The US Department of Energy (DOE) has published the findings of 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 report was prepared by RTI International for DOE’s LED Systems Reliability Consortium. In the testing described in the report, 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.
Among the findings:
- All of the luminaires survived more than 100 cycles of temperature shock (-50°C to 125°C).
- 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 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.
The report is available at www.ssl.energy.gov/tech_reports.html.