Solid State Lighting Reliability by W.D. van Driel & X.J. Fan

Author:W.D. van Driel & X.J. Fan
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

9.1 Introduction

Accelerated Life Test (ALT) is a lifetime prediction methodology commonly used by the industry in the past decades. This method, however, is reaching its limitations with the development of products within emerging technologies requiring long-term reliability. New methodologies are required to shorten the time to market, and accurately predict long-term reliability. A new method to predict long-term reliability by extending ALT methods is presented in this chapter. This will be achieved by a novel numerical-experimental approach, substantiated by a fundamental understanding and description of the degradation mechanisms involved. The purpose of ALT is to induce field failure in the laboratory at a much faster rate, by providing a harsher yet representative environment. In such a test the product is expected to fail in the lab, just as it would have failed in the field, but in much less time. Currently, the industrial trend is towards products with high long-term reliability. These products, contradictorily, are expected to be developed in a shorter time to market. For instance, typical lifetime for automotive electronics is 15 years, LEDs 7,000 h to 50,000–100,000 h, and solar panels more than 20 years. The industrial desire is to be able to qualify these products within 6 weeks, and be able to guarantee the long-term reliability. The established 1,000–6,000 h ALT schemes cannot provide the mentioned industrial needs. Therefore, the state-of-the art ALT is not sufficient anymore and extensions or new methodologies need to be developed.

The common industrial practice is to first identify the dominant failure mechanisms, and then perform tests which accelerate that specific degradation mechanism. This approach needs to be well verified in the sense that only the desired failure mechanism is triggered, and others are suppressed. Further, accelerated tests use harsh environments such as high–low temperatures, high humidity, etc. which affect the material’s response to time-dependent relaxation or diffusion mechanisms as well as mixed interactions such as time and temperature, temperature and humidity.

The reliability information obtained by ALT is fed into models such as Coffin–Manson and Engelmaier, and used to extrapolate the product reliability. In products that require long-term reliability, the extrapolation extends to 10–20 years, where the errors also extrapolate orders of magnitude (Fig. 9.1). Hence, the accuracy and tolerances become crucial. From a research and development perspective, how to accurately correlate the information obtained from ALT to the application is not yet answered. The main goals of our ALT approach are the following:

Fig. 9.1Schematic of stress level vs. lifetime for a hypothetical electronic product

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