Truly rugged and proven reliable: VITA 47 and beyond
All vendors of commercial off-the-shelf (COTS) hardware intended for use in harsh defense and aerospace environments insist that their products are reliable as well as rugged. But without a consistent baseline for comparison, it's almost impossible for system integrators to objectively confirm whether one COTS product is more reliable than another. The VITA 47 standard gives system integrators just such a baseline as it is an American National Standards Institute (ANSI) standard.
ANSI/VITA 47 – introduced in 2005 – defines a set of rigorous environmental, design, and construction, safety, and quality requirements for plug-in COTS hardware that is intended for use in mobile defense and aerospace applications. VITA 47 provides system integrators with a common standard they can use to ensure that the COTS modules they are evaluating provide the functionality needed and that they meet environmental and durability requirements.
The VITA 47 standard outlines requirements for the following environmental factors:
- Temperature, including operating temperature, nonoperating temperature, and high levels of repeated thermal cycling
- Shock and vibration, including operating conditions and bench handling
- Humidity, altitude, and rapid decompression
- Fungus and corrosion resistance
VITA 47 also includes the following design, construction, safety, and quality requirements:
- Workmanship, including soldering to IPC Class 3, conformal coating to IPC Class B, and printed wiring board (PWB) fabrication to IPC Class 3
- Safety, including material restrictions to avoid hazardous or restricted materials, flammability to EN (European standards) or UL (U.S. and Canadian standards) specifications, and toxicity limits
- Quality to the ISO 9001 standard
The requirements defined in the VITA 47 standard are divided into levels within classes, with increasingly stringent test demands at each level. System integrators who require their embedded systems to perform optimally in extreme conditions should seek COTS products that have passed the highest levels in the VITA 47 standard.
Look for long-term reliability
COTS products that pass the highest levels in the VITA 47 standard are proven to meet the most challenging environmental requirements, including operating temperature, nonoperating temperature, and temperature cycling. For the popular conduction-cooled format, the VITA 47 levels are defined as ECC1 to ECC4 (Table 1).
For system integrators that need to assess the long-term reliability of COTS products, temperature (thermal) cycling tests prove particularly important. Even so, while evidence that a given COTS product has been tested to and passed the VITA 47 ECC4 level can provide a system integrator with confidence in the long-term reliability of the product, it doesn’t necessarily provide an indication of expected life span. One of the best ways to determine the expected lifespan of electronics is to develop a “life profile” for the product and then evaluate what that profile means in terms of the VITA 47 thermal cycling requirement.
A life profile estimates the numbers of cycles of various environmental scenarios the electronics is likely to experience – the number of hot days, cold days, and days with ground operations, for example. For airborne systems, it’s also important to consider the temperature ranges when the platform transitions from the ground to a high-altitude environment and back again. The total number of cycles can then be translated into accelerated thermal cycling tests with the corresponding number of cycles and temperature extremes.
As an example, a survey of temperature cycling requirements across 11 defense programs that required 10 to 30 years of service life in harsh environments revealed that the electronics had to sustain an average of 23 cycles of -55 ºC to +105 ºC per year. The VITA 47 C4 test is much more stringent: It requires that electronics be able to sustain performance after 500 cycles of -55 ºC to +105 ºC. This means that, in this case, the VITA 47 ECC4 thermal cycling requirement represents approximately 21.7 years of service life in an average defense program in a harsh environment.
Going beyond VITA 47
While the VITA 47 standard is an important measure for evaluating product ruggedness and reliability, even the most demanding thermal cycling tests won’t deliver the ultimate proof of reliability. The ideal approach is to combine VITA 47 testing with a comprehensive set of reliability analyses, tests, and procedures developed over the course of knowledge gained by designing, building, and testing COTS products.
Best practices dictate that companies should run VITA 47 tests on their standard production modules, verifying compliance with the VITA 47 shock, vibration, and operational temperature requirement as part of a standard qualification process. Such compliance testing involves pushing the equipment to the worst case of VITA 47 expectations.
VITA 47 has proven to be one of the key tools for discovering new ways to deliver the highest levels of ruggedness and proven, long-term reliability. Testing process and research efforts, for example, led to the discovery that lead-free soldered assemblies can be just as reliable, if not more so, than tin-lead soldered assemblies. In addition, it was found that lead-free solder may present opportunities for even better reliability in the future. With tin-lead technologies, smaller solder balls and lower standoff height for components tend to reduce thermal cycling reliability. With lead-free solder, smaller solder joints can actually improve reliability. Such knowledge garnered through testing and research enables the continued miniaturization of components while increasing functionality density, all without negatively affecting reliability.
Another example is seen in improvements for PWB interconnect reliability. All PWBs are not created equal: The more complex the PWB, the smaller the interconnects on the board. The smaller the interconnects, the easier they break. PWB designers and manufacturers must understand the physics behind how electronics behave in harsh conditions over time to ensure that interconnects won’t open or crack.
By studying the physics of failure at the lowest levels, a COTS vendor can understand how to design, analyze, test, and manufacture PWBs with highly reliable interconnects, even on extremely complex high-density interconnect designs.
The results of reliability tests have revealed how to best implement microvias on a PWB. Detailed knowledge about the tradeoffs between the density of interconnects and the reliability of interconnects enables the COTS vendor to optimize microvia density and reliability on a board in a way that goes beyond what is typically provided by other vendors.
Moreover, combining VITA 47 testing with detailed debugging and deep-dive root cause analysis enables designers to identify and eliminate failures before they can cause consequences in the field. Catching and correcting design flaws that might otherwise not be exposed before deployment in an embedded system is an important benefit of extensive analysis and testing.
Look for advanced reliability testing
System integrators evaluating COTS products for use in defense and aerospace systems should take a hard look at each vendor’s reliability design, analysis, and testing procedures. Only vendors that commit significant resources, expertise, and time to understanding the physics of failure and systematically incorporating the resulting knowledge into their products can offer consistent reliability. The VITA 47 standard gives system integrators a baseline by which to objectively compare the reliability of COTS products from various vendors. Products that have passed the highest levels of the VITA 47 standard provide the highest level of reliability in the most extreme environments.
Curtiss-Wright Defense Solutions www.curtisswrightds.com