After the development of the first high-brightness blue LED by Shuji Nakamura in 1994 which was based on InGaN (Indium Gallium Nitride) (2), light emitting diodes (LEDs) have started to be used as lighting sources in various lighting applications. They are also used in display backlighting, communications, medical services, signage and general illumination (3).
Thanks to the small design of LEDs, they offer outstanding flexibility that enables the manufaturer to minimize the size of the fixtures to achieve very smart designs. Moreover, LEDs have high energy efficiency that results in lower power consumption and low current operation, longer life time, higher performance, wider range of controllable color temperatures, wider operating range and no low-temperature startup problem. That’s why the use of LEDs in general lighting has increased in the latest years and still in progress.
Over the past years, several developments were made in the history of LEDs which can be divided into three generations: advancements in new fabrication technology and equipment, new phosphor materials and improved heat dissipation technologies. High-brightness LEDs first generation was commercially successful in 1980s. From 1980s to the present, the second generation high-brightness LEDs became very popular in the world LED market. The third generation is arriving in the market that is developed for savings in electrical energy consumption and a reduction in environmental pollution.(4)
Despite the technological advances, LEDs still face challenges as price and lack of information regarding reliability. If one single LED fails, then the final product is sometimes treated as a failure. Tests are performed in the product development cycle during the design and development phases into operating life tests and environmental tests by using industrial standards. A lifetime estimate is made with uncertainties such as exponential extrapolation of lifetime, assumption of activation energy and possible failure mechanism shift between test and usage conditions. So once an LED package or LED module encounters failures, unscheduled maintenance will be needed causing high cost for end-users. To sum up, LED package reliability is important for improving LED lighting system reliability.
LEDs efficient thermal management will guarantee the reliability of the whole lighting fixture, and ensures longer lifetime based on real-time application conditions. The light output can change as a result of the operating conditions and temperature in particular. So at higher temperature, quantum efficiency decreases, a drop in the forward voltage causing a decrease in series resistance and a change in color causing degradation of light output. Specially in high-power LEDs, lifetime information and standardized relevant measurements, definitions and reliability related tests are an important source for any designer to deliver the best combination.
Finally, poor LED thermal management can affect three main areas: lumen depreciation over time, color shifting and total fixture efficiency. Those three areas are main factors in determining the reliability of the LED package and, indeed, LED lighting system reliability.
(1) LASANCE, CLEMENS J. M. THERMAL MANAGEMENT FOR LED APPLICATIONS. SPRINGER-VERLAG NEW YORK, 2016.
(2) Nakamura, Shuji. "Development of the Blue Light-Emitting Diode". SPIE Newsroom. Retrieved 28 September 2015.
(3) The article will mainly focus on the LEDs in the general illumination applications.
(4) Some high-power LEDs in the market now can reach up to more than 210 lm/w at specific binning and thermal conditions.