Investigation on Some Parameters Affecting Optical Degradation of LED Packages during High-Temperature Aging

The mechanism of optical degradation for the light emitting diode (LED) packages in the light bars under the high-temperature aging condition, one of the accelerated life tests, is studied experimentally and numerically. Some parameters, including junction temperature, encapsulant materials, delamination, leadframe surface degradation, and encapsulation, are closely investigated. It is found that the encapsulants (with or without phosphors) and interface delamination have no effect on the optical degradation during the high-temperature aging. On the contrary, junction temperature, leadframe surface degradation, and encapsulation play important roles on this degradation. Optical degradation of the LED packages is found to be mainly caused by silver chloride, which is generated on the leadframe surfaces with an exposure of silver finish in light and heating conditions after interface delamination happening during the high-temperature aging. The high junction temperature could speed up the generation of silver chloride. Moreover, the blue LED package with an encapsulant (silicone) surprisingly give a lower lumen maintenance than one without encapsulant during the high-temperature aging, because of more light absorption on the darkening leadframe surface by total light reflection from silicone encapsulant. Some important parameters affecting the thermal reliability of the LED packages have been identified and the corresponding optical degradation mechanism has been clarified.