How Long Do LEDs Last?

Nisa explains the “second-half syndrome.”

As president of LED Lighting Technologies, Dr. M. Nisa Khan consults in the solid-state lighting industry and educates consumers about LED lighting. She has a bachelor’s degree in physics and mathematics, and master’s and Ph.D. degrees in electrical engineering. Email her at nisa.khan@iem-asset.com.
 
Popularity begets popularity. Particularly today, because of the countless electronic communication systems that quickly dispatch popular news and information across the globe. Ask Tiger Woods. Today, the news travels – and aggrandizes – quickly.
In the scientific arena, popularity describes a matter, concept or idea that races across a given field, and later, perhaps, the nation or world. However, with science, a rise in popularity does not indicate a rise in validity.
For example, when a colleague recently remarked that LEDs last forever, I lightheartedly, but truthfully said only half will last a portion of forever. My reason? Lighting-industry professionals comprehend that a specific lamp series may be publicized to have a three-year life, but also recognize that only half the lamps are expected to last that long. The remainder may last only a fraction of three years. Some call this the “second-half” syndrome.
The “LED lamps operate interminably or almost forever” belief unfurls because casual observers read LED manufacturers’ claims of 100,000-hour lifespan. Unfortunately, such claims and beliefs have become popular, but nothing could be further from the truth. A classic fallibility example is the partially darkened, LED-arrayed, traffic lights we sometimes see.
Too often, the “second-half” failure syndrome frustrates signage and luminaire makers, especially if the lamps burn out within months or days of installation.
If prosaic traffic-signal LEDs experience lifespan problems, what can we expect from modern, but not-so-well-tested LEDs, especially the much-lauded, white ones?
First-half lamps
For many reasons, some scientific communities, as well as many ordinary people, believe LED lamps last a long time, or have such potential longevity. One reason is advertising claims; another is that LED indicator lamps on early electronic gear still produce light. Further, credible, laboratory tests (accelerated lifespan) of current LED lamps extrapolate lifetimes which should exceed 100,000 hours.
However, lighting professionals’ views differ from that of the public. They recognize, for example, that a manufacturer’s production run of two million, 100,000-hour lifespan, LED lamps produces roughly one million that will endure. Here, lifespan is akin to Mean Time Between Failure (MTBF). Some describe the long-life lamps as “first-half lamps,” because they prevail.
Therefore, assessing such a lamp-life characteristic shows that LED manufacturers shouldn’t claim a consistent lifespan for general-purpose, LED lamps, such as those sold for signage or illumination applications.
Preferably, manufacturers would test a production run of general-purpose LEDs for all relevant light characteristics and announce that half, at least, had endured as expected. Meaning, no light characteristics had dropped below published specifications.
This is rarely the case.
To justify advertised lifetimes, diligent manufacturers perform certain equivalent tests via some sampling, as well as accelerated temperature and aging tests. Such claims may be valid for smaller quantities, say for clusters of one hundred, rather than for tens of thousands, or millions.
Truth is, equivalency tests and samplings don’t always scale, because significant, temperature-related, performance variations exist among LEDs built from the same compound semiconductor epitaxial wafer, as well qas those from wafer batches. Other process variations may also cause performance variations within the entire lamp manufacturing line, but to lesser degrees.
Connections matter
Aside from manufacturing-line variations, consider that other LED lifetime and failure points may affect different LED types and, of course, the applied driving methods can affect endurance. Also, higher ambient temperature, humidity and operating current cause LEDs to degrade faster.
Generally, two types of LED lamps exist in the marketplace: high-power units that use elevated current (200mA or higher) or low-power units which use modest currents (tens of milliamps). Designated LEDs have their own current-voltage requirements and corresponding light-current characteristics. Expectedly, all diodes burn out if you apply excessive current, meaning, you shouldn’t direct high current into low-current lamps.
Because excessive current flow will burn them out, LEDs need to be properly wired. If you accurately choose your voltages, resistors and currents, both series and parallel connections are feasible. If you choose parallel, all the LEDs must be sufficiently identical in relation to operating current, resistance and diode turn-on forward voltage (transistor load time, for practical purposes). However, without the proper binning of equivalent LEDs, such matching is difficult to achieve.
Red, blue and white LEDs inherently require different voltages; therefore, parallel lamp connections may require each LED to have its own series resistance, to limit the current passing through the diode.
High-power LEDs (but not necessarily high-brightness LEDs) require high current, and this causes them to operate at a higher diode-junction temperature, which, in turn, leads to faster aging and a shorter lifetime.
For their various uses, sign manufacturers, and others, require white-light LEDs to produce high lumen output. And, for decent illumination of channel letters, cabinet signs and electronic displays, sign-type lamps should consistently produce several hundreds of lumens throughout their predicted lifespan. However, unlike traditional indicator lamps, signmakers require their lamps to correspond in color rendering and temperature during their entire lifespan. Such requirements inevitably shorten the lamps’ lifetimes.
Most manufacturers of high-power, white LEDs estimate a lifetime between 30,000 to 50,000 hours, to maintain 70% of initial lumen level. This estimate assumes constant operation near 350mA of constant current and a consistent junction temperature not higher than 90° C.
Improved technologies
Improving technologies have produced longer-life LEDs which, to some extent, tolerate higher drive currents and operating temperatures. Presently, certain manufacturers offer LEDs rated for 100,000 hours while operating at over 700mA with junction temperatures up to 120° C. I suggest you consult with these manufacturers, to see if such LEDs are available and cost effective, at high volume.
As the LED lighting industry develops, the engineers consider both 70% and 50% lumen-maintenance levels as lifespan definitions, but for different applications.
For example, a traditional lightbulb ends its usefulness when the filament breaks and the light ceases. Oppositely, an LED lamp’s efficiency drops over time, which causes the light output to diminish gradually. When it falls below 70% (or 50%) of the original output, the design engineers say it has expired.
Interestingly, the average person can’t to judge when the LED lamp has reached this point, which is the drawback of this standard.
To better understand LED lifespans, ask your LED lamp provider for lifespan data sheets that describe the gradual behavior of the lamps’ luminous-output degradation over time and under specific operating conditions.
Also ask them to provide test data that relates to the specific LED chip or die, or junction temperature, over the lifespan test period. Not all LED manufacturers publish such data, but such information can help you calculate a specific LED-based sign’s life.
In my next column, I’ll write more on finding application solutions, and LEDs’ thermal designs and analysis.