Andreas Zibold received the B.Eng degree in electrical engineering in 2012 from the Baden-Wuerttemberg Cooperative State University, Karlsruhe, Germany. He received the M.Sc. degree in Electrical Engineering and Information Technology in 2014 at the University of Stuttgart, Germany. He is currently working towards the Ph.D degree at the Optoelectronis Department of Fraunhofer IAF, Freiburg, Germany. His research interests are design of GaN-based LED drivers and highly efficient LED modules.
Accelerated LED-Degradation under Exposure to Air Pollutants
LED manufacturers estimate the useful lifetimes of the High-Power white LED between 35000 h and 50000 h. However, LED lifetimes are highly dependent on the junction temperature, applied current and environmental conditions like humidity, corrosive gas and air pollutant exposure. This work studies the effects of common air pollutants in different concentrations on the lifetime of different commercial LED types and analyses the observed degradation mechanisms on the AlGaInN LED chips, the metallization and the package components. A special focus is put on the effects of hydrogen sulfide.
Measurement setup: The reactor chambers are glass vials sealed to the printed circuit board over each individual LED. The PCB with the vials is put into an aluminum frame with PFTE coated walls to homogenize and guide the LED light to a photodiode mounted opposite to each individual LED. To produce the desired pollutant atmosphere, a solution is prepared, which is introduced to the vials through holes in the PCB. The vials are filled to 50 % of their volume. The holes in the PCB are sealed afterwards to prevent air leakage.
Experimental results and degradation analysis: Four different LED types are investigated: Chip-scale-packaged, glass encapsulated and standard Mid- and High-Power LEDs with silicone encapsulation. The LEDs show visible degradation effects like tarnishing of the silver reflector, delamination of the converter from the chip and cracks in the encapsulation material. In the case of hydrogen sulfide, the decay in light output after 380 hours of testing ranges from 27.4 % (glass encapsulation) to 88 % (Mid-Power package) at a concentration of 5 % thioacetamide (acts as hydrogen sulfide source) in the vials. The results show that the gas permeability of the encapsulation has to be improved to prevent tarnishing of the silver reflector. Phenyl- and methyl-based silicones are widely used as encapsulation materials for LEDs. Phenyl-based silicones offer a ten times lower gas permeability and at the same time a higher optical index than methyl-based silicones, which facilitates outcoupling of the generated light. Methyl-based silicones are general purpose encapsulation materials and are used because of their lower price compared to Phenyl-based Silicones. The glass encapsulated LED did not show a higher level of resilience against hydrogen sulfide because the glass encapsulation does not seal the chip completely. Thus the air pollutants can attack through unsealed areas.
Outlook & Conclusion: A measurement setup for determining the influence of pollutants to LEDs has been presented. The setup includes the possibility for in-situ light output measurement. Different degradation effects on LEDs under hydrogen sulfide and other common pollutants have been described and analyzed.
Degradation due to pollutants is highly dependent on the packaging. Especially gas permeability and inertness are important factors for encapsulation and packaging materials. The gas permeability of glasses and ceramics is several orders of magnitude lower than for silicones and epoxy. At the same time, glasses and ceramics are inert to many chemical substances. Consequently, we propose hermetically sealed LED-packages as a possible solution for applications in harsh environments.