A significant challenge facing Ultra-Small UAV and Light Vehicle military system designers is the removal of heat from embedded computers. Fanless, sealed enclosures are usually required in harsh environments to maximize reliability and protect electronics from moisture, dust, insects and corrosive chemicals. Whether on the ground or at low to medium altitudes, onboard computers must be dependable in order for the mission to succeed.
Intel-based SBCs—not just the Core family of processors but Atom as well—continue to gain popularity in military and avionics applications. The trick is to adopt smaller, lighter, sealed boxes that don’t cause excessive heat build-up inside. Higher internal temperatures reduce the long-term reliability (mean time between failures or MBTF) of mission-critical electronics. In extreme cases, 20-30W processing platforms can experience thermal runaway where the thermal design is inadequate, causing reboots and shutdowns at inopportune moments. There is no denying the appeal of the Atom family of processors, with their reduced power consumption, for new designs and upgrade programs. However, even the 5 to 7W dissipated by the lowest power Z- series and E- series Atom platforms can wreak havoc on sealed boxes, particularly when I/O cards and other heat generators are in the board stack.
Defining the Mission
With the empty weight of small tactical UAVs dropping below 100 pounds—and in some cases even 10 pounds—for surveillance and reconnaissance missions, computer system integrators are starting to move away from traditional backplane and card cage architectures toward stackable designs in order to reduce bulk and weight. The payload should not represent a high percentage of the overall weight of the aircraft. The problem resulting from this approach is that traditional thermal dissipation mechanisms of stackable SBCs involve inefficient heat sinks, which increase heat inside the enclosure, resulting in reduced MTBF for the computer system as well as reduced maximum operating temperature range.
As if torn from the pages of the VME playbook, stackable SBCs are now embarking along the conduction cooling path. With a simplifying assumption of stacking I/O cards in only one direction—above the host SBC rather than both above and below—heat-generating components such as the CPU are placed on the bottom side of the SBC, rather than the traditional top side. This allows the use of a simple, thin thermal transfer plate on the bottom side, which allows system designers to attach SBCs directly to enclosures, while providing an optimum means of dissipating the heat generated by the SBC. A significant side benefit is that expansion I/O cards can be easily mounted on top of the SBC without fear of interference with the now-eliminated heat sink. This breakthrough for stackable SBCs of upward-only stacking reduces size, weight, power and cost (SWaPaC) compared to previous stacking or card cage architectures and can be applied easily to sheet metal, milled aluminum, or cast enclosures (Figure 1).
Figure 1
The bottom-mounted heat transfer plate leads to an optimal thin and light solution.
Minimizing the temperature rise across series elements keeps sensitive electronics from exceeding maximum junction temperature ratings. This article examines a specific Atom Z530 SBC with thermal test results in a sealed box. With this type of solution, sealed box designs for small UAVs and light tactical vehicles alike can now tap the proven PC/104 I/O card ecosystem as well as the new SUMIT-based PCI Express I/O card market.
Taking the Heat
Due to the highly insulative nature of air (low thermal conductivity), sealed enclosures are not inherently a good match for CPUs or other electronics that utilize common board-mounted heat sinks. This is because the heat must be transferred from the heat sink through the air to the enclosure wall. The air acts as an insulator (thermal resistor), causing undesirable temperature build-up. The temperature difference between the outside ambient air and the air next to sensitive electronics can easily reach 15°C or even more. Subjecting SBCs and I/O cards to excessive temperature for extended periods of time can drastically lower MTBF, an unacceptable situation for mission-critical computers subjected to harsh environments.
The question becomes: How to design high-performance SBCs for effective heat removal in sealed boxes, while providing ruggedness and reliability? Previously, the benefits of conduction cooling in VME markets were not available to users of stackable SFF SBCs. With a new innovative approach, Diamond Systems brings conduction cooling to the PC/104 market so that Atom SBCs and legacy I/O cards can be used in sealed boxes without compromising ambient operating temperature or system reliability. As illustrated in Figure 2, Diamond’s Aurora SBC uses a heatspreader plate on the bottom of the SBC, which attaches to the processor and chipset by way of thermal gap pads. The test results show conclusively that the heatspreader keeps the processor well below its maximum temperature rating.
Figure 2
The heat spreader plate leads to a thin and light solution.
Mounting for Thermal Conduction
The top of Figure 2 shows the side view of the Aurora circuit board assembly. The two hottest components (Atom Z530 processor and US15W chipset) of the green board are on the bottom surface, coupled directly to the gray heat spreader plate by way of the blue Z-axis compliant thermal interface material. A total of 6 watts is conducted through the plate to the system enclosure, held together by four #6-32 screws in a standardized 2.8-inch square pattern.
Figure 2
The heat spreader plate leads to a thin and light solution.
Diamond System engineers conducted a set of tests to model the expected operating environment of a rugged, sealed system with the processor and chipset thermally connected to the enclosure wall. The test results show that the SBC operates successfully in simulated worst-case conditions of thin aluminum walls (1/16-inch thick). In practice, cast or milled aluminum is often much thicker, which improves the thermal conduction and radiation even more.
A 2W power resistor load was placed inside the test enclosure to simulate the effect of other heat sources such as I/O cards in the box. Four thermocouples were used to monitor the temperature at distinct locations—suspended in the temperature chamber air, attached to the bottom aluminum plate, inside the enclosure, and attached to the processor chip. After power and I/O cables were connected, the box was carefully sealed to prevent the air circulation of the chamber from inadvertently cooling the electronics inside. Finally, test software was used to exercise the SBC to full utilization and to monitor the on-die temperature. Test results were logged outside the chamber (Figure 3).
Figure 3
Four thermocouples were used to record temperatures at different locations.
First Test: Thermal Runaway
In order to simulate a system manufacturer’s environment, it is essential to run the SBC at close to 100 percent utilization to mimic image capture and logging to solid state disk (SSD) or transmitting over the network. The key parameter to test for is the Atom Z530 processor’s transistor junction temperature (Tj): This temperature must never exceed the component’s maximum rating of 90°C.
The first test was defined as no thermal connection between the SBC and the enclosure, in order to approximate how a conventional CPU heat sink would perform in the absence of air flow. This was accomplished by mounting the SBC on insulators. Load resistors were used to simulate the presence of an I/O card in the enclosure generating additional heat into the inside air.
The temperature rise of any thermal dissipation mechanism is defined as thermal resistance (in degrees C per watt) times the power dissipated (in watts). Heat sink datasheets show very steep thermal resistance slopes when air flow is low. Even for a low-power Atom processor, lack of air flow can easily lead to a temperature rise of 10 – 15 degrees just in the heat sink alone.
In this case, the temperature rose 11 degrees from the enclosure to the inside air, and another 19 degrees across the heat sink, putting the processor well past the Tj max rating of 90°C. The test was stopped quickly to reduce the risk of permanent damage to the SBC.
Second Test: Thermal Conduction
This time, the SBC and heat spreader were mounted directly to the enclosure, which was sitting on a thin aluminum plate. Again, 2W load resistors were used. Rather than the 6W of processor and chipset heat being dumped into the internal air, it was conducted to the enclosure, where it could be directly removed by the forced air regulation of the temperature chamber. In a real-world environment, this cooling would be performed by radiation (natural convection) into the outside air, or further thermal conduction to a thick metal shelf.
This time, the test results were encouraging. Instead of a temperature rise of 33°C from the outside air to the on-die processor transistor junctions, the rise was only 10°C. This dramatic reduction occurred because the bulk of the heat was being removed directly, rather than building up inside the enclosure. Figure 4 shows how conduction cooling provides two parallel conduction paths for heat, rather than one using a traditional PC/104 stack. Also, compared to VME systems, there is one fewer series element for temperature build-up—the guide rails for slide-in SBCs.
Figure 4
Conduction cooling provides two parallel conduction paths (right) for heat, rather than the one in a traditional PC/104 stack (left).
In practice, thicker aluminum enclosure walls, such as a cast and/or machined enclosure, combined with mounting on a bulkhead or other surface with a temperature at 71°C (160°F) or lower, would reduce Tj even further below the maximum rating.
Traditional Heatsinks Not Enough
The heatspreader plate is very effective in providing a thermal conductive path from the processor and chipset (6W total) to the enclosure surface, which in turn dissipates the heat to the outer environment (temperature chamber simulating a UAV or vehicle environment). The thermal junction limit is observed when conductive cooling is used, even in the presence of 2W of additional thermal loads inside the enclosure (power resistors).
However, the maximum thermal junction specification is violated when there is no conductive path for the heat to the outer enclosure. This is because the 6W from the CPU is dumped into the inside air along with the 2W thermal load. This test shows that traditional heatsinks are effective only in systems with vents and/or cooling fans, where forced or free-flowing air actively removes heat.
The test results show conclusively that Atom SBCs with traditional CPU and chipset heatsinks cannot be used in sealed enclosures whose surfaces reach 70°C, since the internal temperature rise is nearly unbounded (thermal runaway). The additive nature of the series elements drove the junction temperature to 32°C above the enclosure temperature even with only 2W of additional thermal load. Reconnaissance and surveillance missions cannot be put in jeopardy due to inadequate thermal design. The Aurora SBC is the first SUMIT-ISM single board computer (PC/104 size with PCI Express and ISA expansion) to use conduction cooling. This approach greatly simplifies sealed box designs and enhances the functionality and processor speed possible within small enclosures without risking reliability.
Diamond Systems
Mountain View, CA.
(650) 810-2500.
[www.diamondsystems.com].
