Republication of Postings from the U.S. Department of Energy (DOE) Solid-State Lighting Program
by Jim Brodrick, U.S. Department of Energy
Next to energy efficiency, long life is probably the most publicized of solid-state lighting’s (SSL) potential advantages, and plays a key role in any cost-benefit analysis. But understanding how LED lighting products fail, and communicating it effectively, aren’t easy matters — and are further complicated by the cost and impracticality of traditional life testing. The problem is laid out in a U.S. Department of Energy (DOE) SSL Technology Fact Sheet, Lifetime and Reliability, which summarizes what’s known about the topic, including typical causes of failure for LED lighting products, the difference between lifetime and reliability, and currently available methods for measuring and reporting a product’s lifetime.
Many folks are collaborating to better understand the issue of SSL lifetime — including an industry working group known as the LED Systems Reliability Consortium (LSRC), which is meeting this week at DOE’s 11th annual SSL R&D Workshop, in Tampa, FL. A spinoff of an earlier DOE working group that developed the report LED Luminaire Lifetime: Recommendations for Testing and Reporting, the LSRC recently completed a new study utilizing a highly accelerated life-test method — called the “hammer test” — intended to produce failures in SSL luminaires in a reasonable test period, with the goal of providing insight into potential failure modes. Entitled Hammer Testing Findings for Solid-State Lighting Luminaires, the new report was prepared for the LSRC by RTI International and is just the first step in the LSRC’s ongoing work on this important topic, which is helping to forge a common understanding and may influence future standards.
All lighting products eventually reach the end of their useful life. For conventional, lamp-based lighting systems, this most often occurs when the lamp burns out, which is why lifetime focus has traditionally been confined to the lamp itself. But LED lighting isn’t only lamp-based, and performance is affected by a host of system components, the failure of any of which — not just the LEDs, but also the electronics, thermal management, optics, wires, connectors, and seals — can lead to failure of the entire product. In addition to catastrophic failure (i.e., total burnout), there’s also the type of failure that can occur when the light produced is unacceptable in quantity (lumen maintenance) or quality, due to degradation or shift in luminous flux, luminous intensity distribution, color temperature, color rendering, or efficacy.
Among the components that can cause failure in an LED lighting system, most of the attention has focused on the LED packages, which rarely fail catastrophically. Of the various other failure causes, lumen depreciation has received the most attention and is often used as a proxy for LED lamp or luminaire lifetime ratings, even though — for the reasons just stated — it’s not an accurate proxy. The most widely used criterion for lumen maintenance failure is when a product reaches “L70” — that is, when its light output has dropped to 70 percent of what it was when the product was new. Because failures among a set of installed products don’t all occur at once, lumen maintenance ratings are usually based on the time at which 50 percent of a sampling of products have reached L70 — a point denoted as “L70-B50.”
But depending on the application, L70 may be too much — or too little — lumen depreciation. And there are other ways to convey lumen maintenance performance, such as identifying the expected lumen maintenance at a fixed time interval (e.g., 25,000 hours) — which may allow for more-effective comparisons between products, especially when the calculated L70 value exceeds the intended product-use cycle or the anticipated lifetime of another system component.
In the recent hammer test, commercial indoor SSL luminaires were subjected to extreme environmental stressors, including temperature cycling, temperature and humidity soak, and high-temperature bake, with power cycled to provide electrical stress as well. All of the luminaires survived more than 100 cycles of temperature shock (-50º C to 125º C), and nearly half survived more than 300 cycles. The failures that were observed typically occurred in the driver circuit, with board-level failures being most common. The 611 LEDs in these luminaires endured nearly 1 million LED-hours of cumulative exposure to the hammer test, with only four failures — two of which were attributed to solder-joint fatigue, and the other two to board-level corrosion.
The findings reinforce the belief that LEDs in lighting systems are highly reliable, even under extreme conditions, and indicate that other luminaire components are more likely to fail first. While the results suggest that SSL luminaires will have a low probability of random failure in the field during normal use, additional work is needed to determine actual wear-out mechanisms, quantify failure modes, and determine acceleration factors, in order to provide estimates of lifetime and reliability in normal operation.
Users may eventually be able to determine the best balance between lifetime, reliability, serviceability, warranty, sustainability, and cost for the lighting application in question. Typically, long life comes at a cost, but its advantages may not be realized if the expected use cycle is less than the lifetime. For example, a building scheduled to be renovated in the next 15 years may not benefit from lighting products with a 30-year lifetime. Instead, it may be better to use a less-expensive product that has a shorter useful life but higher reliability. On the other hand, shorter-lived products generate more waste and compromise sustainability goals.
As the technology matures, the dependence of LED package performance on other components will continue to require that discussions about lifetime be focused at the luminaire level, as lamp performance in different luminaires can vary. Innovative luminaire designs and control strategies — such as variable drive products that maintain lumen output — will further complicate lifetime measurement and reporting. As with many other performance attributes, LEDs have the potential to surpass conventional lighting for longevity — as shown by the negligible changes observed in lumen maintenance and color in the L Prize®-winning product after 25,000 hours of testing. But choosing the right product requires some understanding of expected failure mechanisms, lifetime, reliability, and serviceability, as well as asking the right application-specific questions.
