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패키지 고장 메커니즘 기반의 고출력 백색 LED 가속수명시험

Accelerated Life Tests of High Power White LEDs based on Package Failure Mechanisms

초록/요약

In recent years, the light emitting diode (LED) technology has experienced an impressive progress. High power phosphor-converted white LED (high power LED) has attracted considerable interest in recent years owing to its diverse range of applications, such as outdoor lights and automobile headlamps. For the various application, the reliability of LED package should be guaranteed. LED lifetime is conventionally measured using the Alliance for solid-state illumination systems and technologies(ASSIST), which requires at least 6,000 hours. It is strongly needed to develop a methodology of accelerated life tests(ALT) in order to decrease the research and development period. From reviews of ALT for high power LED, it is well-known that long-term degradations and failures of commercial high power LED package are primarily associated with the package itself rather than the blue LED chip. In the conventional aging test, LED package are exposed to several temperatures and electrical current, to identify both their degradation mechanism and the defects of LED chip and package materials. Recently, humidity is also considered as an effective stress factor for accelerating the failure of LED packages. The purpose of this paper is to suggest of an ALT based on the failure mechanisms of the LED package. In this paper, I have summarized all the experiment results. (1) Accelerated stress test of high power LED package It was found that the saturated water vapor pressure is the most dominant stress factor for the degradation phenomenon in the package for high-power LED. Also, it was proved that the saturated water vapor pressure is an effective acceleration stress of LED package degradation from an acceleration life test. Test conditions were 121 ℃, 100% RH(Unbiased autoclave), and maximum 168 h storage with and without 350 mA. The accelerating tests in both conditions cause optical power loss, reduction of spectrum intensity, increment of device leakage current and thermal resistance of the package. Also, the dark brown color and pore induced by hygro-mechanical stress partially contribute to the degradation of LED package. From these results, it was shown that the saturated water vapor pressure stress is adequate as an acceleration stress for shortening life test time of LED packages. (2) Accelerated life test of high power LED package ALT of high power LEDs were studied. Samples of LED packages were aged at 110℃/85% RH and 130℃/85% RH up to 900 hours under non-biased condition. The test conditions induced a luminous flux decay on LEDs in all the conditions. Aged devices exhibited modification of package silicon color from white to yellowish brown. The instability of the package contributes to the overall degradation of optical lens and structural degradations such as generating bubbles. The degradation mechanisms of lumen decay and reduction of spectrum intensity were ascribed to hygro-mechanical stress which results in package instabilities. Main failure mechanisms have been identified from the different stress conditions (temperature/humidity accelerated tests). MTTF of 12,615 h have been evaluated for real working conditions by applying Arrhenius model and activation energy was found to be 0.33 eV. The extrapolated activation energy was similar to the results of previous studies. In order to compare the results of accelerated life test to normal operation condition, a typical current aging was performed. The light output decrease over time is exponential in nature and MTTFADTof 10,173 h have been estimated for real working condition. The difference between their results is small. As a result of these experiments, it was proved that the high temperature and humidity stress are adequate for ALT of high power LED package to reduce test time.

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목차

제1장 서 론 1
제1절 연구배경 및 목적 1
제2절 연구범위와 절차 4
제3절 연구의 구성 6
제2장 이론적 배경 7
제1절 LED 개요 7
2.1.1. 백색 LED 동작원리와 구성 7
2.1.2 고출력 LED 수명과 시험기준 13
제2절 고출력 LED의 가속시험 15
2.2.1. 가속시험방법 (Accelerated Testing Method) 16
2.2.2. 가속수명시험 (Accelerated Life Test) 20
제3절 고출력 LED의 가속수명시험 고찰 31
2.3.1 전류와 온도를 이용한 가속수명시험 31
2.3.2 습도를 고려한 온도와전류 가속시험 42
2.3.3 고출력 LED 고장모드와 메커니즘 48
제4절 선행연구의 한계와 문제점 50
제3장 고출력 LED 패키지 가속 스트레스 시험 51
제1절 패키지 가속 스트레스 시험설계 51
제2절 패키지 가속 스트레스 시험 52
3.2.1 시험시료 특성과 구조 52
3.2.2 가속 스트레스 시험방법 54
제3절 패키지 가속 스트레스 시험결과 및 고찰 57
3.3.1 광속 열화 57
3.3.2 스펙트럼 강도 열화 59
3.3.3 I-V 특성 열화 61
3.3.4 패키지 재료의 열화 64
3.3.5 열 저항 열화해석 67
3.3.6 검토 및 논의 70
제4장 고출력 LED 패키지의 가속수명시험 72
제1절 패키지고장 가속수명시험 설계 72
4.1.1 패키지고장과 메커니즘 72
4.1.2 시험방법 도출을 위한 품질기능전개 76
제2절 패키지고장 가속수명시험 79
4.2.1 시험시료 특성과 구조 79
4.2.2 패키지고장 가속수명시험 절차와 기준 80
제3절 시험결과 분석 83
4.3.1 광속 열화 83
4.3.2 스펙트럼 강도 열화 85
4.3.3 I-V 특성변화 88
4.3.4 패키지 재료의 열화 89
제4절 고출력 LED 수명예측 93
4.4.1 LED 열화 데이터 분석 93
4.4.2 가속수명시험 분석 98
제5절 시험결과 고찰 106
제5장 결 론 107
참 고 문 헌 111
ABSTRACT 122

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