Led failure mechanisms

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

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

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

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


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, GaAs

TM


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.

TM


Construction of Light Emitting Diodes

Illustration of GaN LED die

Illustration of LED package with PCB and heat sink

TM


LED Development History

• Development of new phosphor materials
• Development of fabrication technology and equipment
• Development of LED package heat dissipation

TM


Semiconductor-related Failure Mechanisms in LEDs
Failure 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
No Light, High Drive Current or High
Electromigration Electrical Overstress
Short Circuit Current Density


Interconnect-related Failure Mechanisms in LEDs
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 Stress
Electrical Overstess-
No Light, High Drive Current/ High
Induced 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)

Package-related Failure Mechanisms in LEDs
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
Prolonged Exposure to UV Photodegradation
Lumen Degradation,
Color Change, High Current-Induced Joule Heating
Encapsulant Yellowing
Dislocation of the Presence of Phosphor Thermal Stress
Encapsulant
Lumen Degradation,
Phosphor Thermal Broadening of High Current-Induced Joule Heating or
Thermal Stress
Quenching Spectrum High Ambient Temperature
(Color Change)
Thermomechanical Stress
Lens Cracking Lumen Degradation Poor Thermal Design
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


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 PCB

More 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.


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.


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.


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.


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


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/UV
Thermal Moderate/good Moderate Poor/moderate
Quantum
>0.9 >0.8 >0.6–0.7
efficiency
Saturation No No No
Stability Good Good Poor


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.

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)


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


Modified Arrhenius Equation
Arrhenius equation fitting thermal quenching data in order to
understand the temperature dependence of photoluminescence and
determine 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


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


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.


Types of Encapsulant Materials in LEDs
1. 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


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 1.6

2) 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

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.


Thermal Encapsulant Yellowing

Sections of Nichia LED Package Material:
Left: unstressed Reference:
Middle: 133 hours at 150oC [11]
Right: 130 minutes at 200oC


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 following
conditions [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

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.)

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.


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.


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]


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]


LED Package Encapsulant Designs (2)

Concept of Phosphor location
in high power LEDs

Reference: [16]

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
efficiency
Thermal 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

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.


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.


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.


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

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


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.

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