Chapter 1Introduction to LED Thermal Managementand Reliability

Michael Pecht, Diganta Das and Moon-Hwan Chang

Abstract Some like it hot, others do not. And those others for sure include thedesigners of products that contain light-emitting diodes (LEDs). This book is aboutthermal management of LEDs and especially LED applications. The main question tobe addressed is: Why do we need thermal management? As Belady put it eloquentlyin 2001 [Belady and Minichiello, Electronics Cooling Magazine, May issue, 2003]:

The ultimate goal of system thermal design is not the prediction of component temperatures,but rather the reduction of thermally associated risk to the product.

Hence, the objectives of a designer are not in the first place to calculate or measuretemperatures, but to keep the lifetime beyond x years, to keep the color point withinmargin y, and to raise the efficiency to z %. And indeed, these objectives, determiningthe quality of LED-based products, are linked to the junction temperature. This is themain reason why a book on LED thermal management starts with an introductorychapter on LED reliability issues.

Parts of this chapter have been sourced from a chapter in a book on Solid StateLighting Reliability [Pecht and Chang, Solid state lighting reliability: componentsto systems, Springer, New York, pp. 43–110, 2013].

1.1 Introduction to Light-Emitting Diodes

Light-emitting diodes (LEDs) are solid-state lighting sources increasingly being usedin display backlighting, communications, medical services, signage, and generalillumination. Due to their versatility, LED application areas include liquid-crystaldisplay (LCD) backlights, displays, transportation equipment lighting, and generallighting. LEDs are used as a light source for LCD backlights, including in mobile

M.-H. Chang (�) · M. Pecht · D. DasCenter for Advanced Life Cycle Engineering (CALCE), University of Maryland,Building 89, Room 1103, College Park, MD 20742, USAe-mail: mhchang@calce.umd.edu

M. Pechte-mail: pecht@calce.umd.edu

D. Dase-mail: digudas@calce.umd.edu

C. J. M. Lasance, A. Poppe (eds.), Thermal Management for LED Applications, 3Solid State Lighting Technology and Application Series 2,DOI 10.1007/978-1-4614-5091-7_1, © Springer Science+Business Media New York 2014

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phones, cameras, portable media players (PMPs), notebooks, monitors, and televi-sions (TVs). Display areas include LED electric scoreboards, outdoor billboards,and signage lighting, such as LED strips and lighting bars. Examples of transporta-tion equipment lighting areas are vehicle/train lighting (e.g., meter backlights, tailand brake lights) [3] and ship/airplane lighting (e.g., flight error lighting and search-lights). General lighting applications are divided into indoor lighting (e.g., LEDlight bulbs, desk lighting, and surface lighting) [4, 5], outdoor lighting (e.g., dec-orative lighting, street/bridge lighting, and stadium lighting), and special lighting(e.g., elevator lighting and appliance lighting) [6, 7]. The use of LEDs in generallighting has increased, starting with street lighting in public areas and moving on tocommercial/business and consumer-level lighting.

LEDs offer design flexibility, ranging from zero-dimensional lighting (dot-scalelighting) to three-dimensional lighting (color dimming using a combination ofcolors), with one-dimensional lighting (line-scale lighting) and two-dimensionallighting (local dimming, i.e., area-scale lighting) in between. LEDs have small exte-rior outline dimensions and offer high energy efficiency that results in lower powerconsumption with low voltage (<4 V) and low current operation (<700 mA). Theyhave longer life—up to 50,000 h and provide higher performance, such as ultra-high speed response time (microsecond-level on-off switching), a wider range ofcontrollable color temperatures, a wider operating temperature range, and no low-temperature startup problems. In addition, LEDs have better impact resistance. LEDsare also eco-friendly products, with low ultra-violet (UV) radiation (higher safety)and no mercury. Interested readers may consult references [8–13] for more details.

LEDs range from a narrow spectral band emitting a single-colored light, suchas red, yellow, green, or blue, to white, to a distribution of luminous intensityand various types and shapes, depending on the color mixing and package design.White light is a mixture of all visible wavelengths, as shown in Fig. 1.1. Ev-ery LED color is represented by unique x – y coordinates, as shown in Fig. 1.2.Red is on the far right, green is on the top left, and blue is on the bottomleft. The International Commission on Illumination (CIE) chromaticity coordi-nates of x, y, and z are a ratio of the red, green, and blue stimulation of lightcompared to the total amount of the red, green, and blue stimulation. By defini-tion, the sum of the normalized tri-stimulus values (x + y + z) is equal to 1. Thewhite area of the chromaticity diagram can be expanded, and boundaries can beadded to create each color range. The color temperatures and the Planckian lo-cus (black-body curve) show how they relate to the chromaticity coordinates [14].1

As the temperature of the black body increases, the chromaticity location movesfrom the red wavelength range toward the center of the diagram in Fig. 1.2.

1 The color temperature of a white light is the temperature of an ideal Planckian black-body radiatorthat radiates a light of comparable hue to that light source. Thus, the color temperature of a white lightof thermal radiation from an ideal black-body radiator is defined as equal to its surface temperaturein kelvins. When the black-body radiator is heated to high temperatures, the heated black bodyemits the color, going from red, to orange, to yellow, to white, and finally to bluish white. ThePlanckian locus starts out in the red, then moves through the orange and yellow, and finally to thewhite region. The color temperature of a light source is regarded as the temperature of a Planckianblack-body radiator that has the same chromaticity coordinates.

1 Introduction to LED Thermal Management and Reliability 5

360 390 420 450 480 510 540 570 600 630 660 690 720 750 7800.000.050.100.150.200.250.300.350.400.450.500.550.600.650.700.750.80

Spec

tral P

ower

(W/n

m)

Wavelength (nm)

Fig. 1.1 Spectral power distribution—white light-emitting diode (LED)

Fig. 1.2 InternationalCommission on Illumination(CIE) 1931 chromaticitydiagram. (Source: [15]© Cambridge UniversityPress, reprinted withpermission)

Color change should be considered in LED applications because LED degradationnot only results in reduced light output, but also in color changes. LED modules arecomposed of many LEDs. This means that if some number of LEDs experience colorchanges, it will be recognized by users. Even if all LEDs degrade at the same level,LED modules need to maintain their initial color, especially for indoor lighting andbacklight applications.

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1.2 Short History of Light-Emitting Diodes

The history of LED development can be divided into three generations with distinctadvancements in new fabrication technology and equipment, new phosphor mate-rials, and improved heat dissipation packaging technologies. LEDs have becomebrighter and color variance more flexible. Also, light efficiency and light efficacyhave been improved. The first commercialized LED was produced in the late 1960s.This first generation of LEDs lasted from the 1960s until the 1980s. In this period,the major application areas were machinery status indicators and alpha-numeric dis-plays. The first commercially successful high-brightness LED was developed byFairchild Co. Ltd. in the 1980s. In the second generation, from the 1990s to thepresent, high-brightness LEDs became very popular in the world LED market. Themain application areas for the second generation included motion displays, LEDflashes, LED back light units (BLU), mobile phones, automotive LED lighting, andarchitecture.

The third generation is now arriving in the market. These LEDs have been de-veloped for substantial savings in electrical energy consumption and a reduction inenvironmental pollution. Future LED application areas are expected to include gen-eral lighting, lighting communication [16], medical/environmental purposes, andcritical applications in system controls. Some examples are:

• Portable LED projector,• Large-size LED backlighting displays,• LED general lighting,• Visible light communication,• Purifiers,• Biomedical sensors,• Artificial sun.

Moore’s law predicts the doubling of the number of Si transistors in a chip every18–24 months. Similarly, for LEDs, luminous output (luminous flux, measured in lm)appears to follow Haitz’s Law, which states that LED flux per package has doubledevery 18–24 months for more than 30 years [9]. This trend in the technologicaladvancement of LEDs is based on industry-driven R&D efforts targeting high-efficiency, low-cost technology solutions that can successfully provide an energysaving alternative to the recent applications of LEDs.

1.3 Introduction to Light-Emitting Diode Manufacturing

The LED supply chain starts from an LED chip and progresses to an LED package,an LED module, and then to a system. LED production starts with a bare wafer, suchas sapphire, GaN, SiC, Si, or GaAs. Many thin epilayers are then grown on the barewafer. Different colors of LEDs can be made by using different types of epiwafers.The types of epiwafer are InGaN/AlGaN for producing blue, green, and UV-range

1 Introduction to LED Thermal Management and Reliability 7

Fig. 1.3 LED packageassembled with printed circuitboard (PCB)

light; InAlGaP for producing red and yellow light; and AlGaAs for producing redor infrared-range light. The LED chip fabrication process involves attaching electriccontact pads to an epiwafer and cutting the epiwafer into LED dies that are thenpackaged.

LEDs are classified into two types: white LEDs and red-green-blue (RGB) LEDs.White LED packages can use red/green/blue/orange/yellow phosphors with blueLED chips to produce white light. The phosphors comprise activators mixed withimpurities at a proper position on the host lattice. The activators determine the energylevel related to the light emission process, thereby determining the color of the lightemitted. The color is determined by an energy gap between the ground and excitationstates of the activators in a crystal structure. RGB LED packages represent red, green,and blue LED packages, as well as LED packages with multi-dies in a single packagethat produce white light using a combination of red, green, and blue LED dies.

A cross-sectional side view of a white LED is shown in Fig. 1.3. An LED packagemounted on a printed circuit board is composed of housing, encapsulant, die, bondwire, die attach, lead frame, metal heat slug, and solder joint. The housing is a bodyfor supporting and protecting the entire structure of an LED device. The housingis usually formed from materials such as polyphthalamide (PPA) or liquid crystalpolymer (LCP). The encapsulant positioned over the housing is a resin material forthe LED package in the shape of a dome. The typical material types for the resinare epoxy or silicon. The die is a compound semiconductor. The lead frame is usedto apply external power to the LED die. The die attach is used to mechanically andthermally connect the chip onto the lead frame. Typical types of die attaches are Agpaste and epoxy paste. Phosphors dispersed in the encapsulant are used to emit thewhite light excited by absorbing a portion of the light from the LED dies.

LED types are placed in the following major categories depending on LED elec-trical power: low power LEDs are under 1 W of power (currents typically near20 mA); medium power LEDs (high-brightness LEDs) between 1 and 3 W of power(currents typically in the 30 mA/75 mA/150 mA range); and high power LEDs(ultra–high brightness LEDs) have more than 3 W of electrical power (currents typ-ically in the 350 mA/750 mA/1000 mA range). The LEDs vary because of the LEDcurrent–voltage curves, which differ between materials.

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1.4 Introduction to Light-Emitting Diode Reliability

Despite exciting innovations driven by technological advances and ecological/energy-saving concerns, the LED industry still faces challenges in attractingwidespread consumption. One issue is price, and another is the lack of informationregarding reliability. The required number of LEDs for general lighting applicationsis a matter of concern where both of these issues converge. It may take anywherefrom tens to sometimes thousands of LEDs to replace one conventional lamp becausethe emission of a single LED covers a limited area. If one single LED fails, then thefinal product is sometimes treated as a failure. For example, the failure of LEDs inan LCD display is very critical, even when only a single LED package experienceschanges in optical properties [17]. Failures of an LED or LEDs appear as dark spots,dark areas, or rainbow areas.

The LED die is a semiconductor and the nature of manufacturing of LED pack-ages is similar to that of microelectronics. However, there are unique functionalrequirements, materials, and interfaces in LEDs resulting in different failure modesand mechanisms. The major causes of failures can be divided into die-related,interconnect-related, and package-related. The die-related failures include severelight output degradation and burned/broken metallization on the die. The interconnectfailures of LED packages are electrical overstress-induced bond wire fracture/wireball bond fatigue, electrical contact metallurgical interdiffusion, and electrostaticdischarge, which leads to catastrophic failures of LEDs. Package-related failuremechanisms include carbonization of the encapsulant, encapsulant yellowing, de-lamination, lens cracking, phosphor thermal quenching, and solder joint fatiguethat results in optical degradation, color change, electrical open/short, and severediscoloration of the encapsulant.

LED manufacturers usually perform tests in the product development cycle duringthe design and development phases. Typical qualification tests of LEDs are catego-rized into operating life tests and environmental tests by using industrial standardssuch as Joint Electron Devices Engineering Council (JEDEC) or Japan Electron-ics and Information Technology Industries Association (JEITA) [18, 19]. Readersinterested in life testing are encouraged to consult [20].

A lifetime estimate is generally made using the Arrhenius model. Activation en-ergy is sensitive to the test load condition, types of materials, and mechanical designof LED packages. This estimate is life with uncertainties such as exponential extrap-olation of lifetime, assumption of activation energy, possible failure mechanism shiftbetween test and usage conditions, and discount of all other failure causes besidestemperature.

LED core technology in terms of structural and reliability analysis is shown inFig. 1.4. To develop a final LED product, manufacturers are required to considereach level in Fig. 1.4 (composed of LED die, LED package, LED module, andsystem), because market share power is based on optimal thermal dissipation, highexternal quantum efficiency, high electrical power conversion efficiency, enhancedperformance, low cost, advanced opto-mechanical design (minimizing rainbow orglare effects), and high reliability.

1 Introduction to LED Thermal Management and Reliability 9

Fig. 1.4 Light-emitting diode (LED) core technology

LED package reliability is important for improving LED lighting system relia-bility, since all other parts, including mechanical parts, power, and electric circuits,can be repaired or replaced by scheduled maintenance before the system experiencesfailure. Once an LED package or LED module encounters failures, this means thatthe LED system needs to undergo unscheduled maintenance, which causes end-usershigh cost for replacement. Many LED failure modes and mechanisms are related tothermal, electrical, and humidity stress.

1.5 The Rationale of Light-Emitting Diode ThermalManagement

End-product manufacturers that use LEDs expect the LED industry to guaranteethe lifetime of LEDs in their usage conditions. Such lifetime information wouldallow LED designers to deliver the best combination of purchase price, lightingperformance, and cost of ownership for the life of the products. One barrier to theacceptance of LEDs in traditional applications is the relatively sparse informationavailable on their reliability [20]. When a higher drive current is applied to LEDs,there is increased light output but that typically comes with increased heat generation.The light output can change as a result of the operating conditions, temperature inparticular [20–27]which is impacted by heat generation and depends on the methodsof dispersion of the heat. For example, light output decreases with a temperaturerise in the LEDs, since the quantum efficiency decreases at higher temperature thatcontributes to more nonradiative recombination events in LEDs [20, 28]. Temperatureincrease results in forward voltage drop due to the decrease of the bandgap energyof the active region of LEDs and also results in the decrease in series resistance.

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The resistance decrease is due to higher acceptor activation occurring at elevatedtemperatures as well as the resulting higher conductivity of the p-type layer andactive layers. In addition to the quantum efficiency drop, the colors of LEDs alsochange with increased temperature. In particular, phosphor-converted LEDs withblue InGaN and yellow phosphors experience light output degradation along withshifts of blue peak wavelength and the peak energy of the phosphors when thetemperature of the LEDs increases.

The shifts of the blue peak wavelength toward longer wavelengths having lowerenergy (i.e., redshifting) are due to the junction temperature dependence of the energygap shrinkage and quantum confined Stark effect, a process which reduces energyof bound states in a quantum well under an applied electric field [29].

To sum up, many important reliability- related features of LEDs are functions oftemperature. As an example, the long-term stability and lifetime of LEDs are typi-cally judged on the basis of measured light output. The measured light output mostlydepends on the junction temperature. Hence, the correctness of light output measure-ments is dependent on the temperature stability of the light output measurement setupand by the accuracy of the temperature measurement (details of laboratory measure-ment of light output are provided in Chap. 5). In the daily field practice there areassociated uncertainties with prediction of the junction temperature because there areonly indirect ways of measuring and converting temperatures from reference pointsto the junction temperature. (Chap. 4 provides details of thermal testing along witha laboratory method of indirect measurement of LEDs’ junction temperature. Thedetailed description of a standardized laboratory thermal testing procedure is pro-vided by the JEDEC JESD51–51 document [30]). Long-term stability analyses ofLEDs need to demonstrate that the thermal conditions of the LEDs have not changedduring the entire aging/testing process in order to enable correct correlation betweenlight output characteristics and RthJA (LED’s junction-to-ambient thermal resistance).Little information has been published about how the light output measurements inreliability studies are performed, but it is suspected that the current RthJA of theLEDs during aging test measurements is often uncontrolled and changes over time.As a consequence, some of the reported light output variations could be attributedto RthJA variations of the test setup. The in-situ measurements briefly described inSect. 5.4.2 of Chap. 5 were aimed to assure this thermal stability requirement andthe thermal transients measurements accompanying the light-output measurementsdescribed there, provided means of following the changes of the overall RthJAof theLEDs being tested. (Further details on this experiment are provided in [31]). An-other way to prevent this uncertainty is to eliminate the potential changes in RthJA

by ensuring that all light output characteristics are presented as a function of thereal junction temperature [32]. The only way that the reliability data provided bydifferent vendors can be assured is by standardizing all relevant measurements anddefinitions.

Besides the standardization of reliability-related tests, an important source ofinformation for a designer is the published data in the data sheets, especiallythermal data such as junction-to-ambient and junction-to-case thermal resistances.The designer needs these data to ensure that the maximum allowable temperatures

1 Introduction to LED Thermal Management and Reliability 11

prescribed by the vendors are not exceeded. It is necessary for these data to be stan-dardized, because lower thermal resistance is a major selection criterion. Lasanceand Poppe [33–36] and Poppe et al. [32] discuss the need for more sophisticatedthermal characterization and standardization of LEDs and LED-based products. Thereason is that progress in these fields has not kept pace with the exponential growthin applications. This situation became a serious problem for leading manufacturerswho are focusing on a sustainable business for the future and are willing to publishreliable thermal data. Unfortunately, due to the lack of globally accepted standards,manufacturers could publish whatever they wanted. The lack of standards also meanta problem for the experienced user because the thermal data that are published areoften of limited use in practice when accuracy is at stake, and accuracy is needed forestimation of expected performance and lifetime. Remarkably, the situation was notmuch different from the one that the integrated circuit (IC)-world was facing almost20 years ago [32, 37–41]. Around 1990 it became clear that thermal characterizationof IC packages was problematic. Manufacturers all over the world were using differ-ent standards. Even within a single manufacturer, intolerable differences showed up.To solve the thermal characterization problems, manufacturers must publish thermaldata in such a way that the end-user can use this data. End-users are responsiblefor the specifications of the thermal environment to which the LEDs are exposed.Provided that the manufacturers want to cooperate, it would be easy to apply thestandard protocols used by IC business.

In addition to standardization itself and suggestions for improved test setups,Poppe and Lasance discussed [33–36] the role of thermal characterization, the defini-tion of thermal resistance, the different goals of manufacturers and system designers,the similarities and differences between LED and IC thermal characterization, thedrawbacks of the current thermal data in data sheets, and an overview of the questionsthat an LED thermal standardization body should address. The first steps in this re-gard have been recently made by JEDEC through the publication of the JESD51–5xseries of standards [30, 42, 43]. Further information about standardization of LEDthermal characterization is provided in Chap. 6.

1.6 Conclusions

The conventional way to predict the lifetime of LEDs employs the Arrhenius modelto extrapolate test results at high temperature to expected operating temperatures. Amajor problem is that the Arrhenius model is not adequate to represent the failuresof LEDs. Light output degradation is the major failure mode of LEDs, and it resultsfrom hygromechanical and electrical stresses, in addition to thermal stresses. A morerealistic method of LED lifetime estimation needs to reflect total consideration oftemperature, the level of forward current, relative humidity, mechanical stress, andmaterials.

The overall reliability of LED packages is related to interconnect failures, semi-conductor failures, and package failures. Interconnect failures are responsible for

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broken bond wire/lifted ball, electrical metallurgical interdiffusion, and electrostaticdischarge. LED semiconductor failures are manifested as die cracking, defect anddislocation generation and movement, dopant diffusion, and electromigration. Pack-age failures involve mechanical interaction with LED chips, die adhesives, heat slugs,lead frames, and encapsulants. The failure mechanisms responsible for package fail-ures include carbonization of the encapsulant, delamination, encapsulant yellowing,phosphor thermal quenching, and lens degradation.

Cooperation between thermal, electrical, and optical standards bodies and profes-sional societies is required to arrive at globally accepted thermal standards to measurejunction and reference temperatures to ensure a fair comparison of published per-formance and reliability data. Since the end user needs total reliability of the finalproducts, reliability research of LED packages has to be expanded to the reliabilitystudy of the complete LED-based system, including the luminaires and electronics.

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