Cooling electronic boards can be a huge challenge, especially where powerful systems need to be ready for fanless operation. Dissipating heat using a purely mechanical coupling is expensive and consumes valuable real estate, not to mention that the boards need to be specially designed for this cooling concept from the start.
The transfer of excess heat from its place of origin to the heat-absorbing medium is much like thermal resistance found in a series of resistors. Every layer adds thickness to the total thermal resistance, depending on its material and surface. For an IC, for instance, this is the package filling, the heat conducting film, and the heatsink. Obviously, the higher the thermal conductivity of the layers involved in the transfer is, the more efficient is the cooling.
Traditional ways to improve thermal management include simple convection cooling, where air flow is guided along the surface to be cooled. But you also run the risk of that air introducing impurities and liquids into the device that can damage the system. Although filtering equipment is available, you have then added more components that could potentially wear out and require additional maintenance over the life of the system. Simple convection cooling isn’t so simple anymore.
Conduction cooling, on the other hand, doesn’t require air flow. It essentially uses the system structure and existing components to turn the enclosure into a radiator, drawing heat away from the system to reduce thermal effects. Positioning is key in this cooling method, since system components need to be placed correctly to facilitate heat transfer. And, there needs to be enough contact points and dissipation area to accommodate all the heat generated.
Building On Tradition
While conventional conduction cooling is useful for individual printed-circuit boards (PCBs), 19-in. racks with multiple boards preclude the cards’ surfaces to connect directly to the enclosure wall. The increasingly dense embedded systems being built today are requiring more and more components, boards, and complexity, which all equals more heat that needs to be dissipated from deeper within the system.
System designers need to find alternative methods to get the heat out of the system, such as building upon the principles of conduction cooling. This could help improve heat dissipation in areas such as the contact surface between the plug-in board and guide rails to draw heat out from deeper within the system.
In one implementation of this design, an aluminum board assembly fits over the critical heat areas, essentially extending the overall conduction-cooling principles. Another way uses the actual component contact surfaces on the PCB to transfer heat through additional copper layers inside the PCB. Of course, both impact the available surface for component placement and determine the number of copper layers, so these methods need to be undertaken at the onset of PCB design.
But what about existing boards? Can we use these concepts for already designed implementations? If we accommodate for slightly more rack space than required for traditional boards, it can be done.
By using the guide rail area as an extension of the conduction-cooling method, the designer can clamp the PCB to an aluminum frame, which totally encloses the card. Even components at the bottom side can be thermally coupled to the frame, resulting in exceptional heat management.
Standard 3U Compact PCI boards can be installed in a special rack system and operate without convection cooling. And the benefits, compared to common conductive cooling technology, are great, too. Being standard products manufactured in larger volumes, the boards are considerably less expensive. A card also can host more functionality because the cooling infrastructure does not take away precious space.
Extending Cooling For Rugged Requirements
Many rugged applications require embedded systems to operate in extended temperature ranges. In addition to its inherent shock, vibration, and electromagnetic compatibility (EMC) attributes designed specifically for rugged applications, ANSI/VITA 59: RSE (Rugged System-On-Module Express in preparation with VITA, or ESMexpress) specifies an advanced cooling method standard.
The fanless cooling concept used in ESMexpress systems is a kind of hybrid configuration. By utilizing elements of both conduction and convection cooling, ESMexpress utilizes the best of both methods, allowing systems to remain reliable even in extended temperature ranges.
Reliable components are key to harsh, mission-critical applications. If an embedded system is being used in an application where computer failure would lead to a production standstill or loss of life, VITA 59-compliant products and the rugged characteristics the components embrace could be critical to system operability in rugged applications.
The Issue Is Just Heating Up
So, how will designers truly implement these alternative cooling methods? Will liquid cooling surpass all current methods? And just how much heat can be dissipated with these newer methods that build on the proven conduction-cooling concept?
The final result remains to be seen, but the future is clear. Thermal management in embedded systems will always be at the forefront of design, especially as components shrink in size, while adding more functionality, as full systems are required to process more data at fast rates and as embedded systems move into more rugged applications with extreme temperature requirements.
Barbara Schmitz, chief marketing officer of MEN Mikro Elektronik since 1992, is responsible for public relations and product positioning as well as development and coordination of global sales channels. She graduated from the University of Erlangen-Nürnberg. Later, she studied business economics in a correspondence course at the Bad Harzburg business school and followed an apprenticeship in Marketing and Communications in Nuremberg.