Discussion on reliability issues for UVB and UVC LED-based devices

The usage of UV LEDs is getting attractive for application such as phototherapy, plant growth, and disinfection due to the wavelength selective, narrow-band emission and a high potential for miniaturization of LED devices. Besides these benefits, the demands on optical power and long-term stability for these applications can often be well satisfied. For UVB LEDs most promising applications are in the field of medical skin therapy and novel concepts of horticulture and plant growth (irradiation of plants for the generation of phytamines or to reduce hormone-like mixtures). UVC applications focus on disinfection of air, surfaces and water at 265 nm or 280 nm. Each application field requires an individual UV dose, which is connected with the optical power output of the LED, and thus the number of LEDs and their long term stability. Typical doses for skin irradiation is 20 mJ/cm² at 310 nm and for water disinfection 20-60 mJ/cm² at 280 nm depending which target reduction factor log reduction of germicides is required. In this work a discussion on different factors influencing the reliability of LED modules, summarizing several years of research in this field will be given. Degradation effects are shown depending on LED design itself as well as the device assembly architecture including different mounting techniques. The most promising assembly technique was tested by a sample series of twice 400 single LED packages with a total yield of 87.7 % after mounting of UVB LEDs in single LED cases, cascading to an array on a main board by secondary soldering and burn-in of 48 hour at 50mA. In total 4% of the yield loss results by soldering issues of the LED on submount as well as another 8 % yield loss was measured after cascading of single LED packages on main board. Due to the burn-in process additional twenty UVB LEDs were lost. These reliability issues will be discussed using selected "state-of-the-art" LED device structures and examples of testing these LED devices in UV lighting lamp systems built at OSA opto Light will be given.