Features

Reliability test plans are critical to the success of any new electronic medical device or technology. The tests selected must be stressful enough to identify defects but also show a correlation to realistic use environments. The preferred test plan development approach is to use a combination of industry standards and Physics of Failure (PoF) techniques. This results in an optimized test plan that is acceptable to regulatory bodies, management, and users. But even the most streamlined qualification testing can take months or years, and can still result in a design that may not meet reliability requirements.

Fig. 1 – Environmental Variations in Transport - ation Conditions, data courtesy of Xerox Product Performance: Survivability.
Significant opportunities exist for medical electronics. There is an increasing public awareness and perception of issues with medical devices due to high profile recalls and adverse events. Furthermore, the perception that one’s life may be dependent upon these products creates a powerful emotional effect, which means that assuring reliability becomes of critical importance.

Once a product has been designed, data files are created that manufacturers use to build the product. Now, software tools exist that can take those very same manufacturing files and build the product “virtually.” The virtual product can then be subjected to the anticipated user environment to determine the reliability and identify any weak areas. No longer are months required to build prototypes of the product and subject them to reliability tests — design changes can now be made and reviewed almost instantly. What-if analyses to optimize a design can be done in a fraction of the time it takes to even get on a typical designer or fabricator’s schedule. Mechanical simulation can be run in minutes, not days, providing the answers needed to build a product better, faster, and at a reduced cost.

The greatest potential for reliability and quality improvement is also when the cost is lowest: before the first prototype is ever built. The most requested qualification tests have been packaged for the medical device designer and include: virtual shock testing, virtual vibration testing, virtual thermal cycling, and virtual CAF (conductive anodic filament) testing.

Fig. 2 – Lifetime Prediction of Implanted Medical Device with 6-year life with a 5% probability of failure (component failure was the highest contributor).
Medical device designers can use reliability modeling software to get a jump on testing by reducing the risk of failing qualification tests. Using a physics-based automated design analysis (ADA) tool is like testing the design before a single prototype is built. Design weaknesses can be identified early, allowing for iterative design improvements while maintaining an aggressive design schedule. The ADA tool can also identify areas where specific, targeted testing would be beneficial.

Design is always an exercise in tradeoff analysis. Some choices are driven by cost, some by function, some by form, some by risk and safety. The ability to rapidly compare design choices and understand quickly how those choices affect reliability helps ensure that both the likelihood of premature failure and the long-term cost of ownership remains low.

The use of the reliability modeling tool is limited only by the needs of the user. There are a wide range of problems that it can either solve or provide insight on, in regard to designing a reliable product. Some of these uses are identified below:

• Determine thermal cycle test requirements needed to replicate the use environment.

• Determine proper environmental stress screen (ESS) conditions.

• Determine impact of component package modifications or changes to the circuit board laminate.

• Determine impact of changing to Pbfree solder.

• Determine expected failure rates and times for a given set of conditions.

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