Degradation Physics of High Power LEDs in Outdoor Environment and the Role of Phosphor in the degradation process

Preetpal Singh; Cher Ming Tan

doi:10.1038/srep24052

Degradation Physics of High Power LEDs in Outdoor Environment and the Role of Phosphor in the degradation process

Sci Rep. 2016 Apr 7:6:24052. doi: 10.1038/srep24052.

Authors

Preetpal Singh^{1

2}, Cher Ming Tan^{1

2

3

4

5}

Affiliations

¹ Department of Electronics Engineering, Chang Gung University, Wenhua 1st Rd., Guishan Dist., Taoyuan City 33302, Taiwan.
² Centre for Reliability Science and Technology, Chang Gung University, Wenhua 1st Road, Guishan Dist., Taoyuan City 33302, Taiwan.
³ Department of Mechanical Engineering, Ming Chi University of Technology, 84 Gungjuan Rd., Taishan Dist., New Taipei City 24301, Taiwan.
⁴ Department of Urology, Chang Gung Memorial Hospital, Taiwan.
⁵ Institute of Radiation Research, College of Medicine, Chang Gung University, Taiwan.

Abstract

A moisture- electrical - temperature (MET) test is proposed to evaluate the outdoor reliability of high power blue LEDs, with and without phosphor, and to understand the degradation physics of LEDs under the environment of combined humidity, temperature and electrical stresses. The blue LEDs with phosphor will be the high power white LEDs. Scanning acoustic microscopy is used to examine the resulted delamination during this test for the LEDs. The degradation mechanisms of blue LEDs (LEDs without phosphor) and white LEDs (LEDs with phosphor) are found to be different, under both the power on (i.e. with 350 mA through each LED) and power off (i.e. without current supply) conditions. Difference in the coefficient of thermal expansion between the molding part and the lens material as well as the heat generated by the phosphor layer are found to account for the major differences in the degradation mechanisms observed. The findings indicate that the proposed MET test is necessary for the LED industry in evaluating the reliability of LEDs under practical outdoor usage environment.