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Computers for Harsh Environments

Conduction Keeps Computing Cool

Lower power CPUs and chipsets, strategic placement of components on boards along with conduction cooling from chip to chassis, combine to make systems that not only stay cool but also are rugged and resistant to outside contaminants.

COLIN MCCRACKEN, AMERICAN PORTWELL

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Reliable. Quiet. Affordable. Small. With protection from dust and humidity, and flexible mounting options. And easy on the electric bill. System designers expect everything from their box PC. With fewer engineers doing more jobs using slashed development budgets, it is tempting to select a hobbyist-style box PC that has a system fan and air vents. Although such a computer may work well on the lab bench and the price is certainly right, the up-front convenience is traded too conveniently for longer-term reliability risks and potential harm to the system OEM’s reputation. The key issue in creating the ideal reliable box PC is a better system architecture based upon conduction cooling. A solution is needed that can transfer heat from the system processor and chipset directly to the system enclosure and dissipate it from there.

The first step toward improving the reliability of small form factor (SFF) box PCs is to eliminate the weakest links—rotating parts. Many embedded single board computers (SBCs) already come with bootable onboard solid state disk (SSD) options such as Compact Flash, SD revision 1.1, and other tiny options. For larger capacity storage, SATA II SSDs are available in 2.5” and 1.8” form factors.

The harder habit to kick is the rotating fan built with ball bearings. Some SFF SBCs have support in the BIOS firmware for “smart fans,” reading the system temperature and reducing the fan speed (RPM) during periods of lighter processor utilization and heat dissipation, but the fan noise remains. Although sleeve-based fans are quieter and offer reasonable mean time between failures (MTBF), most of the noise still comes from air passing over the fan blades, which can’t be eliminated. In addition, fans require system air vents, with the adverse effect of providing a path for dust and humidity to enter the box PC. Filters can reduce the effects of dust, but the underlying reliability challenge remains. A better overall solution would include replacing the fan and vents with an alternative metal conduction path.

Enter Conduction Cooling

Large form factor SBCs in heavy enclosures have been able to successfully remove processor and chipset heat for many years through thermal conduction to exterior metal surfaces, where the sheer amount of mass absorbs heat like a heatsink, and natural air convection in the environment removes that heat from the enclosure surface.

Several years ago, small form factor box PCs began to emerge, with “fins” on their extruded top covers in order to handle the then-current 10-20W processor/chipset platforms. The sharp ridges along the extrusions transfer heat to the external ambient air much more efficiently than flat enclosures. Due to the SBC convention of placing processors and chipsets on the top surfaces of the boards, “fan-less” box PC manufacturers resorted to either NRE-intensive custom heat pipes or tall copper “chimneys” to remove heat to the extruded lids. Adverse side effects include a longer path for heat to travel (greater temperature rise over the thermal resistance), and a broken airtight seal of the thermal interface material (pad or grease) whenever the top cover needs to be opened. If the lid is not reinstalled properly with additional thermal compound, the thermal resistance can increase due to gaps or air bubbles, unwittingly compromising the long-term reliability.

Re-Thinking SBC Chip Placement

The next improvement to the system architecture comes from placing the hot processor and chipset on the bottom of the SBC. Circuit board layouts are becoming more common with a plan for heat conduction to the metal enclosure, learning the important lesson from computer-on-module (COM) standards. Figure 1 shows one such SBC, with top and bottom views flipped around a vertical imaginary line between the two photos. As shown, the processor and its chipset are located toward the center of the bottom surface (right side of the figure), and four mounting holes are provided in a nearly square pattern for a passive heat spreader plate to be installed. In turn, the plate will make good contact with the metal enclosure for further heat transfer. The result is a reliable fanless solution that, unlike COM products, does not require a custom carrier board in order to achieve conduction cooling.

Figure 1
Portwell’s NANO-6040 SBC features the processor and chipset on the bottom side (right image, toward the center). Mounting holes for the heat spreader can be seen adjacent to the major components.

In the course of designing such an SBC, the processor and chipset are placed first during board layout. Depending on the thickness of the heat spreader plate, two Z-axis height keepouts are established on the bottom side—one for components that reside under the plate, and the other for the rest of the bottom surface to avoid components and connectors touching the bottom of the enclosure. In Figure 1, the SD card socket is just beyond the edge of the heat spreader plate, and the elevation at the top of the socket is well below the top of the plate so that the board can be mounted on top of standoffs in the corners of the enclosure with clearance between the socket and the enclosure itself.

Figure 1
Portwell’s NANO-6040 SBC features the processor and chipset on the bottom side (right image, toward the center). Mounting holes for the heat spreader can be seen adjacent to the major components.

The tall I/O connectors, 4-pin DC power input connector, LVDS connector, two SATA II ports with power connectors, PCIe x1 expansion slot, and PCI Express MiniCard (a.k.a. “mini PCIe”) socket for Wi-Fi and Bluetooth combo modules, are shown on the left side of Figure 1. Being on the top surface of the SBC, they are easy to access during the initial system integration and in the field for upgrades without disturbing the thermal interface between the processor/chipset and the thermal conduction path to the enclosure.

Figure 1
Portwell’s NANO-6040 SBC features the processor and chipset on the bottom side (right image, toward the center). Mounting holes for the heat spreader can be seen adjacent to the major components.

Enter Ultra-Low-Power Atom Processors

The next system architecture breakthrough involves the use of the latest compact, low-power Atom family of embedded processors. Together with the “Topcliff” EG20 I/O hub (chipset), the “Tunnel Creek” Atom E6xx series processors deliver I/O flexibility including an onboard graphics controller, memory controller and expansion interfaces, all within a power envelope of 5-6W at 1.6 GHz. This meets or exceeds the performance of previous generation SBCs used in box PCs based upon VIA Eden, AMD Geode LX 800, and Intel Pentium M / Celeron M plus 852 / 855 and 910 / 915 chipsets. 

For an even higher level of reliability, the Tunnel Creek family also includes processors ending with a “T” to designate extended temperature operation, even up to the 1.6 GHz E680T processor. The T processors are rated from -40° to +85°C. Older generation SBCs could reach the maximum specification limit for operating temperature with even modest software workloads. The T processors add significant temperature margin when the processor occasionally reaches +60° or +70°C, for example.

With less power to dissipate using the 5-6W Atom E-series platform, the enclosure can be implemented with less expensive folded sheet metal and flat plates; fins are not necessary. The amount of metal mass for heat absorption and re-radiation can be reduced as well. Lower weight and flat surfaces also increase the number of applications and mounting scenarios that can be achieved within the very diverse embedded systems market.

Thermal Imaging Confirms Reduced Build-up

Still within the SBC design stage, thermal imaging tools are used to confirm that enough copper has been used to implement the power and ground planes of the board to spread the heat across the board (Figure 2).

Figure 2
A screen capture of the thermal imaging confirms heat spreading across the board with modest temperature build-up at the processor and chipset.

In the figure, the red and white colors toward the middle of the board correspond to temperatures above 50°C. Of course, the very sources of the heat are the processor and chipset themselves, located there. As much as the fiberglass circuit board and the copper plane layers can permit, that heat is spread laterally (X- and Y-dimensions) toward the edges of the board, which keeps the ICs relatively cool by preventing too much build-up at the center of the board.

Transferring Heat from the Bottom Up

The heat spreader plate conducts heat vertically (in the Z-dimension) away from the hot area as well, downward toward the enclosure. As shown in Figure 3, the enclosure is designed to conduct the heat across the bottom surface where it rises naturally up the sides to dissipate into the surrounding air. This bottom-up approach also adds a measure of safety in that the top surface, the front power button and the rear I/O block are not hot to the touch. The use of flat surfaces rather than extrusions with fins improves the breadth of mounting options, from bench-top to DIN rail to shelf-mount to wall-mount to rack-mount (bracket-based or 2-up per shelf).

Figure 3
The WEBS-2350B box PC is designed to conduct heat across the bottom surface and then up the four sides.

A system architecture driven by conduction cooling can greatly enhance MTBF and reduce audible noise by eliminating system fans. Purpose-built box PCs use the latest Gigahertz-class Atom E-series technology to improve reliability, performance, power consumption and protection from dust and humidity due to the elimination of cooling air vents. System designers can now expect everything from their box PC and more, saving time-to-market and development resources now without having to worry about returns and field failures later.  

 

American Portwell Technology

Fremont, CA.

(510) 403-3399.

[www.portwell.com].