Rapid Light Output Degradation of GaN-Based Packaged LED in the Early Stage of Humidity Test

Serious silicone carbonization has been reported in mid-power white light LEDs during highly-accelerated temperature and humidity stress test (HAST), even though the junction temperature of the LED chip is much lower than the critical temperature of silicone carbonization (300 °C). This paper presents a comprehensive investigation, through which, the effects of Joule heating and phosphor self-heating has been eliminated as main failure mechanisms. As a result, the over-absorption of blue lights by silicone bulk, is considered as the root cause for silicone carbonization. This is mainly due to the scattering effect of water particles inside the silicone materials, which can increase the silicone temperature significantly. Furthermore, finite element thermal simulation has also been performed, confirming the validity of failure mechanism.

Section: Blue Light Over-absorption By Siliconementioning

confidence: 99%

mentioning

confidence: 99%

Rapid Degradation of Mid-Power White-Light LEDs in Saturated Moisture Conditions

Huang

Golubović²,

Koh

et al. 2015

“…Consequently, the anodic dissolution process of Ag slows down or even stops due to insufficient thickness of the moisture layer, and instead the water vapor reaches the dice surface and degrades the phosphor, fills the gap at the interface and diffuse Upon further moisture inception under 95%RH, the delamination becomes more severe as more moisture enters into the package but it cannot diffuse into the silicone gel due to moisture saturation in the silicone [3], and thus it can only fill the gap between the encapsulant and the dice surface and results in more severe delamination of the encapsulant polymer material from the dice surface. The presence of the water layer between the dice and the encapsulant caused more scattering of the light from the dice, and may also cause internal reflection of the light from the dice due to the change in the refractive index of the water layer and encapsulant, both results in further reduction in the lumen output as observed from 10%-15% degradation under 95% RH as shown in Table I.…”

Section: B Identification Of the Degradation Physicsmentioning

confidence: 99%

“…However, no standard is currently available, and either 85%RH at 85 • C or 100% RH at 121 • C is used to evaluate the humidity reliability of LEDs in industry today [1]. For the study of the degradation mechanisms of LEDs due to humidity, Tan et al [3] found that the presence of humidity can result in a rapid decrease in the lumen at the initial short duration due to absorbed moisture into lens that cause light scattering. Prolonged exposure to humidity can result in the degradation of LEDs, and the degradation mechanisms are not limited to corrosion only as in the case of integrated circuit [4].…”

Section: Introductionmentioning

confidence: 99%

Time Evolution Degradation Physics in High Power White LEDs Under High Temperature-Humidity Conditions

Tan

Singh

2014

A high temperature-humidity test is commonly employed to evaluate the humidity reliability of electronic devices. For an integrated circuit, the degradation mechanism under the high temperature-humidity test is metal corrosion, and Peck's model is used for extrapolating the test results at accelerated test conditions to the normal operating condition. Such extrapolation is possible as the underlying degradation physics is invariant from the accelerated test conditions to the normal operating condition for integrated circuits. However, this is not true for high power LEDs, as found in this paper. The degradation in the LEDs undergoes time evolution at either 95% or 85% relative humidity (RH) and 85 • C. We also found that the degradation physics are completely different among the various RH levels from 95% to 70%. The degradation process begins from bond pad contamination and Kirkendall void formation, galvanic dissolution, phosphor dissolution to encapsulant, and die attach delamination. Such time evolution degradation physics renders the inapplicability of the Peck model and presents a challenge in extrapolation of test results to the normal operating condition for lifetime prediction.

“…In order to understand the effects of moistures on the LED products, a few researchers conducted a series of experiments, such as wet-high temperature operation life test (WHTOL) [11], [12], highly-accelerated temperature and humidity stress test (HAST) [13], [14]. Results showed that the optical degradation of LED packages was mainly due to delamination of packages [15], lights scattering of water particles inside the silicone volume [16], bubble generation in the encapsulant [13], and dissolution of phosphors [17]. However, though many researchers concentrated their interests on the reliability of high-power LEDs, there is limited report on the optical degradation mechanisms of MP LED packages aged by moisture.…”

Section: Introductionmentioning

confidence: 99%

Degradation Mechanisms of Mid-Power White-Light LEDs Under High-Temperature–Humidity Conditions

Huang

Golubović²,

Koh

et al. 2015

More and more mid-power white-light LED (MP LED) solutions have been used in outdoor illumination due to their good performance, cost attractiveness, and low energy consumption as compared with conventional lighting solutions. Hence, there is a need for MP LED manufacturers to develop more robust MP LEDs aimed at outdoor applications but still offer a significant cost benefit as compared with currently widely used high-power LEDs. This implies that MP LEDs would be operated in an environment with high humidity and high temperature. This may lead to serious degradation with different failure modes compared with an indoor operation. However, the combined effect of temperature and humidity on the MP LED reliability has not been extensively studied in literature. In this paper, MP LEDs were studied by the wet high-temperature operation life (WHTOL) test in order to understand their degradation mechanisms due to the combined effect of temperature and humidity. It is found that encapsulant discoloration (yellowing) is the major degradation mechanism in the WHTOL test, which will induce serious lumen degradation and color shift. Furthermore, it has been found that electrical degradation in terms of forward voltage increase will also affect the lumen maintenance. Finally, statistical analysis shows that lumen degradation mechanisms in the WHTOL test are similar to a failure in the LM-80-08 test, demonstrating that the WHTOL test is an efficient accelerated degradation test method, which dramatically reduces the test duration compared with the LM-80-08 test.