As military programs continue to place an ever growing reliance on embedded computer systems, developments in rugged computer design must continue to advance to keep pace with this demand. This can be accomplished by understanding how individual elements in rugged designs are critical to ensuring a system can perform in harsh environments.
Among the many elements composing rugged system engineering are thermal management and cableless design. Examining these individual ruggedization techniques will provide better understanding into the role they play to ensure the most durable systems possible.
With heat issues often credited as the largest contributor to system failures, designing rugged systems to meet these thermal challenges is critical. The key to successful thermal design is getting heat into contact with ambient air for convection to the external environment as quickly as possible. By implementing a variety of thermal management techniques, heat can be dissipated fast enough to prevent a phenomenon known as thermal runaway. Thermal runaway is an increase in temperature that changes the conditions in such a way that causes exponential temperature increases, ultimately leading to destruction.
Relying on convection cooling inside a system has its limits as heat passes rather slowly (high thermal resistance) to the surface of the chassis where it can be dissipated. However, advancements in conduction cooling have had a tremendous impact on rugged system design. For example, custom-designed clamshell heat sinks can be fitted to encapsulate each printed circuit card assembly, using wedge locks as the contact point with the card stack to dissipate heat more quickly (lower thermal resistance) to the chassis. The clamshells, which essentially work as heat spreaders, and the thermally conductive gap pad significantly reduce thermal issues.
Heat spreaders are also being 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 stand-alone rugged boxes.
When complete convection cooling is not possible — or when designing the cooling properties of system enclosures — thermal designs must consider the thermal boundary layer. The thermal boundary layer is a layer of warm stag-ant air that builds up between the fins of heat sinks, blocking air movement. This build-up of air effectively reduces the useful dissipative surface area of a heat sink.
The turbulence of the air affects the thickness of the boundary layer and, therefore, the rate of heat transfer. For example, the more air molecules that contact the surface of the heat sink, the greater the dissipative effect. The boundary layers of rugged devices need to be carefully evaluated as it is not uncommon for embedded systems to be installed in a stagnant air environment. As such, boundary layers can greatly influence the effectiveness of heat sinks and other convection cooling methods.
Derating Ensures Optimal Performance
Rugged system designs often must specify extremely rugged components that have warranties guaranteeing performance up to +85°C. To ensure optimal performance for these components, derating can be employed. A technique where electronic devices are operated at less than their rated maximum power dissipation, derating takes into account the case/body temperature, the ambient temperature, and the type of cooling mechanism used. Derating increases the margin of safety between part design limits and applied stresses, thereby providing extra protection for the part. By applying derating for an electrical or electronic component, its degradation rate is reduced and the reliability and life expectancy are improved.
Managing Cold Temperatures
Although thermal management mainly surrounds the task of moving heat to the outer chassis, when designing for rugged conditions, managing cold temperatures must also be considered. Cold temperatures can cause a variety of problems with electronic devices, including problems with voltage sags and clock frequencies, creating timing difficulties. Descending to cold temperatures also causes rapid contractions, which can cause problems with powering on a device. To help mitigate these problems, screening each electrical component to ensure it is rated to operate from -40°C to +85°C is imperative.
Thermal Management for Cockpit Control Panels
For over a decade, Parvus has supplied the U.S. Navy with rugged cockpit computer subsystems for some of it’s tactical aircraft. In its latest follow-on contract, Parvus developed special cockpit control panels for key electronic defense warfare aircraft. The contract supports the Naval Surface Warfare Center’s multi-mission upgrade program to include electronic warfare and laser targeting.
To provide enhanced thermal ruggedization for these control panels, three thermal “envelopes” had to be managed. This included the stagnant air inside the main subsystem chassis, the air between the main chassis and the Air Transport Rack (ATR) aluminum chassis and the ambient air. To ensure heat could rapidly transition these “envelopes”, engineers carefully managed the selection of rugged components, implemented conduction cooling where possible and provided components with the most heat direct conduction links to ambient air. For example, the main CPU has a direct heat pipe link to a heat sink exposed to ambient air. The main thermal management goal in this deployment was to keep the thermal resistance of each heat source to ambient air as low as possible and practical.
Cables are often a troublesome feature in rugged computers as any kind of movement from shock, vibration, or the expansion and contraction from fluctuating temperatures can cause a cable to disconnect, chafe and/or break. Therefore, cable free designs are of increasing importance as they decrease component count and increases reliability.
Board-to-board connectors are a cableless option that decrease shock and vibration while still ensuring signal integrity. Advancements in the design of board-to-board connectors now provide increased board space for high-density power components placed between PCBs. Rugged board-to-board connectors can also feature a fully insulated bottom plate that protects the contacts, making the device highly resistant to vibration, drop shock, and short circuits of printed tracks on the PCB.
Creating cable-less designs also entails understanding how the system is going to move as a whole. For example, when designing the DuraCOR computer subsystems, Parvus engineers found that shock absorbers inside the system actually caused more problems. They allowed two components to move against each other, causing more vibration inside the box. By removing the shock isolators, the entire assembly of components acted as one mass-decreasing vibration.
Flex circuitry is another key element in designing rugged systems as it permits more movement, tolerance and the exacting quality requirements needed for today’s high-reliability systems. Flex circuits provide improved vibration resistance, because under intense, broad-spectrum vibration, the low mass of flex circuits reduces internal stresses at solder joints. Additionally, for rugged applications where space and weight are at a premium, flex circuits provide compact, low mass packaging for optimal efficiency. By using more ruggedized connectors, flex circuits offer improved connection reliability.
A case in point is the design of the DuraCOR 810-Duo, where a flex circuit was designed to handle a difficult design challenge. In this application, the use of board-to-board connection was not possible and signal integrity was a paramount design requirement. In this rugged subsystem, the flex cable provided some of the advantages of a cable and a PCB to provide a solid design solution.
Ruggedizing Techniques Continue to Evolve
In order to remain current with the best methods of rugged design practices, engineers should continue to expose themselves to several areas of design. By gaining a better understanding of mechanical engineering practices, electronic engineers in particular can better learn how to solve electronics design challenges, especially when dealing with rugged design requirements. The same can be said for mechanical engineers as they are tasked with the rugged packaging of electronics.
As customers continue to push the limits of computing system requirements, rugged computer subsystems will continue to evolve to endure the toughest conditions imaginable.
This article was written by Jared Francom, Lead project Engineer, Parvus Corporation (Salt Lake City, UT). For more information, Click Here