In the PC world, “case modding” is quite common to enhance the thermal behavior of computers. You can buy a mainboard completely dipped into cooling oil, with a pump making the liquid circulate. This rather extravagant idea would make embedded systems designers smile, but it would not solve any of their design problems.
Facing space-conscious requirements, they need to use smaller form factors, and they are continually being pressured to do more within that reduced space. As if that is not enough, embedded computers now also conquer markets that bring in extreme application demands. These include medical, industrial and especially mobile applications.
Beside small footprint and ruggedization, any system needs specific functions. In the most ideal case, enough flexibility is left for technological upgrades, which in turn makes component compatibility a nice-to-have treat. This sounds as if the costs of embedded designs will soon be exploding because they have to go into so many details. This is where COMs come in.
Computer-On-Modules (COM), or “System-On-Modules” (SOM), incorporate a complete computer on a plug-on mezzanine card. The carrier board customizes the system: it is individually tailored to each application, including specific I/O.
Since the introduction of the COM concept, embedded system designers have reaped a variety of benefits from that technology. Making it easier to replace modules of compatible form, fit and function has enhanced design flexibility, extended application versatility, reduced development costs, and optimized time-to-market.
But despite standardization efforts, these benefits still were not readily accessible for rugged system performance in harsh operating environments. Mobile and safety-critical applications face the same constraints as less harsh applications, but the costs of failure are higher.
The transportation industry, which encompasses both land and air vehicles, has stringent standards itself in terms of the needed ruggedization and manufacturing specifications. Mobile applications, found in areas as diverse as medical engineering, commercial vehicles and ships, are rugged by nature. A growing number of cranes, construction machinery, trucks and tractors are equipped with small, robust human-machine interfaces.
Medical engineering includes safety-critical systems such as movable or portable systems for imaging or to monitor, anaesthetize or treat patients. Outdoor computers are used for telecommunication, in traffic monitoring, on oilrigs or in the security sector for video surveillance applications.
Many environmental aspects come to mind for electronics systems. Computers are subjected to high levels of shock and vibration. For the COM concept with multiple PCBs and board-to-board connections, this is a serious issue. Other factors are dust or moisture, for instance in outdoor applications, but also EMC and finally heat as one of the most typical problems.
Heat needs to be drawn away from the components to prevent them from overheating. And increasingly dense embedded systems with more and more components and complexity equal more heat to be dissipated from deeper within the system. For semiconductors there is a maximum junction temperature above which the semiconductor ceases to work.
Convection cooling is the easiest method to cool board assemblies, by guiding an air flow along the surface to be cooled. The mechanical set-up is simple, but flowing air can bring impurities and liquids into the device, which can cause damage. The complex filtering equipment necessary to reduce this risk and the cooling fans themselves, having a limited lifetime, create the need for maintenance.
Conduction cooling optimizes the thermal contact to lead the heat from the source to the outer wall of the enclosure. Suitable materials minimize the thermal transfer resistance from the electronic components to the enclosure wall. The boards inside the housing need to be placed in the right way, and the heat-conducting cooling blocks need to have such masses that optimum heat transport is guaranteed.
System designers with a mobile, critical application in mind are likely to build upon the principles of conduction cooling, because it is more calculable and reliable. The bad news is: the methods necessary to implement this need to be undertaken at the onset of PCB design, including custom aluminum parts or additional copper layers inside the PCB to transfer heat.