Face it. The ubiquitous Intel® x86 architecture has been around a (relatively) long time, and it provides myriad benefits, especially in portability and ease of legacy code use. However, though x86 thrives and drives benign commercial benchtop and desktop environments, what about the rugged designs required in the military arena? Sometimes a customer-requested military temp component is simply not manufactured, or it is extremely price-prohibitive, thus commercial wares must be used. But converting a commercial x86 design to a rugged one is not a straightforward proposition.
The following discussion centers around these issues:
- At what stage should designs be adapted from commercial to rugged?
- Thermal considerations for the Intel® Atom™ and Core™ i7
- Building in shock and vibe tolerance
- System-level allowances for commercial-to-military adaptation
Timing is everything: When to transform commercial into rugged
Curtiss-Wright Controls Embedded Computing and GE Intelligent Platforms, two military embedded industry heavyweights, both employ an “adapt first, not last” mantra when ruggedizing commercial designs. They are also both quick to point out that ruggedizing a commercial design differs from merely using ruggedized commercial components.
Curtiss-Wright, an Affiliate member of the Intel® Embedded Alliance, utilizes a “built rugged” concept when it comes to using commercial wares in rugged designs, meaning “designing and manufacturing circuit cards to be rugged (i.e. harsh environment capable) at the outset. We have always had this philosophy, and we have used and still use commercial temperature components,” explains Ivan Straznicky, Principal Engineer and Technical Fellow at Curtiss-Wright.
A similar outlook is adhered to at GE Intelligent Platforms, an Associate member of the Intel® Embedded Alliance. “Our philosophy is ‘designed to be rugged’: We don’t believe it is possible to create a truly rugged product ‘after the fact.’ Ruggedness needs to be designed in from the very outset,” says Frank T. Willis, Director, Military/Aerospace Product Management at GE. He further explains, “Adapting a commercial design is not the same as adapting commercial hardware. It’s incredibly difficult to adapt a commercial design [often 0 ˚C to 60 ˚C] to rugged requirements [GE’s rugged temps are typically -40 ˚C to +71 ˚C]. … The design approaches are generally so different that it makes more sense to start [the design] over,” he adds.
Design procedure must begin with analysis of thermal and structural components, in addition to component selection and signal integrity consideration, requiring advance determination of PWB routing and layout protocol, Willis explains. “These factors are more difficult – in many cases impossible – to add later in the product manufacturing cycle. … This can only be done from the ground up, not by just adding a metal frame to a board and hoping for the best.” GE also uses commercial components as necessary within their harsh environment designs, and both companies use careful testing to ensure reliable commercial-to-rugged hardware adaptation. (See also Part 2 of this series, “Adapting commercial x86 embedded designs to harsh environments – Can it pass the test (every single time)?” for details on commercial-to-rugged testing practices.)
Intel® Atom™ and i7™: Some like it hot … or maybe not as much
As Intel’s relatively newborn, commercial-temp versions of the Atom™ and Core i7™ processors gain market traction, their thermal design capacities and limits must be considered when using them in ruggedized warfighter technologies.
Adapting commercial processors suited for forced-air cooling in low ambient temps to a conduction-cooled design can be a “very involved” process, says Straznicky. “The conduction cooling solutions employed must have high efficiency, i.e. the thermal resistance from processor to ambient must be low.” The most critical heat removal path is the contact resistance between the card edge and the chassis rail contact, and a 5-10 ˚C thermal contact resistance reduction allows a 5-10 ˚C chassis rail increase to raise permissible ambient temperatures, he adds.
Meanwhile, Willis says both the Intel® Atom™ and the Core i7™ can be used effectively in conduction- or rugged air-cooled solutions, but Atom’s 10 W power dissipation makes it the easiest to manage of the two. Effective thermal conductivity for the 30 to 95 W Core i7™, though, necessitates “proprietary heatsinking methods … between the part itself and the heatsink (not forgetting a good heat path through the PCB to the heatsink also).”
Shaking it all up: Shock and vibe
“Shock and vibration consideration for rugged products using commercial components is much more than simply tweaking commercial designs,” asserts Straznicky. Some of the unique x86 card failure mechanisms and modes induced by shock and vibe include socket contact fretting corrosion or pad cratering underneath processor solder balls. Other types of mechanical failures can additionally occur with various processor types during environmental testing (Figure 1).
Figure 1 | Starting at top left, going clockwise, failure analysis shown: 1) Excessive local strains induce pad cratering underneath the BGA in vibration tests; 2) Direct module exposure to 500 hours of salt fog results in a salt bridge; 3) A vibration test produces excessive micromotion and resultant connector contact fretting corrosion; and 4) Insufficient cooling fried this processor during a high-temp test. Figure courtesy of Curtiss-Wright.
Learning about vibe and shock effects to the circuit card and processor is pivotal in the rugged equation, as is their mitigation. Thus, Straznicky offers these insights:
- “Circuit card stiffeners are ubiquitous on conduction cards to reduce displacements, strains, and stresses imparted by shock and vibration.
- Lighter materials such as aluminum (0.0975 lb/in3) are used instead of copper (0.321 lb/in3) to conduct heat, even though copper has a much higher thermal conductivity.
- Shock and vibration isolators are often used on or as chassis mounts to reduce the shock/vibration levels imparted to the chassis and enclosed cards.
- Chassis parts are structurally solid and brazed together to provide a rigid construction that will not fatigue and reduces vibration amplification.”
Additionally, pins versus mounting points is also an important consideration: Merely meeting electrical requirements via pins doesn’t work, as enhanced vibe and shock capability begins with components featuring several mounting points instead, Willis explains.
Systemic issues for commercial-to-rugged adaptation
When mitigating high component temperatures when allowances have been made for adapted commercial products within a rugged system, Willis suggests these steps: 1) Make sure that the PWB-to-component thermal interfaces are effectively designed, as mentioned earlier. 2) Check that the thermal resistivity is minimized by the chassis-to-PWB card edge interface.
“Two-phase or spray cooling can cool the highest power densities,” adds Straznicky. Single-phase liquid cooling can also effectively cool extremely high power levels, in excess of 650 W on a 6U liquid flow through card, for example. Other methods have also shown promise in rugged designs (Table 1).
Table 1 | Several cooling methods have proven effective in modern ruggedized designs.
Worth the trouble?
Though an ideal rugged design would contain only mil temp components, the practice of adapting commercial temp wares into rugged designs will always be required, says Straznicky, but it might cost vendors. So … is it truly worth it?
Written by Sharon Schnakenburg-Hess, an assistant managing editor at OpenSystems Media®, by special arrangement with the Intel® Embedded Alliance.
1. “VITA 47: Environmental considerations for VITA technologies,” by Ivan Straznicky, Curtiss-Wright Controls Embedded Computing, www.vmecritical.com/articles/id/?3748
2. “Conduction Cooling Techniques for Rugged Computers,” by Serge Tissot, Kontron, www.mil-embedded.com/articles/id/?4476