Cooling rugged, high-performance embedded systems has always been a tough assignment. The Intel® Atom™ microprocessors with their incredibly low TDP (thermal design power) ease the task of cooling a system, but the conventional approach of relying on convection cooling with an appropriate heat sink bonded to the processor simply won’t work in extreme military and aerospace environments, particularly when there’s no air to remove the heat—and there simply is no air to provide cooling for many aerospace applications. However, even in terrestrial applications where there’s plenty of air in the surrounding environment, component count and density have reached the point where most of the air has been squeezed out of the product enclosure so that you need to think more carefully about extracting component-generated heat and conveying it to the environment. Makers of single-board computers destined to be used in rugged environments have addressed this problem for years and their heat-extracting solutions are very instructive.
All ruggedized, heat-extracting approaches involve metal—usually aluminum or copper—appropriately shaped and properly bonded to a board’s heat-generating semiconductor components and to the external environment. Metal is a much better heat conductor than air. For example, aluminum has a thermal conductivity of 250 W/mK at 25° C compared to air, which has a thermal conductivity of 0.024 W/mK at 25° C. That’s more than a 10,000x difference in thermal conductivity. Copper’s even better with a thermal conductivity of 401 W/mK at 25° C although copper costs more than aluminum.
The most effective and most common way to create a whole-board heat sink appears to be the creation of a milled aluminum plate designed to match a circuit board’s topology so that the metal can be brought close to each chip on the board. A thermal film (often silicone-based) then serves as a thermal gasket and as an electrical insulator between the components on the board and the metal heat sink. The thermal film conducts the heat away from the components and conveys it to the external environment. This is precisely the approach used by makers of ruggedized, single-board computers such as Kontron and MEN Mikro Elektronik GmbH. Kontron is a Premier member of Intel® Embedded Alliance and MEN Mikro Elektronik GmbH is an Affiliate member.
You might assume that there’s much less need to worry about removing heat from board-level systems based on the Intel® Atom™ microprocessor and an Intel® US15W System Controller Hub because of their low TDP. Although it’s true that the processor’s low TDP makes cooling much less of a hassle, some thermal engineering is still required for rugged environments. For example, here’s a photo of the MM1 ESMini™ COM (computer on module) board from MEN Mikro Elektronik GmbH. The MM1 is a single-board computer based on the Intel® Atom™ processor. The board has a total power consumption of between 5W and 10W depending on the version and clock frequency of the Intel® Atom™ processor supplied on the board. In addition to the processing power provided by the Intel® Atom™ microprocessor, the MM1 carries as much as 1 Gbyte of DDR2 SDRAM and provides one PCIe port, as many as two Fast Ethernet ports, eight USB 2.0 ports, two UARTs, as many as two CAN (controller area network) interface ports—which have become increasingly popular for automotive, industrial, and medical applications—and Intel® HD audio.
The MM1 ESMini™ COM is designed to be used in either convention- or conduction-cooled environments and is screened to operate over a temperature range of -40° to +85° C. Although the above photo doesn’t show it, the MEN Mikro Elektronik GmbH MM1 COM board is designed for and is supplied with a covered, metal frame that acts as both a heat sink and an EMI shield. The following photo shows the MM1 board with its thermal coverings in place.
The MM1’s cover and frame can be removed for operation in moderate environments with forced-air cooling, but forms the foundation of a board-level cooling system in harsher thermal environments. For environments where air cooling is practical but heat sinking is required, you can bond a heat sink directly to the top cover. For harsher environments, perhaps airless, you can thermally bond the MM1’s metal cover to a metal cold frame that conveys the heat to the environment.
What do you think is the best way to convey heat from ruggedized embedded designs? Have you found one way that’s better than others?
Note: This Intel® Roving Reporter blog post is based on these two excellent, recent articles: New ruggedization enables commercial-grade products in harsh environments by Joe Eicher of Kontron America and Conduction Cooled Assembly (CCA) Solutions for Standard Boards by Detlev Schaadt of MEN Mikro Elektronik GmbH.
Roving Reporter (Intel Contractor)
Intel® Embedded Alliance