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Thermal management of electronics

01 August 2008

The efficiency and performance of mission-critical electronics can be significantly affected by operating temperature. Fluid based thermal management technologies are very sophisticated and they demand high-integrity connections that can be made and broken in an instant without contamination or spillage. We review the technologies and illustrate them with examples from a specialist’s portfolio

Traditionally, most electronics used in the military sector have been air cooled, using natural convection via metal heat sinks. This passive approach can accommodate the cooling requirements of fairly simple electronics, but where greater power densities or reduced available space are encountered - often associated with faster operating speed - or where there is a requirement to maintain the thermal environment within a close temperature range, then frequently this type of system proves inadequate.

Recent significant increases in data processing speed, moving from parallel to very high speed serial bus technology in CPCI, VME, VPX, VXS, and MicroTCA architectures, have created an opportunity for new and innovative high power enclosures. With high speed, comes high power, increasing junction temperatures and therefore a greater need for efficient cooling.

There are a variety of cooling techniques that can be applied. The first to consider is forced convection, using fans alongside the heat sinks to increase air flow and the cooling rate. A further step up from this is the fluid phase change system, where closed loop heat pipes allow the rapid exchange of heat through evaporation and condensation of the piped refrigerant. Ultimately, there are liquid cooled systems where coolant is pumped through cold plates which, with an associated heat exchanger, cool the system.

Generally, liquid cooled technology is reserved for applications involving high heat density. Here, forced convection or phase change systems are challenged by the power demands and are unable to dissipate the heat generated. But fast and secure connection of the coolant circuits is vital, and this is an area where fluid connector technology is increasingly playing an important role.

Heat pipes and cold plates
Embedded heat pipe assemblies transport heat away from areas where power is concentrated. They facilitate this transfer passively by employing an evaporator and condenser in a closed loop, spreading heat evenly within the unit‘s base or transporting it to peripheral fins. Both small and large diameter heat pipe assemblies are available and these function much in the same manner; however, the larger diameter assemblies are capable of transferring much greater thermal loads because of the increased volume of refrigerant and the circumferential surface area of the pipe.

Liquid cold plate heat sinks provide the best thermal performance per unit volume and counter nearly every drawback associated with air cooling by dissipating more heat with considerably less flow, better thermal performance and less local acoustic noise. Cold plates take advantage of the increased thermal conduction properties of liquid by actively circulating fluids past a heat source through a closed loop system. These liquid passageways are formed by sealing machined channels or adding copper or other tubing to a metal base plate.

Micro channel cold plates provide the superior performance of traditional liquid cooling in a much smaller footprint by forcing fluid through a network of miniature passageways in a cold plate, mounted directly above the heat source. These compact active techniques are suitable for high performance microprocessors and other high heat flux density applications. The major advantage of micro channel heat sinks is the high heat transfer coefficients, which are up to 60 times higher than conventional heat exchangers. Micro channel heat sink technology will have a major impact on the future of electronic packaging and design.

However, due to the construction and size of the micro bore, the associated fluid pressures are extremely high, and high-pressure pumps are required. Consequently, the design and integrity of the fluid connections will be a concern in the future, with multiple small diameter, high pressure lines needing to be connected safely and reliably.

Spray cooling
The technology of spray cooling is relatively new and under continuing development. It involves spraying a fine mist of non-conducting fluid over the areas to be cooled. The phase change associated with evaporation of the fluid on contact with a hot surface provides a very efficient direct cooling effect. Evaporate is captured and cooled via a heat exchanger and returned to the circuit in a closed loop. Again, high-integrity fluid connections are required for each ‘spray’ and ‘receive’ device, as well as the circuits between the main pump and heat exchanger system. Moreover, all these connections require effective management.

With all fluid thermal management systems there is increasing complexity associated with pumps and external heat exchangers, all of which present specific fluid connection challenges - for example, micro couplings for board mounting or large connectors for the main feed. The environmental and operating conditions of military vehicles and aircraft demand high-integrity, quick-release connections that prevent the introduction of fluids to a closed loop circuit as well as avoiding fluid spillage on disconnection. Mechanical vibration and temperature fluctuations have significant effects on the overall success of such connection systems, so care must be taken when designing them and specifying components for such demanding applications.

Fluid connector specialist, Stäubli, has wide experience of this sector and has provided reliable and efficient connection systems for the most demanding applications.

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