Reliability Qualification Methods
“Qualification” is the formal, legal, demonstration of survival of a series of stress tests conforming to recognised international standards, during which specific characteristics are measured to determine the product behaviour within or beyond the specified end-of-life limits. In the components industry, the established standards include those that were drafted by BellCORE (now Telcordia) and more recently IEC –increasingly favoured by telecommunications service providers. In the electronics industries the standards are JEDEC, IEC, CECC, EIA, AEC, MIL. Meeting the requirements of the “standards” does not, however, absolve the supplier company from proving that their products are fit-for-purpose. Hence there is the need for long term reliability assurance.
Single Telcordia standards do not fully cover the range of products now offered by photonics component suppliers. Therefore, Qualification Plans require intelligent and knowledgeable interpretation of those standards and adoption and modification of relevant tests. Applying a full Telcordia specification to a subsystem level product imposes a draconian range of tests to be applied (adding cost, time and irrelevance). Sometimes over-severe stresses may destroy the high-level product. Therefore Telcordia and IEC stress tests are more effectively applied at the level of build at which the tests remain relevant. Thus, product reliability assurance is built up from reliability tests on the building block elements that make up the product. Provided that the design, technology and processing of the individual elements remain unchanged, the qualification data is then stored in a library of parts and is reusable as the Buliding Blocks of future more complex products. This concept of Building blocks qualification was devised in the 1990s by Nihal Sinnadurai whilst VP and Global Director of Reliability of Bookham Technology, in order to streamline yet keep effective and authentic the Qualification of complex products such as photonics transponders, tuneable transmitters and multiplexers. The concept was extended also to complex electronics subsystems and systems.
The reliability models and the associated acceleration factors for new technologies are verified as new technologies and processes are introduced. The reliability tests emulate long term behaviour by applying overstress and accelerated ageing conditions beyond normal operating environmental conditions, such as by elevated temperature overstress to accelerate thermally-induced failures in a known way in accordance with the Arrhenius model. Reliability Assurance tests are in addition to “Qualification” and generate information on FIT rates and the essential statistics required by those deploying systems in the field. Because qualification tests and reliability assurance tests are intended to consume actual life, and demonstrate the product does not fail such tests are deemed “destructive”, i.e. the components cannot be sold on.