December 23, 2013…While lifetime in LEDs is how long an LED lasts until it emits no light, it is the useful lifetime that is important. A number of organizations have come out with procedures that give a good estimate of average lifetime based on a test of lumen maintenance. This is used to estimate useful lifetime usually referred to LM-79. The useful lifetime according to the DOE is then the the time required for the luminous flux of a percentage of the population (e.g., 50% or B50 number) to drop below a desired level (e.g., lumen maintenance drops below 70% or L70 value). This definition includes both catastrophic failures (i.e., when no light is emitted) as well as lumen maintenance failures. This testing takes a considerable amount of time 6,000 to 12,000 hours. Another issue in examining luminaire lifetimes that a luminaire with many components may have a different lifetime than the LEDs within it. Each of these components could have a different rate of failure, so a system level approach must be taken.
The DOE formed the LED Systems Reliability Consortium (LSRC), which consists of lighting companies, national laboratories, research institutes, universities, and other interested parties to further understand the reliability of LED luminaires. The DOE says that developing an understanding of SSL luminaire lifetime requires building a database of likely failure modes. Since field failure data are generally not widely available, these failures must be created through highly accelerated life tests (HALT). The LSRC decided to perform HALT testing on select luminaires donated by luminaire manufacturing companies.
A panel consisting of Dr. Xuejun Fan (Lamar University), Dr. Abhijit Dasgupta (University of Maryland), and Dr. Lynn Davis (RTI International) was commissioned to develop a “Hammer Test” that would serve as an initial HALT protocol for these luminaires. Dr. Willem van Driel of Philips also provided valuable inputs to the panel. The Hammer Test is The Hammer Test was to serve as a HALT method that would produce failures in SSL luminaires in a reasonable test period (defined as less than 2,000 hours. The DOE says that it was not intended to be a universal accelerated life test (ALT) for luminaires, but instead was designed solely to provide insights into potential failure modes.
The Department of Energy has published the detailed protocol for “Hammer Testing luminaires along with an explanation of their usefulness and rational. The DOE also published the results for the Hammer Tests on seven different luminaire models.
The Hammer Test consists of four phases of testing. Stage 1 is the Steady-state temperature humidity biased life test consisting of 6 hours at 85 degrees C and 85 percent relative humidity. In this stage, power is applied to the devices on a 1-hour cycle—the devices are switched on for an hour and then off for an hour for a total of 6 hours. Stage 2 is the temperature shock test consisting of 15 hours cycling at -50 Degrees Celsius to +125 degrees Celsius with a hold time at each temperature extreme for 30 minutes. Stage 3 testing consists of steady-state temperature humidity biased life test consisting of 6 hours at 85 degrees C and 85% RH. Stage 4 is high-temperature operational lifetime testing consisting of 15 hours at a constant 120 degrees C with power cycled on an hourly basis.
The panel compared the results of the Hammer Tests to 296 days of performance testing of the luminaires to act a control. At the end of each 42-hour loop of the Hammer Test, each luminaire’s performance was screened both visually and with a small integrating sphere containing a calibrated Minolta CL-200A Chroma Meter. After every 5 loops of Hammer Test, more quantitative analysis of each luminaire was conducted using the 65? integrating sphere and the testing procedures outlined in LM-79.
The LSRC performed the Hammer Test on seven different commercial luminaire models, provided by different manufacturers or independently purchased. Six of the luminaire models were 6 downlights or similar products, and one luminaire model was a 2×2 troffer. The total sample populations of Luminaires A, C, D, and E were split; half (typically three samples each) were placed into the Hammer Test and the other half were used as controls. The control luminaires were placed in the ceiling of an office building and operated continuously for 296 days. After 296 days (7100 hours) of control testing, in general, the luminous flux for the control population remained stable in the first control test cycle. The average luminous flux measured for the samples of Luminaires D and E changed by less than 1% of the original reading. The luminous flux of the three luminaires in the control group decreasing by an average of 10.2%.
All luminaires examined in the study survived more than 100 cycles of temperature shock (–50°C to 125°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 hours of cumulative exposure to the Hammer Test with only four failures. Two of these LED failures were attributed to solder joint fatigue and the other two failures were due to board-level corrosion.
The DOE concluded that SSL luminaires are robust and can withstand the extreme stresses of the Hammer Test. Furthermore, the level of performance demonstrated by the luminaires examined in this Hammer Test protocol suggests that SSL luminaires will have a low probability of random failure in the field during normal use, and that properly designed and installed SSL luminaires are likely to have long lifetimes under normal operating conditions. However the DOE says additional work is needed to determine actual wear-out mechanisms, quantify failure modes, and determine acceleration factors for SSL luminaires. This information is necessary in order to provide estimates of lifetime and reliability.