Ming-Hung Chen

Failure Mechanisms Associated With Lens Shape of High-Power LED Modules in Aging Test

High-power light-emitting diode (LED) modules encapsulated with different lens shapes after a thermal-aging test were studied experimentally and numerically. Samples from different manufacturers were aged at 80degC, 100degC, and 120degC under a constant driving voltage of 3.2 V. The results showed that the LED modules encapsulated with a hemispherical-shaped plastic lens exhibited a better lifetime due to better thermal dissipation than those with cylindrical-or elliptical-shaped plastic lenses. Results also showed that the optical power of the LED modules increased after removing the plastic lens because degradation of the lens material decreased the amount of light. The key module package-related failure modes under thermal-aging were identified as the degradation of the plastic lens and lens material. A finite-element method (FEM) simulation showed that thermal and major principle stress distributions of the high-power LED modules were dependent on aging temperature. Both experimental and FEM simulated results clearly indicated that a uniformly thermal dissipation to minimize the thermal effect along the thermal path from the LED chip to the plastic lens is essential to extend the operating life of high-power LED modules.

High Thermal Stability of Phosphor-Converted White Light-Emitting Diodes Employing Ce:YAG-Doped Glass

High thermal stability of phosphor-converted white light-emitting diodes (PC-WLEDs) using Ce:YAG-doped glass (CeYDG), instead of conventional Ce:YAG-doped silicone (CeYDS), as a PC layer is proposed and fabricated. The proposed CeYDG possesses host stability as glass and retains desired fluorescence as Ce:YAG. The CeYDG employed in the PC-WLED test demonstrates better performances than conventional CeYDS, including lumen loss, chromaticity shift, transmittance loss, and peak emission intensity undergoing three industry-standard reliability tests at either high (8 wt%) or low (2 wt%) doping concentrations of Ce:YAG. While the CeYDG reveals better stability than the CeYDS, we expect even better performance after refining the glass composition and fabrication process for CeYDG due to current processing facility limitations. However, this study clearly demonstrates the feasibility and advantages of adapting glass as a PC layer in PC-WLED modules that can potentially provide higher reliability and better performance for high-end LEDs, particularly in the area where strict reliability is highly required and in the environment where silicone fails to stand for long.

Investigation of Ce:YAG Doping Effect on Thermal Aging for High-Power Phosphor-Converted White-Light-Emitting Diodes

In this paper, high-power phosphor-converted white-light-emitting diodes (PC-LEDs) with selected concentration and thickness of cerium-doped yttrium aluminum garnet (Ce:YAG) phosphor-doped silicones are investigated to study the thermal-degradation effect of the Ce:YAG phosphor-silicone layer. The experimental results showed that the lumen loss, chromaticity (CIE shift), and spectrum intensity reduction increase as the concentration of Ce:YAG phosphor-doped silicone increases. Although silicone degradation attributed to the final thermal degradation, it is not a dominant factor until a much thicker silicone is employed in PC-LEDs. The major degradation mechanism of the PC-LEDs results from the higher doping concentration of Ce:YAG in silicone. We found that 94% lumen loss was attributed to 5.5 wt% Ce:YAG doping and only 6% of the lumen loss was due to a 1-mm thickness of silicone degradation. However, the negligible differences of measured fluorescent lifetimes among the test samples before and after thermal aging (at 150 degC for 500 h) eliminated any significant nonradioactive quenching processes that existed in the aged samples. The emission spectra indicate that a higher doping concentration in silicone causes a higher degree of loss at the emission wavelength of Ce:YAG. Therefore, minimizing any unwanted interactions, such as refractive index and thermal-expansion mismatches, between the phosphor and the silicone during thermal aging is a new direction of addressing thermal reliability for high-power PC-LEDs. From practical points of view, we found that a lower doping concentration of the Ce:YAG phosphor in thin silicone is a better choice in terms of having less thermal degradation for use in packaging of the high-power PC-LEDs modules and is essential to extend the operating lifetime of the phosphor-based white LED modules.

Decay Mechanisms of Radiation Pattern and Optical Spectrum of High-Power LED Modules in Aging Test

Decay of radiation pattern and optical spectrum of high-power LED modules fabricated by different manufacturers after a thermal-aging test were investigated experimentally and numerically. Samples were aged at 65degC, 85degC, and 95degC under a constant current of 350 mA. The results showed that the radiation pattern of the LED modules at the two view angles of plusmn(45deg ~ 75deg) decreased more than the other angles as aging time increased. This was due to the reduction of optical power from corner shape of lens. Due to the degradation of lens material after thermal aging, the center wavelength of the LED spectrum shifted 5 nm. Furthermore, the radius curvature of plastic lens was observed to have 6-70 mum contraction as aging times increased. The key module package related to the decrease of power density, the change of radiation pattern, and the shift of optical spectrum in high-power LED modules under thermal aging were due to the degradation of lens material and lens structure. Both experimental and simulated results clearly indicated that improving the lens structure and lens material is essential to extend the operating life of the high-power LED modules. This study may provide practical LED package guidelines in low-cost consumer applications.

High thermal stability of high-power phosphor based white-light-emitting diodes employing Ce:YAG-doped glass

High thermal stability of high-power phosphor-converted white-light-emitting diodes (PC-WLEDs) incorporating a Ce:YAG-doped glass as the phosphor layer is demonstrated. An exploring study of lumen loss, chromaticity shift, and transmittance loss of high-power PC-WLEDs with Ce:YAG-doped glass and Ce:YAG-doped silicone under thermal aging at 150°C for 500 hours was performed and compared. The results showed that the high-power PC-WLEDs with 6 wt% of Ce:YAG-doped glass exhibited 60% less lumen loss, 50% lower chromaticity (CIE) shift, and 20 % smaller transmittance loss than with the Ce:YAG-doped silicone. This clearly indicates that the Ce:YAG-doped glass exhibits higher thermal stability than the Ce:YAG-doped silicone after thermal aging. A better thermal stability of glass phosphor layer may be beneficial to the many applications where the LEDs with high-power and high reliability are demanded.

1.55-µm Fiber Grating Laser Utilizing an Uncoated Tapered Hemispherical-End Fiber Microlens

An uncoated tapered hemispherical-end fiber microlens, formed by etching a single-mode fiber in a hydroflouride solution, is spliced with a fiber grating to form an external cavity for a 1.55 µm fiber grating laser (FGL). The fiber microlens provides a coupling efficiency of more than 65% between the semiconductor laser and the fiber grating external cavity. The FGL has spectral characteristics of a single line (λ0=1555.6 nm), a single longitudinal mode (SMSR>30 dB), and a high output power (>1 mW). However, the SMSR shows a current-dependent oscillation, which can be consistently explained by calculating the rate equations of an equivalent external cavity model on the FGL.

1.55-μm non-anti-reflection-coated fiber grating laser for single-longitudinal mode operation

A 1.55-μm fiber grating laser (FGL) was fabricated by optically packaging a non-anti-reflection (AR) coated Fabry–Perot (FP) laser to a lensed fiber grating. The FGL demonstrates a single-longitudinal mode operation with a side-mode suppression ratio (SMSR) of up to 40 dB. The SMSR oscillates and diminishes to <30 dB as the increase of injection current above 38 mA, and the tilt of the fiber approaches ∼5° away from the facet normal of the FP laser. We have performed numerical simulations on the single-longitudinal mode operation for the FGL. The SMSR for the FGL increases over 40 dB as the increase of the fiber grating reflectivity (Rg) above 0.7 with non-AR-coated FP laser facet. Our calculations also show that the strong current-dependent SMSR oscillation is from the mode selection by the fiber grating external cavity and the heating effect in the FP laser.

High humidity resistance of high-power white-light-emitting diode modules employing Ce:YAG doped glass

The reliability study of thermal shock and damp heat tests for high-power white-light-emitting diode modules (WLEDMs) incorporating Ce:YAG doped glass, instead of conventional Ce:YAG doped silicone, as a phosphor layer is presented. The Ce:YAG doped glass as a phosphor layer is used for the glass to possess high transition temperature (Tg) of 750°C that can exhibit higher thermal stability and humidity resistance than conventional silicone. A comparison study of lumen loss, chromaticity shift, and transmittance loss of high-power WLEDWs with Ce:YAG doped glass and Ce:YAG doped silicone under thermal shock and damp heat tests was performed. The damp heat results showed that the high-power WLEDWs with Ce:YAG doped glass at 2∼8 wt% doping concentrations exhibited 67∼69%, 49∼65%, and 35∼67% better improvements than doped silicone for the lumen loss, chromaticity shift, and transmittance loss, respectively. The damp heat test demonstrated the larger improvement of glass doped in transmittance loss. The thermal shock results showed that the high-power WLEDWs with Ce:YAG doped glass at 2∼8 wt% doping concentrations exhibited 57∼68%, 53∼58%, and 58∼67% better improvements than doped silicone for the lumen loss, chromaticity shift, and transmittance loss, respectively. These results demonstrated that the Ce:YAG doped glass exhibited higher humidity resistance than the Ce:YAG doped silicone under damp heat and thermal shock tests. A better thermal stability and humidity resistance of doped glass phosphor layer may be beneficial to the many applications where the LED modules with high-power and high reliability are demanded.

Decay of Lumen and Chromaticity of High-Power Phosphor-Converted White-Light-Emitting Diodes in Thermal Aging

In this paper, we would like to report the following two subjects:(1) Thermal decay mechanisms of phosphor-doped silicone in high power phosphor-converted white light emitting diode (PC-WLED) module and (2) Thermal aging variations of light profile and output power of blue LED modules having a polycarbonate lens and silicone as an encapsulant. Although silicone degradation attributed to the final thermal degradation, it is not a dominant factor until a much thicker silicone is employed in PC-LEDs. The major degradation mechanism of the PC-LEDs results from the higher doping concentration of Ce:YAG in silicone. However, the negligible difference of fluorescent lifetimes among the test samples before and after thermal aging (at 150°C for 500hrs) eliminated any significant quenching processes that existed in our aged samples. The emission spectra suggest that a higher doping concentration in silicone causes a higher degree of loss at the emission wavelength of Ce:YAG, namely 570nm. Therefore, minimizing any mismatch of the refractive index, thermal expansion , and chemistry between the phosphor and the silicone is a new sign of improving thermal reliability for high power PC-LEDs. Thermal aging variations of light profile and output power of LED modules fabricated by three manufacturers (namely, Type I, II, and III) were investigated experimentally and numerically. Both experimental results and simulation results suggested that improving the lens/ encapsulant materials and packaging designs are essential to not only greatly extend the product lifetime but also enhance the light quality of LED modules as illumination sources. Index Terms -High-powered phosphor-converted white-light-emitting diodes (PC-WLEDs), lumen loss, chromaticity shift, silicone, polycarbonate, thermal aging, etc.

Decay of radiation pattern and spectrum of high-power LED modules in aging test

High-power light-emitting diodes (LEDs) modules encapsulated with different lens shapes after a thermal-aging test were studied experimentally and numerically. Samples from different manufacturers were aged at 65, 85, and 95degC under a constant driving current of 350 mA. The results showed that the optical power of the LED modules at the two view angles of plusmn (45deg~75deg) decreased more than the other view angles as the aging time increased. This was due to the reduction of radiation pattern from the corner effect of lens shape, resulted in lower output power. The simulation of the corner effect of lens shape is in good agreement with the experiment result. Results also showed that the center wavelength of the LED spectrum shift 5 nm after thermal aging 600 hours at 95degC because of degradation the lens material. The key module package related failure modes under thermal-aging may be due to the corner effect of lens shape and the degradation of the lens material. Therefore, improving the lens structure and lens material is essential to extend the operating life of the phosphor-based white LEDs modules.