A degradation mechanism analysis methodology is proposed for the mid-power white-light LED packages in this paper. Based on the degradation data obtained from a series of aging tests that are performed on the individual packaging material, the degradation kinetics of lumen output and spectral power distribution of the LED packages are investigated using optical simulation. As a result, although the reflectivity of the packaging materials decreased severely for the blue lights (i.e., 450 nm), the simulation showed that lights at this wavelength were little absorbed in the LED package. More specifically, it is found that: 1) the degradation of the blue lights is mainly due to blue chip deterioration, while rarely affected by the degradation of the silicone encapsulant and other packaging materials; 2) the degradation of the down-converted lights is significantly attributed to the degradation of the blue chips, the phosphors, the lead frames, and the package housing; and 3) the degradation of the silicone encapsulant contributes about 1.35% to the total lumen degradation within 168 h, while has no more contribution to the lumen degradation with further aging duration. The simulation results have been validated by experiments and successfully applied to the degradation mechanism analysis of LED packages in LM-80-08 tests.

In this paper, lumen degradation is described by using a modified Brownian motion process for mid-power white-light LED packages, which were aged under step stress accelerated degradation test (SSADT). First, a SSADT model has been established based on the theory of equivalent accumulative damage. Then, a method was proposed to improve the accuracy of the parameter estimation by carefully modifying the estimator, which was proposed in the previous research. Experimental data show that parameters estimated by using SSADT model are very close to those estimated by using constant stress accelerated degradation test (CSADT) model, indicating the feasibility of the SSADT model. The experiment also indicates that SSADT can be used as an alternative to CSADT, as it enables comparable estimation accuracy, while using less testing time, a smaller sample size and less test capacity.