-on. Packaging engineers first need to calculate a system’s thermal requirements, then make design decisions accordingly. Selection of the appropriate thermal solution also hinges on selecting a manufacturing partner that is experienced across the range of platforms and form factors who can serve as advisor. Ideally this type of manufacturing partnership provides a significant competitive advantage to designers, with all critical levels of expertise combined into a single engineering resource.
Increasingly complex military systems are driving increased challenges in integrating effective thermal management methodologies. Making early choices about power dissipation, design layouts, paths for air flow and overall thermal performance has become essential to developing rugged systems suitable for mission-critical military environments. On the positive side, advancements and the broad scope of thermal methodologies benefit packaging engineers who have a deep understanding of each type of available solution.
Rugged military applications demand it all: higher performance with higher speed and density components, coupled with smaller form factor boards and reduced system footprints. As designers continue to push the envelope with increased functionality, new thermal management options will continue to evolve to satisfy requirements for more efficient cooling solutions to match new standards specifications or increased ruggedness. Managing the complex cooling issues associated with many of these unique or extreme environments requires broad thermal expertise, and a thorough understanding of supporting design choices including working with a strong partner that can provide extensive expertise in how and when to provide proven semi- and full-custom solutions.
In part 2, we will review examples, illustrating how thermal design
choices meet the computing challenges of specific military program
Predicting thermal runaway
Avionic & military applications need -55 C operation
About the author
David O’Mara is product manager at Kontron. David has a diverse engineering background that includes extensive experience with military and aerospace electronics packaging, pressure and accelerometer sensor design, and high pressure solid state physics. He graduated from UCLA in 1985 with a Bachelor of Science degree in Physics.
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