PC/104 In the Military

By Paul Rosenfeld

Diamond Systems Corporation

Since its inception in 1991, PC/104 technology has been embraced by military systems engineers for a wide range of applications, from pilotless drones to missile launchers to military radios. Many engineers select PC/104 because its ultra-small (3.55” x 3.775”) form factor enables an off-the-shelf solution for applications that may previously have been possible only with a custom CPU design. For example, the small size allows PC/104 CPUs, and in fact, complete multi-board systems to be placed in the nose cone of a missile or fit nicely inside a small UAV.

Other engineers select PC/104 technology because of the ruggedness inherent in its stacking architecture (See Figure 1). This unique approach to multi-board systems provides for a shock- and vibration-resistant off-the-shelf computing solution by eliminating backplanes and metal card cages, making PC/104 ideal for military vehicles such as tanks or even HUMVEEs. Finally, the light weight of PC/104 systems make PC/104 an ideal architecture for systems carried by soldiers, such as military radios.

There are other features that are not inherent to PC/104 itself, but are provided by many PC/104 suppliers, that make the technology even more well suited for military applications. The inherent ruggedness in the PC/104 mechanical design is enhanced by using boards that have been designed and tested for extended temperature operation. Many PC/104 boards are rated from -40°C to +85°C. Cold temperature is a requirement for operation in an aircraft at high altitude. High temperatures can occur in a sealed enclosure sitting on the ground in a desert country. Developing boards that run reliably at such extreme temperatures is a challenge when fundamental elements such as the CPU and chipset may not be rated at such extreme temperatures. Dissipating heat from a 5W to 15W CPU under such circumstances takes skill and an outstanding design.

Diamond Systems, among others, supplies a number of PC/104 CPUs and I/O boards rated to run over the full extended temperature range of -40°C to +85°C. While many of these boards use some components rated by their manufacturers for only 0 to 70°C, there are certain design techniques that can make extended temperature operation feasible. First, board suppliers have developed broad experience with a wide range of passive components to understand which components can be expected to perform well outside their rated temperature range. The key to designing with passive components is to analyze the temperature effects on their key parameters and then design the circuit to operate reliably under worst case conditions. In addition, more expensive tantalum capacitors are preferred over the aluminum electrolytic capacitors found on commercial grade boards.

Component selection is only part of the battle. Board design must incorporate very conservative timing parameters, particularly with respect to low temperature operation. More attention is paid to matching trace lengths and matching impedence on multi-pin interfaces between components. This can be particularly important on high speed interfaces such as the front-side bus between the CPU and northbridge or the memory bus between the northbridge and the memory components.

For ICs, a common but not universal rule is that derating, such as downclocking processors or using logic chips at speeds much slower than their specified performance, can often yield an increase in reliable operating temperature range. In this case performance may sometimes be sacrificed in favor of reliability; this is a judgment that must be made by both the designer and customer.

Finally, these boards may have more layers, providing greater spacing between traces and more room for copper layers to pick up and dissipate some of the heat. As a final step, the memory refresh rate may be manipulated in the BIOS to prevent memory failures at the temperature extremes.

But it is not enough to just design a product according to these types of design rules. From the very earliest prototype, products must be exercised over the full temperature range. Design tweaks aimed specifically to broaden the operating temperature range of the product may often be tried out. It is important to have extensive experience testing multiple boards operating for considerable time at the temperature extremes to gain sufficient confidence to specify a product over the full range and guarantee operation at the extremes for the duration of the warranty period. But even this may not always be enough. In some cases, there may be sufficient variances in component performance that make it desirable to test each and every product shipped over the full temperature extremes. Some PC/104 vendors, including Diamond Systems, offer optional extended temperature screening of each product shipped to provide the highest level of confidence (See Figure 2).

A second feature that is highly valued by the military is resistance to shock and vibration. The stacking PC/104 architecture provides the basic elements that enhance the performance of a PC/104 stack in such applications. When implemented with four metal standoffs at each of the four corner mounting holes, a PC/104 stack is extremely rugged. But a rugged stack that stays together is not enough if the system fails due to jumpers coming off or socketed memory DIMMs vibrating loose. In general, all sockets should be eliminated in a rugged design. For example, in one real-world application, a DIMM memory module that was held down by a strap vibrated sufficiently within its socket that the socket contacts lost tension over time, resulting in intermittent memory failures in the field a year or more after initial deployment – a very difficult and expensive problem to diagnose and repair.

In this case replacement of the DIMM socket with a different type solved the problem. But the use of soldered–on memory, as implemented on Diamond Systems CPU boards, represents the best possible solution, since it eliminates the source of the problem rather than trying to overcome it.

Similarly, a configuration jumper (or shunt) on a board may represent a cost of less than a penny. But once again, jumpers are available with differing tension and plating characteristics. And after being subject to continuous shock and vibration, shunts lose their grip over time. The most secure way to fix a board configuration in a production system (which is unlikely ever to change) is to hardwire the jumper, either with a soldered wire strap or, preferably, with a zero ohm resistor. Many PC/104 suppliers, including Diamond Systems, make such a configuration option available to their customers by building footprints for these configuration resistors right onto the PCB.

Figure 4 shows Diamond Systems new Elektra PC/104 CPU which incorporates many of the ruggedness features outlined above, including extended temperature operation, soldered memory and optional hardwired configuration jumpers along with on-board data acquisition. [Note that this can be a caption]

Let’s look at how these capabilities available with many PC/104 solutions play a role in a real-life application.

A leading defense contractor was developing a Weapons Station to remotely control light and medium caliber weapons that can be installed on any type of military vehicle. The system required a small form factor computing platform with full analog, digital, and serial I/O and a display interface.

The customer selected a PC/104 CPU with IDE flashdisk storage from Diamond Systems due to its extended operating temperature range and integrated data acquisition capabilities. Since the vehicle would likely see action in extreme environments such as water, sand, high temperatures etc., the customer requested that Diamond conformally coat the PCB and flash disk.

It was also anticipated that the system would be exposed to extreme shock and vibration. Soldered memory on the CPU board was essential for this application. However, the customer requirements called for changes to the standard product in order pass MIL-STD-810 shock and vibration standards. Diamond Systems replaced standard connectors with latching connectors with increased contact plating. The PC/104 connector was replaced by a high reliability MIL style connector. Selector shunts were replaced with zero-Ohm resistors to eliminate any possible dislodging of the shunts due to vibration or misapplication. Finally, to counteract the effects of extreme vibration over and above that of the MIL 810 vibration specification, all of the BGA chips were under-filled to eliminate any solder bond cracking between the board and the BGA balls.

In order to assure consistent and reliable thermal management in this rugged, high vibration environment, Diamond Systems’ engineers worked with the customer to create a custom heat sink that would equalize vibration resistance utilizing several legs positioned about the PCB. The heat sink utilized a thermal contact pad to eliminate the effects of shock and vibration from being transferred from the heat sink to the BGA chips. After extensive testing, the solution successfully passed all of the higher vibration test requirements.

The application described above shows dramatically how the capabilities inherent in the PC/104 architecture – rugged inter-board connections and small size – make PC/104 architecture suitable for vehicle-based and airborne military applications. But they also demonstrate how particular features and capabilities added by suppliers of PC/104 CPU and I/O boards go the extra mile to fit the specific needs and requirements of military systems. Design considerations such as component selection, PCB design, and component derating contribute to a product’s reliability. Extra features, such as an extended operating temperature range, conformal coating, additional ruggedness through elimination of jumpers, latching connectors and soldered-on memory, and burn-in or temperature screening of each board shipped add to the ruggedness of the products. These techniques ensure that PC/104 technology continues to maintain a leadership position in military applications challenged by limited space and harsh operating environments.