Accelerated life testing
A form of testing that reduces the time till results are known, ALT provides a means to estimate the failure rate over time of a product without resorting to normal use conditions and the associated duration.
For example, solar photovoltaic cells should operate for 25 years without failure. The product development time is less than a year for a new panel and the team wants to estimate the reliability of the cells over the 25-year duration.
Why is it better than other methods?
A simplistic way to determine the reliability is to set up and operate solar systems for 25 years and track the number of failures over time. This isn’t practical or useful.
Another method is to only test units as above for one year then use the available information to make a decision. While product failures may occur (most likely due to manufacturing and installation errors) it will not be useful for delimitation of coatings or sealants, solder joint fatigue, PV cell degradation, and other longer term failure mechanisms. While we could make an estimate, it would not be very accurate beyond the one year of use replicated in the testing.
Two words: Time compression
Accelerated testing uses a variety of methods to compress time. Using the solar cell example, if UV radiation degrades one of the coatings, we can ‘accelerate’ this by exposing the cells and coatings to UV radiation more often per day than in normal use. If in normal use the panel would have 10 hours of direct daylight per 24 hour day, then using UV lamps, we could expose the units 24 hour per day, for a 2.4 x acceleration. Thus instead of 25 years, we would require little more than 10 years for the testing.
Still not good enough.
If we explore the UV damage mechanism, it may be part of a chemical reaction that clouds the coating. This chemical reaction most likely can be accelerated with temperature (Arrhenius rate equation – remember your chemistry). With a little work, we find the expected coating temperature during use is 40°C or less 90 percent of the time. By increasing the temperature (and not melting the coating – watch those phase transitions) we may find another 10x of acceleration. The specifics of the acceleration is related to the activation energy of the specific chemical reaction, the testing temperature, and the use temperature.
Use ALT to its best ability
Therefore, with a little understanding of the use conditions (10 hours of daylight per day) and the specific failure mechanism in question (UV and temperature driven chemical reaction) we can achieve a 24x acceleration factor. This will still take a little more than a year to fully test out to 25 years, yet the team may be able to use the data out to 24 years as a rough estimate and update when the final results become available.
This is just a short example and mainly an example of my approach to accelerated life testing. There is a lot more to ALT design, yet the most important by far is the understanding of the failure mechanism.
Reading a Datasheet (article)
Sources of Reliability Data (article)
Sample Size – success testing (article)