# Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
## 3 Results and Discussions
### Packaging Analyses  
Figure 8a depicts photographs of four representative samples corresponding to before and after aging for 3840 h in three cases, with sample number of analyzed samples as #6 (CSA-1), #16 (CSA-2), and #23 (CSA-3), respectively. Ratios of the optical power after 3840 h aging to the optical power before aging for #6, #16, and #23 are computed as 90.8%, 90.2%, and 80.7%, corresponding to 9.2%, 9.8%, and 19.3% optical decay, respectively. From the appearance in Figure 8a, the sample in CSA-2 exhibits remarkable yellow-browning in the optical lens, whereas that in CSA-3 demonstrates distinct darkening on the surface of the silver-coated reflective layer. Considering the optical decay and the surficial change, we rationalize that the darkening on the surface of the silver-coated reflective layer induces more serious degradation of the optical power than the yellow-browning in the silicone-based optical lens does. Figure 8: (**a**) Photographs of four representative samples; and (**b**) angular intensity distributions of hppc-WLEDs. Angular intensity distributions look like a pair of bat wings for peanut-shaped LEDs with maximum intensities at \(\pm\)60\({}^{\circ}\) (Figure 8b). The largeness of view angles in these peanut-shaped LEDs clearly differs from that in conventional hemisphere-shaped LEDs [38]. Angles at which the maximum intensity occurs remain unchanged for all cases, whereas intensities in three aged cases display monotonic decreases at all tested angles as the current stress increases. These decreases may be attributed to the systematic degradation inhpp-WLEDs, including the yellow-browning of optical lenses, the darkening of silver-coated reflective layers, and the decomposition of phosphor-silicone mixtures. We employ a white LED (with the current driven at 350 mA and the heat-sink temperature maintained at 25 \({}^{\circ}\)C) to measure and compare transmissivities of silicone epoxy lenses and reflectivities of silver-coated reflective layers in an unaged device and three 3840 h-aged devices in CSA-1, CSA-2, and CSA-3, respectively. As shown in Figure 9a, all transmissivities of aged lenses experience a decrease in comparison with the unaged one at measured wavelengths ranging between 380 nm and 780 nm. Clearly, the transmissivity of the lens in CSA-2 lowers most severely among those of lenses in all aged cases. The reason for this loss lies in that the device in CSA-2 exhibits a prominent yellow-browning silicone epoxy lens. Besides, variations of reflectivities on surfaces of silver-coated reflective layers before and after aging are shown in Figure 9b. The reflectivity in CSA-3 is generally lower than those in CSA-1 and CSA-2 within the visible wavelength range due to the most severe darkening of the silver-coated reflective layer. Therefore, both decreases of the transmissivity of the silicone epoxy lens and the reflectivity of the silver-coated reflective layer are found to be strongly related to the optical power decays after aging. Figure 9: Before and after 3840 h aging in CSA-1, CSA-2, and CSA-3, respectively: (**a**) transmissivities of silicone epoxy lenses; and (**b**) reflectivities of silver-coated reflective layers. To further investigate the degradation of silver-coated reflective layers, we employ a field emission scanning electron microscopy (FM-SEM) on surfaces of silver-coated reflective layers to observe microstructural changes of these surfaces before and after 3840 h aging, as shown in Figure 10. Results indicate that, in comparison with the surface of the un-aged sample (Figure 10a), dark areas are generated on those of aged silver-coated reflective layers (Figure 10b-d). Generally, the formation of dark areas can be primarily attributed to the carbonization effect of plastic encapsulation materials on surfaces of silver-coated reflective layers [39; 40]. This effect reduces the intensity of the reflected light because the impinging light is absorbed by these dark areas.