When failure is not an option: Test and qualification help ensure reliability in mission-critical environments

Military and aerospace equipment requirements are generally far more stringent than those found in consumer, automotive, industrial, and similar applications. In most cases, anything less than 100 percent operation is not an option for military/aerospace applications, since even the slightest failure could have catastrophic and deadly results. Reliability is critical.

Yet military and aerospace applications typically encompass a number of the harshest environments: extreme high and low temperature ranges – from the intense cold of the Antarctic to the world’s hottest deserts; the highest levels of shock and vibration – from explosive detonation to the rigors of a space launch; high atmospheric humidity and even total immersion; resistance to nuclear and solar radiation as well as the total vacuum of space; and the high pressures of the deepest oceans (Figure 1).

This puts an enormous burden on the components and systems used in these environments, since they need to strike a balance between being always available to meet safety-critical standards and operating under the extreme conditions of these intense applications. Component testing, characterization, and validation are mandatory in developing a reliable embedded computing system. Key aspects include guaranteed by design vs. qualified by test, along with temperature and semiconductor considerations.

Dependable components are key

Unlike their commercial or industrial counterparts, military products are specifically designed to exacting specifications and the tightest tolerances from the outset. For example, electronic and mechanical components are first selected for extended environmental operation, such as temperature, vibration, shock, and humidity.

For example, if military temperature grade components (-55 °C to +125 °C) are not available, then industrial temperature range devices (-40 °C to +85 °C) are selected. If – and only if – these two temperature grades are not available, then commercial temp range (0 to +70 °C) devices are used, and rigorously inspected and prescreened to the levels needed to ensure reliable product operation in the intended application during the system’s mission life cycle.

For high humidity operation, plastic encapsulated devices must be held and stored in a humidity-controlled storage cabinet prior to board assembly to reduce moisture entry into the devices. A specifically chosen conformal coating is used after board assembly to reduce susceptibility to moisture during the system’s deployment.

Then, depending on the grade and history of the components chosen, many are subjected to additional mechanical testing, such as package tolerances and hermeticity properties, Particle Impact Noise Detection (PIND) and lead plating and bonding, as well as temperature and upscreening tests, especially for commercial components that may be operating outside of their stated standard temperature range. Sometimes radiation screening is part of the equation (see sidebar on component testing, next page). This entire prescreening process, known as "guard-banding," ensures all the components operate well within the expected and tested parameters during system deployment. In light of today’s diminishing sources of military-grade components, upscreening remains one of the more reliable processes to pre-qualify electronics prior to end use.

You want qualified or guaranteed?

Upscreening at the component level remains a viable process approach if individual components and mechanical devices are 100 percent inspected, measured, rated, and sorted based on real data collected in real time, and not rated based upon statistically generated data from some predecided levels of expected yield probabilities. If premature failures are seen early during the manufacturing process, failure and risk mitigation steps must be taken to ensure higher yields at the next assembly. Then tighter process controls, such as 100 percent component inspection, including prescreening, should be put in place to reduce the chance of repeated failure (Figure 2). Electronic boards and modules using individually prescreened components are effectively and operationally guaranteed by design to produce a reliable yield during manufacture, and ultimately, reliably pass the demanding rigors of end use applications.

A complete platform is essentially a system of systems and is therefore only as dependable as its least reliable (non-redundant) component in an event chain. Unfortunately, the indiscriminate usage of upscreening has taken this once-reliable methodology into areas where it can potentially no longer guarantee success. Using off-the-shelf, untested, and uncharacterized components from brokers or other uncontrolled sources and upscreening only at higher levels of integration – such as at a board level or subsystem only – can effectively mask individual components operating at levels beyond their guard-band, meaning a module is destined for early failure. Modules using this late screening process are only qualified by test, meaning there are no process controls to guarantee future yield.

Without properly characterizing a board’s individual components, an upscreen board may pass some preset levels of Highly Accelerated Stress Screening (HASS) testing one day with flying colors and 100 percent yield and then miserably fail the next day. In most instances, no one knows what happened or why. Is this the degree of confidence end-users are looking for in safety-critical applications?

Temperature rising

Heat generated within a system can be a dangerous source of failure in any system or subsystem. (Remember that temperature and reliability are inversely proportional.) The potential for extreme temperatures and rapid, exaggerated, constant temperature cycling ranges combined with the extreme cost of an irreparable catastrophic failure within a critical application require close consideration.

As boards and systems become more densely configured, heat-generating characteristics of onboard electronics can create numerous problems at much lower temperature swings if a system is unable to aggressively dissipate heat from the active devices. Subsystem reliability decreases by approximately half with just a 10 °C rise in temperature in today’s complex "system-on-a-board" embedded designs.

Recurring heating and cooling cycles put severe mechanical stresses on system components, threatening long-term system reliability. Matching the Thermal Coefficients of Expansion (TCEs) for system components and the printed wiring boards on which they are mounted cuts the risk of having boards and components with significantly different TCEs. Unmatched TCEs can cause adjacent sections to contract and expand at different rates, resulting in heat-related electrical and mechanical failures.

Semiconductor considerations

Because excessive heat can exacerbate semiconductor package deterioration as well, leading to premature system failure, managing system temperature extremes is critical to ensuring reliability. Two common conditions of deterioration directly affected by inadequate thermal management are metal electromigration and electrostatic discharge.

One of the most common failure modes in modern Metal Oxide Substrate (MOS) semiconductors, metal electromigration occurs when a chain of metallic molecules forms, which can bridge thin insulating oxide layers and cause internal shorts. Advances in semiconductor designs – such as higher line densities that decrease width geometries and increase device functionality – greatly impact metal electromigration. These higher line densities generate a larger current density (charge per unit volume), increasing the resulting Electromagnetic Field (EMF). Over time, increases in the MOS devices’ EMF can induce metal ions from the metallization lines within the semiconductor to move or migrate, leading to the short.

We’ve all received a shock in our daily lives, sometimes even from a piece of electronic equipment, referred to as Electrostatic Discharge (ESD). While ESD in a computing system tends to mirror the larger and recognizable single discharge event (shock) we commonly experience, there are smaller static discharges within a computing system that can partially damage the oxide layer and cause dormant or hidden (latent) defects. If not detected, mitigated, or circumvented, these latent defects will cause premature failure in deployed systems. Continuous application of an EMF across a damaged, pitted insulating layer, coupled with higher device die temperatures, will accelerate metal ion migration across the partially failed insulation layers, resulting in premature system failure.

Qualifying the system: Imperative for critical apps

The products used in military and aerospace applications are expected to perform flawlessly for extended periods with little or no maintenance – especially during times of mission-, safety-, or life-critical operation. The users of these electronic subsystems therefore mandate the immediate, highly reliable operation of this equipment when deployed in the most severe environments and applications. Stringent testing and qualification procedures down to the very component are essential to ensuring that operational reliability. Guaranteed by design vs. qualified by test, in addition to temperature and semiconductor considerations, are vital to this assurance. CS

Doug Patterson is vice president of worldwide sales and marketing for Aitech Defense Systems, Inc. He has more than 20 years of experience in marketing and business development as well as product management in telecommunications and harsh environment electronics. He serves on VITA’s board of directors and holds three patents in advanced metered mailing systems and nonvolatile memory redundancy mapping. Doug holds a BSEE from BEI/Sacred Heart University. He can be contacted at [email protected].

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