By: Ray Tabladillo & Ben McMillan, Parvus
As customers continue to push the limits of computing system requirements, rugged computer systems will continue to evolve to endure the toughest conditions as long as good engineering practices are implemented and maintained in the process.
Technological developments that reduce complexity while enhancing usability are always a welcome addition to rugged applications, since fewer parts means improved reliability. The need for these developments is especially evidenced by the increasing demand for rugged computing systems where reliable high-performance computing is required. Rugged electronics systems must deliver uncompromising performance in demanding environmental conditions, including extreme temperatures, shock, vibration and humidity amidst altitude, fungus, salt fog, explosive decompression, immersion and sand/dust exposure, if the application so requires it.
In addition, demands continue for these systems to possess enhanced system integrity to satisfy electromagnetic interference/compatibility (EMI/EMC) requirements while operating over a wide range of power conditions. Rugged computing products are also benefiting from the latest developments in processors and electronics, demanding lean engineering practices that have garnered proven results.
Processor Choice Key for Rugged Computing Design
One of the greatest hurdles for rugged computing designers is including greater embedded processing power while maintaining low power consumption. The Pentium M and Celeron M processors have been popular choices for rugged systems, as they are designed from the ground up to deliver high performance with low power consumption. Availability of Intel’s recent Atom processor family injects additional possibilities for rugged stand-alone boxes. This low-power processor family has a thermal design power (TDP) starting as low as 0.65 watts, scaling up 13 watts with processor speeds running as high as 1.8 GHz speeds. Having exceptional performance-per-watt, the Atom is a good choice for a wide array of low-power, size-constrained applications. Recent additions to the Atom family, including multicore processors, promise to continue to improve the performance and efficiency of embedded applications.
While many applications can still be managed by energy-efficient single-core processors, new technology developments are demanding higher processing power with low power consumption. Multicore processing technology consequently sees a significant boost in deployment within stand-alone rugged boxes. For demanding applications, mobile Core 2 Duo or Core i7 processors, as examples, provide attractive solutions. While presenting a challenge to manage the 10-55 watts of thermal design power (TDP) of these processors, they offer a level of performance that pushes the boundaries of traditional rugged computing required by new applications.
Solid-State Drives Provide Proven Results
Connected to the processor selection, the ongoing development in solid-state drives is another catalyst helping advance rugged computing design. Based on flash technology, solid-state drives are rugged and supply proven performance in extreme conditions. For this reason, single-level cell (SLC) solid-state drives have become the leading data storage technology for almost all mission-critical military applications. With no moving parts, these devices are not hindered by seek time, latency, nor other electromechanical delays found in traditional hard drives. The drawbacks commonly associated with solid-state drives are being reduced, as random access speeds rival—and now beat—other media. Retention and re-writing cycles have dramatically increased, and many drives offer erase-all functions that comply with military declassification standards for security-sensitive applications. Solid-state drives are able to withstand extreme shock and vibration environments and, due to improved wear-leveling algorithms, boast very high mean times between failure (MTBF)—a major boon to rugged computing systems.
Combing solid-state drives with processors ranging from the Intel Atom to the Core 2 Duo, the Eurotech Group, including Parvus Corporation, has developed test-proven solutions that meet and supersede the requirements for use in extreme military environments, among other industrial and commercial application solutions. Use of the following design techniques has resulted in small packages with significant processing and expansion capabilities for extremely demanding environments.
Ruggedizing Internal Components at the PCB Level
Ruggedization begins internal to the system at the electronics level where components sustain the rigors of motion and shock, both physical and thermal. These types of ruggedization techniques must be implemented to ensure the processor can withstand extreme conditions. One such technique is underfilling—the process of injecting a specialized adhesive underneath BGAs to keep the chips from moving during vibration and shock (Figure 1). Its application provides a strong mechanical bond between the BGA component and the corresponding connection to the circuit board, protecting the solder joints from mechanical stress. Care must be taken, however, to select an underfill with an appropriate coefficient of thermal expansion (CTE). Otherwise the components will be subjected to undue stress during thermal cycling. Underfill adhesives can also aid the transfer of heat from the BGA component to the board while also mitigating tin whiskers on lead-free boards. Testing has proven that the underfilling technique provides remarkable improvements in long-term vibration resilience.
Another critical method to produce robust electronics involves potting (Figure 2). Potting can be performed by completely encapsulating an electronic device or by staking it down, to provide protection against shock and vibration. It can further ensure security of sensitive designs, as well as create a barrier against moisture, fungus, dust and corrosion. Enhancing circuit reliability by eliminating leakage from high-voltage circuits, protecting against voltage arcs and short circuits, and by preventing the formation of tin whiskers, potting becomes essential to rugged design. Potting materials come in a multitude of varieties affected by requirements including thermal, outgassing, electrical and thermal isolation or conduction capabilities, and manufacturing application requirements to name a few. Consequently, selection of the correct potting materials is a key engineering decision to ensure its function depending on the location it is applied and the environment for which the end product is destined.
As a final step, conformal coating material is applied to electronic circuitry to protect it from moisture, dust, chemicals and temperature extremes. This process improves and extends the working life of the board and helps ensure safety and reliability. These coatings “conform” to the contours of the board and its components, creating a thin protective layer that is both lightweight and flexible. For circuit boards that are not conformal coated, extreme environmental conditions could cause corrosion, mold growth and current leakage, resulting in board failure. Taking extra precautions to ensure that the board circuitry can endure harsh conditions is paramount in designing and building rugged computing systems that will last through the life of the product.
Rugged Design Relies on Thermal Management
Thermal management remains a major component of any rugged electronic design as heat issues are often the largest contributors to failures. Consequently, advances in thermal management will continue to rank as one of the most important trends in rugged computing design since processors continue to change along with the demand for higher processing capabilities at faster speeds.
Historically, there have been more embedded systems based on low-end, pre-Pentium processors than newer multicore processors. As a result, corresponding OEM or off-the-shelf enclosures have not been designed with new thermal requirements in mind. Therefore, with the integration of faster—and hotter—processors, chassis must become more sophisticated and must be considered to more suitably accommodate thermal issues. Simple chassis may have once comprised flat thin exteriors, while newer processor requirements for cooling demand considerations for increasing surface area using fin technology and increasing mass to improve the capability of the chassis itself to act as a heat sink and thermal conduit.
Gaining a better understanding of what combination of thermal products and techniques help transfer heat while maintaining cost, weight and system integrity, will prove to be one of the most important elements in rugged computing design. This influences chassis and internal package design to accommodate hardware and system design features to manage the heat while still maintaining the physical integrity and size and weight constraints of the unit. Consequently, the ability to analyze requirements quickly and identify potential solutions remains a key skill for the design engineer.
Some of the cooling techniques that have helped rugged computing systems maintain reliability without significant weight gain include embedded heat pipes, heat sinks, new thermal interface materials and heat spreaders (Figure 3). The inclusion of heat spreaders incorporated on top of devices to help transport heat from its source has drastically reduced thermal issues in embedded designs. These heat spreaders use a variety of materials and physical forms and are now designed to accommodate a number of thermal options, such as top-mounted heat sinks, fan heat sinks and heat pipes to effectively cool microprocessors. Innovative heat pipe/heat spreader combinations are proving especially effective in the thermal management of rugged computing systems to the point where mounting orientations may become negligible for excellent performance.
Although not a new cooling technique, the use of embedded heat pipes in conduction frames can dissipate large amounts of heat with very little temperature difference, eliminating the need for any input power for active cooling or the inclusion of moving parts. These passive cooling methods are more reliable than fan cooled designs and are currently more affordable than spray or liquid cooled chassis priced at the high end of the market. In the end, considerations for galvanic corrosion, cost and time-to-market are among the many constraints that may affect the choices for thermal management.
Keep EMI/EMC Management Core to System Design
Additionally, systems must meet certain EMI/EMC requirements. For military systems, this often includes compliance to MIL-STD-461 for radiated and conducted emissions and radiated and conducted susceptibility. Systems may also need to comply with power characteristics defined in MIL-STD-1275 or MIL-STD-704. To meet these requirements, proper considerations must be taken at the earliest stages of system design; EMI/EMC compliance is rarely achieved when viewed as an afterthought. Important considerations include defining test requirements, selecting an adequate power supply, protecting I/O lines with ESD diodes, designing sealed enclosures with good EMI gaskets, and creating proper test cables (Figure 4). Moreover, proper grounding techniques and good bonding between chassis surfaces are critical in creating an enclosure that acts as a faraday cage. Since external power leads are typically unshielded in test and application, they can be the single largest point of noise and susceptibility. A well-designed filter, located at the point where power enters the system, is critical to an electrically quiet system. This prevents internal noise from exiting the system and protects sensitive electronics from external noise that otherwise might enter the system.
Eliminating Cables Reduces Failures
Cableless technology has also improved reliability and ruggedness by eliminating points of possible failure. Although power cables are usually still necessary, eliminating cumbersome data cables simplifies the system by decreasing components and increasing reliability. Use of I/O breakout boards, rigid flex connectors and board-to-board connectors, provides a cableless internal interconnect scheme that is conducive to a rugged design and signal integrity, while still supporting modular customization. When cables are necessary, options for embedding them in an elastomeric material are available to provide both strain relief to prevent the cables from disconnecting or being severed in vibration or shock while easing manufacturability and handling for maintenance to improve reliability. Teflon-coated cables, for example, are also quite robust and can be selected and tied down with physically mounted zip ties and lacing applied to further prevent any possible mechanical failures during shock and vibration.
Modeling & Simulation Software Save Time, Money
Test results provide important data necessary to design the most robust systems possible for customers who can accept nothing less. In order to verify whether ruggedized systems will perform sufficiently in extreme environments, highly accelerated life tests (HALT) and highly accelerated stress screens (HASS) are typically applied. In the military and defense applications, these tests are typically defined by MIL-STD-810 and sometimes require MIL-S-901D shock test for naval applications. Not all customers require these regulated tests for their applications and they can be defined as required to suit the product’s intended environment. Parvus, for example, has developed its own vibration profiles based on its own experience for standard products to meet extreme conditions for most applications.
Significant time can be spent performing engineering calculations or writing programs to review designs. These have their place in the preliminary design phase, but often analyses can be completed quickly using simulation software that will correlate to testing. For example, in relation to thermal constraints previously mentioned, thermal modeling software is available for making precise decisions in conduction, natural convection cooling and even thermal flow simulations. By identifying and analyzing potential cooling issues, thermal modeling software can provide important thermal management capabilities by ensuring new thermal devices will meet specific standards. Furthermore, simulation programs such as Simulation Premium, a version of COSMOS that has been integrated into SolidWorks, allow engineers to subject their designs to real-world stresses including shock, vibration and even nonlinear stress analyses, helping significantly shorten the design cycle by reducing or eliminating need for redesign on the front end in order to get products to market faster. By running a variety of analyses, engineers can quickly determine where potential points of failure could exist when subjected to shock, thermal and vibration tests prior to physical testing.
With many companies wanting to streamline the design process and attempting to choose systems that are easy to manage for the long haul, defense contractors among many other companies are increasingly moving toward using COTS products. Engineers need to maintain a clear understanding of how to evaluate any required component within the system and design with many of the considerations previously reviewed. Of equal importance with the change to using COTS components, is the increased trend toward managing regulating directives such as the restriction of hazardous substances (ROHS). Having a goal to limit harmful materials, these types of regulations can significantly affect the design process. Learning how to take a design made for industrial and possibly commercial applications and transform it for more rugged applications, the engineer must have a broad understanding of what is suitable for rugged applications and combine this knowledge with available analysis tools in his repertoire. Furthermore, the designer must consider what is allowable for the system and know what and how to replace any components not well suited to shock, vibration and thermal limits among the gamut of environmental concerns.
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