Understanding the transport phenomena leading to tarnishing of the reflecting silver layer causing reduced light output of LEDs

Tarnishing of the reflective silver layer in LED packages is an important failure mechanism, leading to both a decrease in luminous flux by up to 75% and a change in color spectrum. The key research question that needs to be answered to improve package integrity, is how corrosive species, such as H
₂
S, SO
₂
, and Clcan reach the silver surface. A clear dependence on the presence of water in the system can be expected, as water will enhance both the transport and the corrosion process. Free films of commercially available optical grade silicone were prepared to gain insight on the transport properties of water through the bulk. Additionally, films containing phosphor particles were prepared to study the influence of their interfaces to the silicone matrix on transport. For the analysis, two techniques - wet cup and dynamic vapor sorption (DVS) - were employed. Furthermore, samples were prepared by sputtering silver on glass plates, followed by applying the silicone resin, containing gold wires and phosphor particles. These samples were exposed to H
₂
S, while the state of the silver was constantly monitored by a camera, giving us the opportunity to observe the transport of species through bulk and/or interfaces. In this work, we show results of the water permeation and content in optical grade silicone. We compare the results on diffusivity obtained by wet cup and DVS measurements. Furthermore we introduce a setup for studying the predominating transport path of corrosive species in LEDs.