Lumen recovery in high-power LEDs under prolonged outdoor operation

After unveiling the underlying physics of the initial rapid lumen degradation in packaged high-power LEDs under MET test, this chapter discusses the physics of an unexpected lumen recovery in the second stage. As this recovery occurs in between the initial and final stages, physical verification is difficult and extensive use of finite element modeling via ANSYS software is employed with subsequent mechanism verification using confocal scanning acoustic microscopy. Higher moisture absorption by die attachment results in reducing moisture content in the silicone encapsulant and thus lumen degradation recovery in the LEDs. This moisture absorption in die attachment will lead to degradation of thermal resistance in LED packages, and hence this recovery of lumen degradation is actually the beginning of a long-term degradation stage.