The long-term reliability of LED (light emitting diodes) semiconductor die provides a very strong value for most applications. If packaged properly, LEDs emit light for a much longer time period than almost every other alternative light source technology. LEDs are semiconductor light sources. Because LEDs operate under the laws of solid state physics, reliability behavior is characterized in a semiconductor context. LEDs are not subject to catastrophic failure when packaged properly and operated within the design parameters.

Light generation in LEDs and incandescent bulbs are distinctly different processes. With incandescent bulbs, light is generated by heating a tungsten filament to high temperatures, and leads to macroscopic materials degradation and loss. Within an LED, light is generated through electron-hole recombination and is a materials conserving process. The crystalline structure of the LED is unchanged in the light emission process, although microscopic changes can occur. At normal low drive currents and voltages, these microscopic changes are small. The macroscopic versus microscopic nature of the degradation processes also leads to different failure mechanisms. Incandescent bulbs fail suddenly and catastrophically when the filament ruptures. The performance of LEDs with time typically follows a predictable degradation of light output with time. In some instances, LEDs can fail catastrophically due to environmental (high Al-content LEDs) or packaging related effects (stress).

The most common lighting industry characterization is rated life. Rated life is defined as the length of operational time, under standard conditions, that 50% of a large sample of devices will fail catastrophically. For example, one can expect that half of the incandescent lamps rated at 1000 hours will fail after 1000 hours of use. At 1000 hours, there is a 50/50 chance that any bulb's filament will break. No reference is made to the degradation of light output over the rated life.

The semiconductor industry also refers to catastrophic failure as a definition of device life. In this case, the term used is MTBF (mean time between failure). MTBF is a well defined and rigorously calculated statistical term. LED physics dictates that this number reflect the time before one can expect a single failure. Once the initial failure occurs, one restarts to time zero, and it will take another MTBF period before another failure can be expected. Memory ICs are characterized under these same conditions. Although this definition is appropriate, it is not terribly useful in light output sensitive applications. It also does not integrate many of the device handling, assembly, device drive conditions, and environmental factors that contribute to premature LED failure. No reference is made to light output degradation.

The mean time between failure (MTBF) of high quality LEDs properly packaged, is on the order of millions of hours. Therefore, a rated life projection provides little useful information. However, all LEDs experience light output degradation. It is a matter of useful life — how long will the light output level remain useful?

Because every application is different, there is no simple answer to this question. A determination of the minimum acceptable light level, associated drive requirements, and applicable reliability parameters desired needs to be carefully reviewed and serve as the basis for the LED device packaging design to ensure the minimum light level is not breeched over the useful life. In combination with this review a clear understanding of the package design in terms of mechanical, material and other factors must be reviewed and determined for potential component package related degredation issues ranging from moisture induced corrosion, dark-line-defect growth, strain-optic effect, optical transparency, and other chemical and physical effects. In practice, LED die and their life can be extended or diminished based on these packaging choices. LEDs can be packaged to exceed 100,000 hours of life while being cycled from –55 to 125 ºC, or can be packaged as an indicator lamp (5mm/T-1 3 /4) to last 100,000 hours at ambient temperature.

Existing light output reliability data provides characteristic LED packaged component performance under several drive conditions and at various temperatures, over time.Various packaged LED lamp manufacturers publish this type of information. Packaged LED operating life is noted by these suppliers as characterzed by the degradation of LED intensity over time for that particular package style. The limiting factor in all of these characterizations is almost always package-related versus LED die-related constraints.

When the LED packaged component degrades to half of its original intensity after 100,000 hours, for example, it is at the end of its useful design life although it will continue to operate at diminished levels. LEDs can also be operated in high shock and vibration modes, over wide temperature variations and environments, and can be cycled on/off without excessive degradation. As for the actual LED die, the useful life far exceeds packaged performance component levels.