Led failure mechanisms

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The increasing demand for light emitting diodes (LEDs) has been driven by a number of application categories, including display backlighting, communications, medical services, signage, and general illumination. The construction of LEDs is somewhat similar to microelectronics, but there are functional requirements, materials, and interfaces in LEDs that make their failure modes and mechanisms unique. This web seminar will present a review for industry and academic research on LED failure mechanisms and reliability to help LED developers and end-product manufacturers focus resources in an effective manner. The focus is on the reliability of LEDs at the die and package levels. The driving factors for precipitating these mechanisms will be discussed to help the developers and users of LEDs control the mechanisms and assess reliability. We will concentrate on the phosphor thermal quenching mechanism to illustrate the uniqueness of LEDs compared with other semiconductor devices.

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Led failure mechanisms

  1. 1. Light Emitting Diode Failure Mechanisms Diganta Das, PhD Center for Advanced Life Cycle Engineering (CALCE) University of Maryland, College Park, MD, USA diganta@umd.edu (www.calce.umd.edu) University of Maryland Copyright © 2012 CALCE
  2. 2. University of Maryland: 2012• Started in 1856.• About 48,000 students.• Ranked 10th best value among public institutions in the USA (Kiplinger)• Ranked 9th among engineering programs at public universities in the USA (U.S. News and World Report).• Ranked 8th in engineering in the U.S. (Wall Street Journal). Center for Advanced Life Cycle Engineering 2
  3. 3. CALCE Overview• The Center for Advanced Life Cycle Engineering (CALCE) formally started in 1984, as a NSF Center of Excellence in systems reliability.• One of the world’s most advanced and comprehensive testing and failure analysis laboratories• Funded at $4M by over 150 of the world’s leading companies• Supported by over 100 faculty, visiting scientists and research assistants• Received NSF innovation award in 2009 Center for Advanced Life Cycle Engineering 3 University of Maryland http://www.calce.umd.edu Innovation Award Winner Copyright © 2012 CALCE
  4. 4. Why LEDs for Lighting? • Design flexibility – Zero to three dimensional lighting (dot-scale, line-scale, local dimming lighting and color dimming) – Small exterior outline dimensions (< 20mm × 20mm) • High energy efficiency – Low power consumption (energy savings) – Low voltage operation (< 4V) – Low current operation (< 700mA) • High performance – Ultra-high-speed response time (micro-second-level on-off switching) – Wide range of controllable color temperature (4,500K to 12,000K) – Wide operating temperature rating (-20˚C to 85˚C) – No low-temperature startup problems • Eco-friendly product – No mercury – In 2007, the Energy Independence and Security Act set standards for U.S. that cannot be met by common incandescent bulbs.calce Center for Advanced Life Cycle Engineering 4 University of Maryland TM Copyright © 2011 CALCE
  5. 5. LED Supply Chain LED Module/ System: LED lamp, BLU, Display, etc. Optical System and Power Board RGB-UV White LED LED RGBOY packaging Phosphor packaging LED Chip Chip Fabrication Epiwafer: (In, Al) GaN (B, G, UV), InAlGaP (R,Y), AlGaAs (R,IR) Epitaxy growth Wafer: Sapphire, GaN, SiC, Si, GaAscalce Center for Advanced Life Cycle Engineering 5 University of Maryland TM Copyright © 2011 CALCE
  6. 6. Haitz’s “Law” for Light Emitting Diode Flux • Moore’s Law: the number of transistors in a silicon chip doubles every 18-24 months. • Haitz’s “Law”: LED flux per package has doubled every 18-24 months for the past of more than 30 years. D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, “Illumination with Solid State Lighting Technology”, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 2, pp. 310-320, 2002.calce Center for Advanced Life Cycle Engineering 6 University of Maryland TM Copyright © 2011 CALCE
  7. 7. Construction of Light Emitting Diodes Illustration of GaN LED die Illustration of LED package with PCB and heat sinkcalce Center for Advanced Life Cycle Engineering 7 University of Maryland TM Copyright © 2011 CALCE
  8. 8. LED Development History • Development of new phosphor materials • Development of fabrication technology and equipment • Development of LED package heat dissipationcalce Center for Advanced Life Cycle Engineering 8 University of Maryland TM Copyright © 2011 CALCE
  9. 9. Semiconductor-related Failure Mechanisms in LEDsFailure Mechanism Failure Mode Failure Cause Effect on Device High Ambient Temperature High Current-Induced Lumen Degradation, Thermomechanical Die Cracking Joule Heating No Light Stress Poor Sawing and Grinding Process Lumen Degradation, Defect and Increase in Reverse Dislocation High Current-Induced Thermomechanical Leakage Current, and, Generation and Joule Heating Stress Increase in Parasitic Movement Series Resistance Poor Fabrication Process of Lumen Degradation, p-n Junction Increase in Series Dopant Diffusion High Current-Induced Thermal Stress Resistance and/ or Joule Heating Forward Current High Ambient Temperature No Light, High Drive Current or High Electromigration Electrical Overstress Short Circuit Current Density Center for Advanced Life Cycle Engineering 9 University of Maryland Copyright © 2012 CALCE
  10. 10. Interconnect-related Failure Mechanisms in LEDsFailure Mechanism Failure Mode Failure Cause Effect on Device Thermal Cycling Induced Deformation Wire Ball Bond No Light, Mismatch in Material Thermomechanical Stress Fatigue Open Circuit Properties (e.g., CTEs, Young’s Modulus) Moisture Ingress Hygro-mechanical StressElectrical Overstess- No Light, High Drive Current/ HighInduced Bond Wire Electrical Overstress Open Circuit Peak Transient Current Fracture High Drive Current or Lumen Degradation, Electrical Overstress Electrical Contact High Pulsed/ Transient Increase in Parasitic Metallurgical Current Series Resistance, Interdiffusion or Short Circuit High Temperature Thermal stress Poor Material Properties Thermal Resistance (e.g., poor thermal Electrostatic No Light, Increase conductivity of substrate) Discharge Open Circuit High Voltage (Reverse Electrical Overstress Biased Pulse) Center for Advanced Life Cycle Engineering 10 University of Maryland Copyright © 2012 CALCE
  11. 11. Package-related Failure Mechanisms in LEDsFailure Mechanism Failure Mode Failure Cause Effect on Device Carbonization of High Current-Induced Joule heating or High Lumen Degradation Electrical Overstress Encapsulant Ambient Temperature Mismatch in Material Properties (CTEs and CMEs) Thermomechanical Stress Delamination Lumen Degradation Interface Contamination Moisture Ingress Hygro-mechanical Stress Prolonged Exposure to UV Photodegradation Lumen Degradation, Color Change, High Current-Induced Joule HeatingEncapsulant Yellowing Dislocation of the Presence of Phosphor Thermal Stress Encapsulant High Ambient Temperature Lumen Degradation, Phosphor Thermal Broadening of High Current-Induced Joule Heating or Thermal Stress Quenching Spectrum High Ambient Temperature (Color Change) High Ambient Temperature Thermomechanical Stress Lens Cracking Lumen Degradation Poor Thermal Design Moisture Ingress Hygro-mechanical Stress Mechanical Stress Lumen Degradation, Mismatch in material properties or Cyclic Creep and Stress Solder Joint Fatigue Forward Voltage Thermal Cycling Induced High Temperature Relaxation increase Gradient Fracture of Brittle Intermetallic Compounds Center for Advanced Life Cycle Engineering 11 University of Maryland Copyright © 2012 CALCE
  12. 12. Package-related Failure Mechanisms in LEDs Encapsulant Phosphor LED Die Die attach Bond Wire Housing Lead Frame PCB Solder Joint Heat Slug (Al or Cu) LED package assembled with PCBMore at: Light Emitting Diodes Reliability Review, M.H. Chang, D. Das, P. Varde, M.Pecht, Microelectronics Reliability, Volume 52, Issue 5, Pages 762-782, May 2012. Center for Advanced Life Cycle Engineering 12 University of Maryland Copyright © 2012 CALCE
  13. 13. Phosphor Thermal Quenching: Role of Phosphors in LEDs• Small Phosphors (5 – 20 microns) are dispersed into the encapsulant material such as silicon and epoxy to increase the amount of light output producing white lights.• White LEDs are usually phosphor-converted LEDs (pcLEDs) that utilize short wavelengths emitting from LED dies to excite phosphors (luminescent materials) spread over the inside of the encapsulant.• Phosphors emit light with longer wavelengths and then mix with the remains of the diode light to produce the desired white color.• White color can be tuned by selecting a type of phosphors emitting specific light color. Center for Advanced Life Cycle Engineering 13 University of Maryland Copyright © 2012 CALCE
  14. 14. Phosphor Thermal Quenching: LED Phosphors• LED phosphors are embedded inside the encapsulant that surrounds the LED die.• Phosphors generally consist of a host and an activator (also called a luminescent center).• Rare earth element (REE) phosphors are the most frequently used phosphor activators.• The REE phosphors convert some portion of the short wavelength light from the LED, and the combined LED light with the down- converted light produces the desired white light.• Phosphors doped with REEs such as Ce3+ /Eu2+ emit light with longer wavelengths and the mix with the remainer of the diode light. Center for Advanced Life Cycle Engineering 14 University of Maryland Copyright © 2012 CALCE
  15. 15. Phosphor Thermal Quenching: Impact• Phosphor thermal quenching decreases light output with the increase in nonradiative transition probability due to thermally driven phosphorescence decay.• Phosphor thermal quenching: the efficiency of the phosphor is degraded when the temperature rises. Center for Advanced Life Cycle Engineering 15 University of Maryland Copyright © 2012 CALCE
  16. 16. Phosphor Thermal Quenching: Characteristic Requirements for LED Phosphors• Phosphors used in LEDs must have certain qualities, including the following [18]: − High color rendering index (CRI) − Good color reproducibility − No color shifting − Suitability for high flux devices − Chemical and thermal stability Center for Advanced Life Cycle Engineering 16 University of Maryland Copyright © 2012 CALCE
  17. 17. Phosphor Thermal Quenching: Example of LED Phosphors [18] (Oxy)nitride: Sulfides:Criterion YAG: Ce3+ Ce3+/Eu2+ Ce3+/Eu2+ Low CRI High CRI (full High CRI (full CRI (>4000K) range) range)Excitation Blue Blue/UV Blue/UVThermal Moderate/good Moderate Poor/moderateQuantum >0.9 >0.8 >0.6–0.7efficiencySaturation No No NoStability Good Good Poor Center for Advanced Life Cycle Engineering 17 University of Maryland Copyright © 2012 CALCE
  18. 18. Phosphor Thermal Quenching: Phosphorescence vs. Fluorescence• Phosphorescence has a longer emission pathway (longer excited state lifetime) than fluorescence.• Phosphorescence decay is temperature dependent, while fluorescence decay is independent of temperature. Fluorescence vs. phosphorescence. Center for Advanced Life Cycle Engineering 18 University of Maryland Copyright © 2012 CALCE
  19. 19. Phosphor Thermal Quenching: Failure Causes and Failure Modes• Failure causes: – High drive current and excessive junction temperature, which are attributed to increases in the temperature inside the package.• Failure modes: – Decrease in light output – Color shift – Broadening of full width at half maximum (FWHM) Center for Advanced Life Cycle Engineering 19 University of Maryland Copyright © 2012 CALCE
  20. 20. Phosphor Thermal Quenching: Example of Phosphor Thermal Quenching• Upon heating, the broadening of FWHM is caused by 0.7 Die Phosphor phosphor thermal quenching 0.6 (3) 35.4C 56.3 C (4) ((4)–(6)). Optical power (W/nm) 0.5 80 C 97.8 C• A slight blue shift of the 0.4 (2) 115.2 C (5) 125.7 C emission band is observed for 0.3 phosphors as the temperature 0.2 (1) (6) increases. 0.1 0.0• This short wavelength shift of 350 400 450 500 550 600 650 700 750 800 the phosphor is due to Wavelength (nm) phosphor thermal quenching. Spectra change with temperature rise Center for Advanced Life Cycle Engineering 20 University of Maryland Copyright © 2012 CALCE
  21. 21. Phosphor Thermal Quenching: Modified Arrhenius EquationArrhenius equation fitting thermal quenching data in order tounderstand the temperature dependence of photoluminescence anddetermine the activation energy for thermal quenching: Io I (T )  E 1  c  exp   kT  where: – Io is the initial intensity – I(T) is the intensity at a given temperature T – c is a constant – E is the activation energy for thermal quenching – k is Boltzmann’s constant Center for Advanced Life Cycle Engineering 21 University of Maryland Copyright © 2012 CALCE
  22. 22. Phosphor Thermal Quenching: Solutions• Manufacturer: − Enhance and maintain light extraction efficiency to minimize temperature rise on the inside of LED packages by optimizing  Phosphor material  Phosphor size  Concentration of phosphors  Geometry of phosphor particles− Improvement of thermal design of LED packages• User: − Improvement of thermal design of boards to dissipate the internal heat of LED packages Center for Advanced Life Cycle Engineering 22 University of Maryland Copyright © 2012 CALCE
  23. 23. Encapsulant Yellowing: Introduction• LEDs are encapsulated to prevent mechanical and thermal stress shock and humidity-induced corrosion.• Transparent epoxy resins are generally used as an LED encapsulant.• Epoxy resins have two disadvantages as LED encapsulants: – Cured epoxy resins are usually hard and brittle owing to rigid cross-linked networks. – Epoxy resins degrade under exposure to radiation and high temperatures, resulting in chain scission (which results in radical formation) and discoloration (due to the formation of thermo-oxidative cross-links).• The degradation of epoxy resins under radiation and high temperatures is called encapsulant yellowing. Center for Advanced Life Cycle Engineering 23 University of Maryland Copyright © 2012 CALCE
  24. 24. Encapsulant Yellowing: Types of Encapsulant Materials in LEDs1. Polymer Materials [1][2][3][4] – Epoxy resin – Silicone polymer – Poly methacrylate (PMMA)2. Requirements for LED encapsulant material to enhance light extraction efficiency and reliability [1][5] – Transparency – High refractive index matched with LED die – High temperature resistance – High moisture resistance Center for Advanced Life Cycle Engineering 24 University of Maryland Copyright © 2012 CALCE
  25. 25. Encapsulant Yellowing: Comparison of the Common LED Encapsulants [1][5][6]1) Epoxy resins • Remains transparent and does not show degradation over long time for long- wavelength visible-spectrum and IR LEDs • Epoxy resins lose transparency in LEDs emitting at shorter wavelengths (blue, violet, and UV) • Thermally stable up to temperature of about 120°C • Refractive index is near 1.62) Silicone Polymer • Silicone is thermally stable up to temperature of about 190°C • Silicone is flexible thereby reducing the mechanical stress on the semiconductor chip, but poor adhesion strength and dust abstracting.3) Poly methacrylate (PMMA, acrylic glass) • Relative low refractive index (n=1.49 in the wavelength range 500-650nm) • Limited extraction efficiency when used with high refractive index semiconductors Center for Advanced Life Cycle Engineering 25 University of Maryland Copyright © 2012 CALCE
  26. 26. Encapsulant Yellowing: Failure Causes and Failure Modes• Failure causes: – Prolonged exposure to short wavelength emission (blue/UV radiation), which causes photodegradation (i.e., UV yellowing) – Excessive junction temperature (i.e., thermal yellowing) – Heating of the phosphor particles, increasing the temperature of the encapsulant or the die (i.e., presence of phosphors)• Failure modes: decreased light output due to decreased encapsulant transparency and discoloration of the encapsulant. Center for Advanced Life Cycle Engineering 26 University of Maryland Copyright © 2012 CALCE
  27. 27. Encapsulant Yellowing: Thermal Encapsulant Yellowing Sections of Nichia LED Package Material: Left: unstressed Reference: Middle: 133 hours at 150oC [11] Right: 130 minutes at 200oCCenter for Advanced Life Cycle Engineering 27 University of Maryland Copyright © 2012 CALCE
  28. 28. Encapsulant Yellowing: Photodegradation of Polymer Material• Photodegradation occurs by the activation of the polymer macromolecule provided by absorption of a photon of light by the polymer [7].Degradation of polymer materials takes place under followingconditions [8]:• By increasing molecular mobility of the polymer molecule by raising the temperature to above the glass transition temperature (Tg)• Introduction of chromophores as an additive or an abnormal bond into the molecule which have absorption maxima in a region where the matrix polymer has no absorption band Photodegradation mainly depends on• Amount of radiation• Exposure time Center for Advanced Life Cycle Engineering 28 University of Maryland Copyright © 2012 CALCE
  29. 29. Encapsulant Yellowing: Photodegradation Study (1)• J.L. Down [9] studied the yellowing of epoxy resin by monitoring the absorption at 380 and 600nm on a ultraviolet-visible spectrophotometer.• All absorbance data (A at 380 and 600nm) at timed intervals (t) were subjected to the following calculation and standardization to a film thickness of 0.1mm. At = [A(380nm)t-A(600nm)t]*(0.1mm/F) Where: At =degree of yellowing, A=absorbance, t=time, and F=average film thickness for each sample.• Criteria of the failure: epoxy samples with absorbance (At) greater than 0.25 were unacceptable in color. (Samples with At less than 0.1 were normal. From 0.1 to 0.25 absorbance, uncertainty in color acceptability existed.) Center for Advanced Life Cycle Engineering 29 University of Maryland Copyright © 2012 CALCE
  30. 30. Encapsulant Yellowing: Thermal Encapsulant Yellowing• Though UV exposure plays a role in encapsulant degradation, it has also been shown that degradation can be achieved through purely thermal effects.• N. Narendran et al. [10] reported that the degradation rate of 5mm epoxy-encapsulated YAG:Ce low power type white LEDs was mainly affected by the junction heat rather than the short wavelength radiation.• In the study by Barton et al., [11] the yellowing is related to a combination of ambient temperature and LED self-heating. Their results indicated that junction temperatures of around 150°C were sufficient to change the transparency of the epoxy causing the attenuation of the light output of LEDs. Center for Advanced Life Cycle Engineering 30 University of Maryland Copyright © 2012 CALCE
  31. 31. Encapsulant Yellowing: Effects of Presence of Phosphors in Encapsulant• Narendran et al. [10] reported that 5mm type phosphor-converted white LED degrades faster than the similar type of blue LEDs• If heat and the amount of short radiation were the only reasons for the yellowing of the epoxy, then the blue LED should degrade faster than the white LED because the total amount of short-wavelength radiation would be much higher for the blue LED compared with the white LED at the same drive current.• At any given time only a fraction of the light will travel outward from the phosphor layer.• Since the radiant energy travels through the epoxy region of the white LED more often than in the blue LED, the epoxy would yellow more.• Arik et al. [12] showed that during wavelength conversion, localized heating of the phosphor particles occur. As low as 3mW heat generation on a 20m diameter spherical phosphor particle can lead to temperatures sufficient to contribute to light output degradation. Center for Advanced Life Cycle Engineering 31 University of Maryland Copyright © 2012 CALCE
  32. 32. Encapsulant Yellowing: How Phosphor Scatter Reduces Efficiency?• The encapsulant materials have a maximum refractive index of 1.6 while still maintaining good transparency. The YAG:Ce phosphor has a refractive index of 1.85 in the visible region.• The large difference in refractive indices combined with small particle size and weak absorption results in diffuse scattering of incident and emitted light.• This phosphor scatter reduces efficiency due to – Increased path length for light inside the phosphor, leading to reabsorption losses and decreasing the effective ηq of the phosphor – Randomizing of light directionality passing through the phosphor, leading to longer path lengths and increased contact with high loss areas such as reflectors, phosphor layer, and LED die. Reference: [13] Center for Advanced Life Cycle Engineering 32 University of Maryland Copyright © 2012 CALCE
  33. 33. Encapsulant Yellowing: LED Package Encapsulant Designs (1)• The scattering and trapping of light by phosphor particles increase the probability of light being absorbed by the cup, packaging materials, and LED die.• The efficiency that is reduced by these mechanisms depends on the concentration of phosphor, reflector cup surface roughness, the thickness of phosphor-composite layer, the size of phosphor particles, geometry of the encapsulant, carrier medium, refractive index matched with encapsulant material, and the curvature of encapsulant surface, especially, when the phosphor is not in contact with the die but away from the die. Reference: [13][14] Center for Advanced Life Cycle Engineering 33 University of Maryland Copyright © 2012 CALCE
  34. 34. Encapsulant Yellowing: LED Package Encapsulant Designs (2)Concept of Phosphor locationin high power LEDs Reference: [16] Center for Advanced Life Cycle Engineering 34 University of Maryland Copyright © 2012 CALCE
  35. 35. Encapsulant Yellowing: Summary Challenges Problems Packaging Materials Solutions Light Extraction Refractive index mismatch • High refractive index between LED die and encapsulant encapsulant • Efficient lens/ cup design • High phosphor quantum efficiencyThermal Yellowing Thermal degradation of • Modified epoxy resins or encapsulants induced by silicone based encapsulant high junction temperature • Low thermal resistance between LED die and substrate leadframe UV Yellowing Photodegradation of • UV transparent or silicone encapsulants induced by UV based encapsulant radiation from LED dies and outdooor Center for Advanced Life Cycle Engineering 35 University of Maryland Copyright © 2012 CALCE
  36. 36. LED Reliability Prediction Standard• TM-21-11 is an IESNA standard that recommends a method of projecting the lumen maintenance of LED light sources from the data obtained by LM-80-08 testing. – TM-21-11 method is applied separately for each set of DUT test data collected at each operating (e.g., drive current) and environmental (e.g., case temperature) condition as specified in LM-80-08.• Sample size recommendation: – Recommended number of the sample set is a minimum of 20 units. – For sample size of 10-19 units, the allowed life extrapolation limit is shorter. Center for Advanced Life Cycle Engineering 36 University of Maryland Copyright © 2012 CALCE
  37. 37. Lumen Life Projection Method in TM-21-11 (2)• Curve-fit: perform an exponential least squares curve-fit through the averaged values for the following equation Φ t B exp ‐αt t = operating time in hours; Φ (t) = averaged normalized luminous flux output at time t; B = projected initial constant derived by the least squares curve-fit; α = decay constant derived by the least squares curve-fit. Center for Advanced Life Cycle Engineering 37 University of Maryland Copyright © 2012 CALCE
  38. 38. Lumen Life Projection Method in TM-21-11 (3)• Projection of the lumen maintenance life: ln 0.7 70 α• Data Used for Curve-fit: for data sets of test duration (D) – From 6000 hours up to 10000 hours, the data used for the curve-fits shall be the last 5000 hours of data – For data sets of test duration greater than 10000 hours, the data for the last 50% of the total test duration shall be used for curve-fit. Center for Advanced Life Cycle Engineering 38 University of Maryland Copyright © 2012 CALCE
  39. 39. What is the Problem with this Standard?• The standard completely ignores the failure mechanisms by which LEDs degrade• The standard extrapolates based ONLY on temperature and assumed Arrhenius relationship without any proof of that being an appropriate model• There is exponential progress in the technology of LEDs in performance and the reliability assessment being promoted here is at the level where semiconductor reliability assessment was in the 1960s• We cannot throw away the knowledge from the physics of failure just to get simple calculation Center for Advanced Life Cycle Engineering 39 University of Maryland Copyright © 2012 CALCE
  40. 40. References (1)[1] E. Fred Schubert, “Light-Emitting Diodes”, 2nd Ed., chap. 11, pp. 196-198, Cambridge University Press, 2006[2] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, “Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting”, J. Display Technology, vol.3, no.2, pp.160-175, 2007[3] F. M. Steranka, J. Bhat, D. Collins, L. Cook, M. G. Craford, R. Fletcher, N. Gardner, P. Grillot, W. Goetz, M. Keuper, R. Khare, A. Kim, M. Krames, G. Harbers, M. Ludowise, P. S. Martin, M. Misra, G. Mueller, R. Mueller-Mach , S. Rudaz, Y. C. Shen, D. Steigerwald, S. Stockman, S. Subramanya, T. Trottier, and J. J. Wierer, "High Power LEDs – Technology Status and Market Applications," phys. stat. sol. (a), vol.194, pp.380-388, 2002.[4] Lumileds, “Luxeon Reliability”, Reliability Datasheet RD25, Philips Lumileds, 2006[5] Y. Lin, N. Tran, Y. Zhou, Y. He, and F. Shi, “Materials Challenges and Solutions for the Packaging of High Power LEDs”, 2006 International Microsystems, Packaging, Assembly Conference Taiwan, IMPACT 2006. International, pp.1-4, 2006[6] H.-T. Li, C.-W. Hsu, and K.-C. Chen, “The Study of Thermal Properties and Thermal Resistant Behaviors of Siloxane-modified LED Transparent Encapsulant”, Microsystems, Packaging, Assembly and Circuits Technology, 2007. IMPACT 2007. International, pp.246-249, 2007[7] J.F. Rabek, “Polymer Photodegradation: Mechanisms and Experimental Methods”, chapter 1. pp. 1-6, Chapman& Hall, 1995[8] A. Torikai and H. Hasegawa, “Accelerated photodegradation of poly(vinyl chloride)”, Polymer Degradation and Stability, vol.63, pp.441-445, 1999[9] J.L. Down, “The Yellowing of Epoxy Resin Adhesives: Report on High-Intensity Light Aging”, Studies in Conservation, vol.31, pp.159-170, 1986 Center for Advanced Life Cycle Engineering 40 University of Maryland Copyright © 2012 CALCE
  41. 41. References (2)[10] N. Narendran and L. Deng, “Performance Characteristics of Lighting Emitting Diodes”, Proceeding of the IESNA Annual Conference, 2002, Illuminating Engineering Society of North America, pp.157-164, 2002[11] D.L. Barton and M. Osinski, “Degradation Mechanisms in GaN/ AlGaN/ InGaN LEDs and LDs”, Semiconducting and Insulating Materials, (SIMC-X) Proceedings of the 10th Conference on, pp.259-262, 1998[12] M. Arik, S. Weaver, C.A. Becker, M. Hsing, and A. Srivastava, “Effects of Localized Heat Generations Due to the Color Conversion in Phosphor Conversion in Phosphor Particles and Layers of High Brightness Light Emitting Diodes”, International Electronic Packaging Technical Conference and Exhibition, ASME, Maui, Hawaii, pp.1-9, 2003[13] S.C. Allen and A.J. Steckl, “A nearly ideal phosphor-converted white light-emitting diode”, Applied Physics Letters, vol.92, pp.143309-1-3, 2008[14] N.T. Tran and F.G. Shi, “Simulation and Experimental Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting Diodes”, Microsystems, Packaging, Assembly and Circuits Technology, 2007. IMPACT International, pp.255-257, 2007[15] N. Narendran, Y. Gu, J.P. Freyssinier-Nova, and Y. Zhu, “Extracting phosphor-scattered photons to improve white LED efficiency”, Physica Status Solidi (a), vol.202, no.6, pp. R60-R62, 2005[16] J.K. Kim, H. Luo, E.F. Shubert, J. Cho, C. Sone, and Y. Park, “Strongly Enhanced Phosphor Efficiency in GaInN White Light-Emitting Diodes Using Remote Phosphor Configuration and Diffuse Reflector Cup”, Japanese Journal of Applied Physics, vol.44, no.21, pp. L649-L651, 2005[17] H. Luo, J.K. Kim, E.F. Shubert, J. Cho, C. Sone, and Y. Park, “Analysis of high-power packages for phosphor- based white-light-emitting diodes”, Applied Physics Letters, vol.86, pp.243505-1-3, 2005[18] Philippe Smet (2010), “Luminescence and Luminescent Materials (ppt slides)”, retrieved from http://www.telecom.fpms.ac.be/PhotonDoctoralSchool2010/documents/Luminescence-DocSchoolPhotonics2010- PP97.pdf. Center for Advanced Life Cycle Engineering 41 University of Maryland Copyright © 2012 CALCE
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