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<\/p>\nAccelerated life testing is the process of testing a product by subjecting it to conditions such as stress, strain, temperatures etc. in excess of its normal service parameters in an effort to uncover faults and potential modes of failure in a short amount of time. It is the answer to the frequently asked question of \u201chow long is a new design likely to work under normal use conditions?\u201d or the question of \u201chow can we improve the reliability of systems or components?\u201d<\/p>\n
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There are basically two methods of performing accelerated life testing; the classic life testing (ALT) and Calibrated Accelerated Life Testing (CALT).<\/p>\n
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The presentation gives an overview of the methods for design and execution of accelerated testing with focus on applicability, advantages and disadvantages of the methods. Cases are included to exemplify the application of the methods.<\/p>\n
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Classic accelerated life testing based on acceleration models and acceleration factors are used to predict the \u201clifetime\u201d for a given product in a predefined use environment.<\/p>\n
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In an accelerated life test, the time-to-failure (tfH) for a given wear out failure mechanism is determined at high (accelerated) stress level (SH). Provided that the function G(S) applicable for the wear out failure mechanism under consideration and thus the acceleration factor (AFLH), is known, the time-to-failure (tfL) at a lower stress level (SL), normally the use stress level, can then be calculated as: tfL = AFLH \u00b7 tfH.<\/p>\n
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The basic principle of accelerated testing is illustrated in the following figure.<\/p>\n
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Hence, the design of an ALT the following have to be considered:<\/p>\n
No single acceleration model can address all possible failure mechanisms of complex assemblies and products. Therefore, the challenge is to find the most relevant failure mechanisms to investigate. This may be done by failure mode analysis of the designed product associated with the intended use profile or by performing a HALT (Highly Accelerated Life Test).<\/p>\n
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It should be noted that only wear out failure mechanisms can be addressed with accelerated life testing. Overstress failures are not time dependent in the way that is assumed in the acceleration models commonly used. Rather, they depend on the short term absolute stress level and should be addressed using relevant \u201coverstress\u201d test methods such as HALT. The table below shows examples of common wear out and overstress failure mechanisms.<\/p>\n
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Different acceleration models apply depending on the failure mechanism and thus the relevant stress i.e. the Arrhenius acceleration factor for temperature, the Inverse power law for vibration (high cycle fatigue), the Coffin-Manson acceleration factor another Inverse power law for thermal cycling (Low cycle fatigue) etc.<\/p>\n
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The extent of acceleration is limited by the risk that the stress acceleration may exceed physical properties of product materials and cause unforeseen damage by stimulating irrelevant failure mechanisms.<\/p>\n
The example below describes the tailoring of an ALT for a high power white light emitting diode (LED) which is used as light source in a food analyser.<\/p>\n
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The use and performance requirements for the LED are described as:<\/p>\n
Identification of failure type and mechanism:<\/strong><\/p>\n According to [1] “LEDs rarely fail catastrophically; instead, their light output slowly degrades over time”. This is typically caused by a combination of:<\/p>\n The information given above indicates that one or several temperature dependent wear out failures are dominating the lifetime of the LED. The underlying failure mechanisms are likely to be diffusion, thermal ageing or a combination. It is therefore expected that the Arrhenius acceleration model can be applied and the acceleration factor, AFLH<\/sub> calculated from the equation below:<\/p>\n Typical maximum operating temperatures for GaN based white LEDs are 80 \u00b0C to 100 \u00b0C. This limit is often stated just to ensure a reasonable lifetime and not because of possible overstress. Typical polymers used in LEDs have been tested at 100 \u00b0C and 140 \u00b0C. Thus, a test temperature between 80 \u00b0C and 100 \u00b0C seems to be on the safe side with respect to overstress.<\/p>\n <\/p>\n For an activation energy, Ea<\/sub>, of 0.79 eV and a \u201cpre-exponential factor\u201d, At<\/sub>, 8.00 \u221910-9<\/sup> h, the test time at T = 80 \u00b0C is 1504 hours (~63 days) and 374 hours (~16 days) at T = 100 \u00b0C.<\/p>\n <\/p>\n As the average time-to-failure tf<\/sub> (or “lifetime”) of the LEDs, as a function of temperature T [\u00b0C], can be described by:<\/p>\n CALT (Calibrated Accelerated Life Testing) is a single stress analysis and test strategy used to find a product\u2019s lifetime. The highest stress level is defined as slightly below the \u201cFoolish stress level\u201d, which is where the device instantly breaks down. A device is tested to failure at the highest possible stress level and the time to failure is recorded. The actual failure mode is identified. Test a new device to failure at a lower stress level, again record the time to failure. Perform an analysis of the results and test a third device at a stress level corresponding to the time available for reliability testing. Finally, the expected life at normal stress level is calculated based on all the test results. The figure below (taken from GMW8758) is illustrating this in a graphical way.<\/p>\n\n
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<\/p>\nCALT<\/h3>\n
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