time evolution degradation physics in high power white leds under high temperature-humidity...

742 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 14, NO. 2, JUNE 2014

Time Evolution Degradation Physics inHigh Power White LEDs Under High

Temperature-Humidity ConditionsCher Ming Tan, Senior Member, IEEE, and Preetpal Singh

Abstract—A high temperature-humidity test is commonly em-ployed to evaluate the humidity reliability of electronic devices.For an integrated circuit, the degradation mechanism under thehigh temperature-humidity test is metal corrosion, and Peck’smodel is used for extrapolating the test results at accelerated testconditions to the normal operating condition. Such extrapolationis possible as the underlying degradation physics is invariant fromthe accelerated test conditions to the normal operating conditionfor integrated circuits. However, this is not true for high powerLEDs, as found in this paper. The degradation in the LEDs under-goes time evolution at either 95% or 85% relative humidity (RH)and 85 ◦C. We also found that the degradation physics are com-pletely different among the various RH levels from 95% to 70%.The degradation process begins from bond pad contaminationand Kirkendall void formation, galvanic dissolution, phosphordissolution to encapsulant, and die attach delamination. Such timeevolution degradation physics renders the inapplicability of thePeck model and presents a challenge in extrapolation of test resultsto the normal operating condition for lifetime prediction.

Index Terms—Akaike information criterion, delamination, ex-pectation and maximization, kirkendall voids, galvanic corrosion,silver migration, simulated annealing.

I. INTRODUCTION

S INCE the invention of high power white LEDs, increas-ing applications of such LEDs in various environmental

conditions are emerging. To ensure the durability of LEDsas lighting sources, accelerated life tests methods have beenadopted according to the MIL STD, IEC and JEDEC standards.The life under normal stress can then be derived using theaccelerated models such as Arrhenius model, Peck’s model,Erying model and Coffin-Manson model [1].

Among the various reliability tests for LEDs, thermal degra-dation is studied most extensively because heat is known to beone of the key factors that will affect LEDs performance anddurability. LM-80 presented the approved test method for LEDsunder different high temperatures, and TM-21 is introduced toprovide a method for extrapolation [2]. As more LED luminar-ies are used in outdoor conditions with high humidity such as

Manuscript received November 18, 2013; accepted March 20, 2014. Date ofpublication April 18, 2014; date of current version June 3, 2014.

C. M. Tan is with the School of Electrical and Electronic Engineering,Nanyang Technological University, Singapore 639798 (e-mail: [email protected]).

P. Singh is with Chang Gung University, Taoyuan 333, Taiwan (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TDMR.2014.2318725

street lamps and marine applications etc, the effect of humidityon LED lifetime is also becoming important. However, nostandard is currently available, and either 85%RH at 85 ◦C or100% RH at 121 ◦C is used to evaluate the humidity reliabilityof LEDs in industry today [1].

For the study of the degradation mechanisms of LEDs dueto humidity, Tan et al. [3] found that the presence of humiditycan result in a rapid decrease in the lumen at the initial shortduration due to absorbed moisture into lens that cause lightscattering. Prolonged exposure to humidity can result in thedegradation of LEDs, and the degradation mechanisms are notlimited to corrosion only as in the case of integrated circuit[4]. In fact, it was found qualitatively that the degradationmechanisms of LEDs vary with the test time [5], and thisfinding renders the inapplicability of the common practice inintegrated circuit of using the standard reliability test data at85%RH and 85 ◦C to estimate its lifetime at normal conditionfor LEDs. The purpose of this work is to develop a methodologyto investigate and identify this time varying degradation physicsin LEDs subjected to high temperature and humidity. OsramGolden Dragon Plus LEDs are used as an example for suchinvestigation.

II. EXPERIMENTATION

A total of 60 units of 1 W Osram Golden Dragon PlusWhite high power commercial packaged LEDs are placed ina high temperature-humidity chamber (μ series from Isuzu) foraccelerated life test. The temperature of the chamber is kept at85 ◦C while three RH levels are used with sample size of 20 ateach RH level. The three RH levels are 95%, 85% and 70%.

All the packaged white LEDs are mounted on a metal boardwhich then placed in the temperature-humidity chamber. Toprevent evaporation of the trapped moisture in the LED en-capsulation by the heat generated from the power-on LEDs,all LEDs are not powered during the humidity test. The LEDsare removed from the humidity chamber at specific intervals tohave both their electrical and optical properties measured. Theintervals are fixed at 24 hours.

To ensure no evaporation of moisture from the LEDs’ pack-ages after they are taken out from the chamber for electrical andoptical measurements, all such measurements are done within3 hours and the samples are kept at 25 ◦C from the time theyare taken out from the test chamber.

Before the humidity tests, the electrical I–V characteristicsand optical characteristics, namely the luminous flux and blue

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TAN AND SINGH: TIME EVOLUTION DEGRADATION PHYSICS IN HIGH POWER WHITE LEDs 743

Fig. 1. Optical measurement system used in this paper.

to yellow ratio for each LED are measured using Keithleysource meter model 2651A and optical measurement system,respectively. The optical measurement system is as shownin Fig. 1 which comprises a 1-m-diameter integrating spheremodel SLM-40TS-110902 and a spectroradiometer modelOcean Optics QE6500. This initial set of measurement servesas the reference baseline for each test sample.

During the measurements, the LED under test is powered upwith a nominal constant current of 300 mA as recommended inthe manufacturer’s specification sheet. To ensure consistencyand accuracy of the measurements, a stabilization time of30 seconds is incorporated and the ambient temperature of theLEDs is maintained at 30 ◦C using thermo-electric cooler. Thesetting for all measurements is done according to [6] to preventself-heating during measurement.

The measured luminous flux of the individual LEDs isplotted as a function of time, and the time to degradationto 5%, 10%, and 15% degradation with respect to its initialmeasurement are derived for each LED, similar to the workdone by Tan et al. [4]. The results are shown in Table I fortest conditions of 95%RH, 85%RH and 70%RH at 85 ◦C,respectively.

III. EXPERIMENTAL DATA ANALYSIS

A. Identification of Statistical Distributions Associated WithDegradation Mechanisms

Akaike information criterion (AIC) is used to identifythe number of possible degradation mechanisms in a givenset of measured data, and a global maximization algorithmcalled simulated annealing (SA) in conjunction with theexpectation—maximization (EM) algorithm is employed to de-termine the probability of each unit that belong to each possibledegradation mechanism. The global maximization algorithm isalso used to determine the distribution parameters of statisticaldistribution associated with each possible degradation mecha-nism, followed the work of Tan et al. [4]. We found that thereare three possible degradation mechanisms at each degradation% as shown in Tables II–IV for 95%RH and 85 ◦C. SA plus

TABLE ITIME TO DEGRADATION (HOUR) OF EACH LED

AT 85 ◦C AND 3 RH LEVELS

TABLE IITIME TO 5% DEGRADATION OF EACH LED UNDER 95%RH/85 ◦C AND

ITS RESPECTIVE PROBABILITY (PR) TO EACH POSSIBLE STATISTICAL

DISTRIBUTION ASSOCIATED WITH EACH DEGRADATION MECHANISM

(DM). UNITS 3 AND 12 ARE OUTLIER POINTS

EM also help us to identify that the corresponding statisticaldistributions are 3 parameters lognormal distribution.

While AIC criteria indicates three possible degradationmechanisms from the data, a few units are actually outlierpoints, and with detail examination of Tables II–IV, it is clearthat there is only one dominant degradation mechanism at each% of degradation as indicated in red Bold. The parametersof the distributions associated with the dominant degradationmechanisms at each % degradation level are shown in Table V,and Fig. 2 shows the corresponding lognormal probability plots.The times to degradation are augmented by the incubation timesin order to plot the 3-parameters lognomal distribution onto2-parameters lognormal probability graph paper [7].


TABLE IIITIME TO 10% DEGRADATION OF EACH LED UNDER 95%RH/85 ◦C AND

ITS RESPECTIVE PR TO EACH POSSIBLE STATISTICAL DISTRIBUTION

ASSOCIATED WITH EACH DM. UNITS 3 AND 12 ARE OUTLIER POINTS

TABLE IVTIME TO 15% DEGRADATION OF EACH LED UNDER 95%RH/85 ◦C AND

ITS RESPECTIVE PR TO EACH POSSIBLE STATISTICAL DISTRIBUTION

ASSOCIATED WITH EACH DM. UNITS 16 AND 17 ARE OUTLIERS

TABLE VDISTRIBUTION PARAMETERS OF EACH DOMINANT DEGRADATION

MECHANISM AT 95%RH/85 ◦C

Similarly, only one dominant degradation mechanism existsat each % of degradation for the case of 85%RH and 70%RH,respectively, and their distribution parameters are shown inTables VI and VII.

From Tables V and VI, one can see that the shape param-eters at each % degradation are different, indicating that thedegradation mechanisms at each % degradation are different[7], [8], i.e., the degradation physics is evolving with time. Also,the shape parameters at a given % degradation are different atdifferent %RH, and this implies that no extrapolation is possible

from 95% to 85% to 70% RH. In other words, the test dataat 85%RH and 85 ◦C cannot be used to estimate the life timeof LEDs under lower humidity condition as the acceleration isnot linear [7], [8]. The initiation of the dominant degradationmechanism at different % of degradation can be obtained fromthe corresponding incubation time at the % of degradation.

For the condition of 70%RH and 85 ◦C, the shape parametersat 5% and 10% degradation are close to each other, implyingthat the degradation mechanism is the up to 10% degradation,and a new degradation mechanism evolves after 10% degra-dation. As this condition is closer to the normal operatingcondition, its mechanism will be of greater interest.

B. Identification of the Degradation Physics

For a packaged LED, the degradation can occur either at thechip level or package level. Chip level can further be split intosemiconductor dice and phosphor, and package level can besplit into encapsulant (silicone gel and lens), bonding wires andbond pad.

Non-Destructive Analysis: To investigate the degradationphysics, the first step is to determine if the degradation is atthe chip or package level. Let us focus first on the units withstress condition of 95%RH and 85 ◦C. For the chip level, weexamine the Blue to Yellow ratio (BYR) as well as the idealityfactor and reverse saturation current of the junction in the semi-conductor dice. This BYR is computed by taking the ratio ofthe peak intensity at the blue wavelength over that at the yellowwavelength. Fig. 3 shows the change in the BYR at differentdegradation levels for two units selected arbitrary. These unitsare selected because they show larger change (around 80%increases) in the BYR, making the degradation physics moreobvious. The other units are expected to be the same mechanismas they all belong to the same statistical distribution.

The ideality factors and reverse saturation currents of thejunctions for all the tested LEDs are found to remain constantup to 15% degradation. These values are computed using theextraction method developed by Tan et al. [9]. Therefore, onecan conclude that up to 15% degradation, the junctions have notbeen degraded. This is further confirmed by the insignificantchange in the wavelength of the blue peak for all the units atdifferent % of degradation.

On the other hand, from the extraction results, the seriesresistances (Rs) of the units vary with the % degradation, andthen remain constant beyond 5% degradation. Fig. 4 showstypical variation for the two units selected.

From Figs. 3 and 4, it is obvious that the degradation mech-anism for the first 5% is due to the increase in Rs, and the next5% (i.e., from 5% to 10% degradation) is due to the phosphordegradation as evidenced by the increase in the BYR since theelectrical characteristics of the chips remain intact. The last 5%is due to another different mechanism. This concurs with theabove statistical analysis that the degradation mechanisms aretime varying.

Similar detail analysis on the degradation at 85%RH and 85 ◦Cwill not be meaningful as their degradation mechanisms do notresemble that under normal operating conditions. Instead, wewill now switch to the case of 70%RH and 85 ◦C.


Fig. 2. Lognormal probability plot of time to degradation at 95% and 70% RH at 85 ◦C. 85% RH data is not included to avoid congested lines. The label 70_15in the legend means 70% RH and 15% degradation, and the same applies to other labels in the legends. All the lines show good linear fit as can be seen from thevalues of ρ (coefficient of regression) at the bottom of the graph as computed by Reliasoft.

TABLE VIDISTRIBUTION PARAMETERS OF EACH DOMINANT DEGRADATION

MECHANISM AT 85%RH AND 85 ◦C

TABLE VIIDISTRIBUTION PARAMETERS OF EACH DOMINANT DEGRADATION

MECHANISM AT 70%RH AND 85 ◦C

For the units subjected to 70%RH and 85 ◦C, we also see nochange in the ideality factors and reverse saturation currents ofall the units as well as the peak wavelength of the blue light,indicating no degradation on the semiconductor dice. Fig. 5shows the BYR of two typical units and one can see a slightincrease (less than 10%) during the initial degradation. How-ever, as the shape parameters of the distributions are similar forall level of degradation up to 15%, such increases in the BYR isbelieved to be insignificant to cause the lumen degradation. Onthe other hand, the increases in Rs are significant and consistentas shown in Fig. 6, and one can thus conclude that the increaseof Rs of the Au bonding wire on Ag coating is one of thedominant degradation mechanisms for the LEDs at this stresscondition.

Noting that the increase in Rs from 10% to 15% degradationlevel becomes small, this could imply the presence of anotherdegradation mechanism from 10% degradation level onward.This is in agreement with the statistical analysis shown inTable VII and will be discussed later.

Fig. 3. Blue to yellow spectrum intensity ratio of the emitted light versus %of degradation at stress condition of 95%RH and 85 ◦C for two selected units.

Fig. 4. Change in Rs at different levels of degradation at stress condition of95%RH and 85 ◦C for the two selected units shown in Fig. 3.

Physical Destructive Analysis: To explore the physicalmechanisms of degradation further, we look closer at the in-ternal structure of the LED studied in this work, and it is shownin Fig. 7 as obtained from the Osram’s webpage [10].

The bond pad shown in Fig. 7 is found to be coated withsilver from our Energy Dispersive System (EDS) analysis, andit acts also as a silver mirror. The gold bonding wire is bondeddirectly onto the silver coating.


Fig. 5. Blue to yellow spectrum intensity ratio of the emitted light versus %of degradation at stress condition of 70%RH and 85 ◦C.

Fig. 6. Change in the Rs at different levels of degradation at 70%RH and 85◦C of the two selected units shown in Fig. 5.

Fig. 7. Internal structure of high power LEDs studied in this work [10].

It is known that Ag is a rapid diffuser in Au while Au is aslow diffuser in Ag [11], thus Kirkendall voids can easily beformed at the interface between Au and Ag as shown in [12].At elevated temperature of 85 ◦C as used in our work, suchvoid formation will be further enhanced. When Ag is diffusedinto Au, it is completely dissolved into Au as the Ag-Au binarysystem has perfect solid solubility [13].

Under the temperature-humidity test performed in this work,moisture will enter into the package through the silicone geland lens, the interface between the lead and the package andalso the interface between the lens and the package [14]. This is

Fig. 8. Jeol JSM-5600L SEM micrograph of the Ag coating near the Au bond-ball of (a) a degraded LED (unit #1) at 70% RH and 85 ◦C after 184.6 hours;(b) a fresh LED; (c) the corresponding circled area of (a) on the fresh unit.

also confirmed by the finite element analysis done by Tan et al.[15]. Hence, layers of water will be present at the Au-Aginterface. As the electrode potential of Au is 1.52 V whichis much higher than that of Ag (= 0.8 V) at 25 ◦C [16], theAu-Ag contacts at the bond pad become Galvanic cells during


Fig. 9. Ag coated bondpad of the LED showing the contamination layer. Thebroken Au bonding wire is due to the damage created during decapsulation.

the temperature-humidity test, with Ag as anode, and anodicdissolution of Ag from the coating occur. Such dissolutioncan result in a removal rate that can be as high as 35.6 nm/s[17]. Following the dissolution, Ag migration proceeds via iontransport and electro-deposition [18]. The activation energy ofAg+ diffusion in water is found to be 0.2 eV, and the electro-deposition will result in either Ag clouds or dendrites [19].

For the bond pad structure studied in this work, moisture thatenters into the LED package during the temperature-humiditytest can penetrate through the voids along the peripheral ofthe Au ball at the ball-Ag coating interface. The moisturecan remove the Ag from underneath the ball and weaken thebonding, increases Rs. The dissolved Ag+ can then be re-deposited on the Ag coating around the ball area. All theseare indeed observed as shown in Fig. 8 which shows theSEM micrographs of Ag coating near the bonds of both freshand a degraded LED subjected to 70% RH and 85 ◦C for184.6 hours. The Au bonding balls are intentionally displacedusing air-gun after decapsulation in order to observe the un-derlying area below the ball. The bond pad for degraded LEDunder 95%RH and 85 ◦C cannot be seen as the encapsulantpolymer material on top of the bond pad cannot be removedduring decapping. This is also encountered during failure anal-ysis by others [20], [21] due to the change in the composition ofthe encapsulant polymer material at high temperature. In fact,this change was observed to occur as short as 0.1 seconds if theheat is sufficiently high [20].

The increase in Rs as observed in our experiments areunlikely to be due to the degradation of the semiconductor diceas we have verified that the electrical parameters of the dicesdid not change in all our cases. Such bond weakening process isexpected to be more severe at 95%RH 85oC. In fact, the changein the polymer material on top of the bond pad for the case of95%RH as compared to the case of 70% seems to verify thehigher severity of the bond weakening at 95%RH which leadsto much higher increase in the Rs and higher Joule heating (as

Fig. 10. The same SEM micrograph of Fig. 9, but at 5 KV accelerating voltagesetting.

Fig. 11. EDS spectrum on the contamination layer (indicated in Fig. 9) at 5 KV,showing very little Ag content.

the test current into the LEDs is kept constant) that change thepolymer material on top.

Besides the increase in Rs of the Au-Ag bond, additionallayers of contamination can also be found on the Ag coating,probably due to the change in the composition of the polymermaterial. This is shown in Figs. 9 and 10 for the same degradedunit shown in Fig. 8. From Figs. 9 and 10, one can see that thesome contamination layers are very thin which become obviousonly at 5 KV. Figs. 11 and 12 show the EDS spectrums at a spoton the contamination layer shown in Fig. 9 at 5 KV and 15 KV,respectively, providing evidence that this layer is actually ontop of the Ag coating.

For the units degraded at 95% RH, this severe increase inbond pad resistance stops after 5% degradation, and phosphordegradation takes over as shown in Fig. 3. We postulate thatafter 5% degradation due to bond pad weakening as well as Ag


Fig. 12. EDS spectrum on the contamination layer (indicated in Fig. 9) at15 KV, showing much larger Ag content.

Fig. 13. SEM Micrograph of decapped unit #10 subjected to 95%RH/85 ◦C for 560 hours. The big white patch is the un-dissolved encapsulantwhich covered the bondpads.

mirror contamination, the increase in the Joule heating at theAu ball-Ag contact increases the temperatures of the pad andthe dice, which further change the composition of the polymermaterials and contaminate the Ag coating further. This increasein temperature also vaporize the incoming moisture at the padwhich then flows along the interface between the dice and theencapsulant (including lens) due to capillary tube effect wherethe small gap is created as a result of the differential thermalexpansion between the encapsulant and the dice. Similar phe-nomena is indeed observed by Chen et al. [22] and confirmedby finite element analysis performed by Tan et al. [15].

Consequently, the anodic dissolution process of Ag slowsdown or even stops due to insufficient thickness of the moisturelayer, and instead the water vapor reaches the dice surface anddegrades the phosphor, fills the gap at the interface and diffuse

Fig. 14. Zone in of the red-circled area in Fig. 13, showing the non-uniformdissolution of the phosphor on the dice surface. Some dissolution occurs in themiddle of phosphor layer, showing the sign of vapor flowing along the interfaceand reaches the middle part. Part A is the GaN dice and Part B is the phosphoras confirmed by our following EDS spectrums.

Fig. 15. EDS spectrum of Part A at 10 KV, showing that it is a GaN dicesurface.

into the encapsulant. This postulation is supported by theextension of the changed encapsulant polymer material on thedice surface and the degradation of the phosphor layer as shownin Figs. 13–16 below. Also, the decapping of the degradedunits under 70%RH shows a good adhesion of the encapsulantmaterial onto the dice, and clear separation of the encapsu-lant material from the bond pad as shown in Figs. 9 and 10earlier.

Upon further moisture inception under 95%RH, the delam-ination becomes more severe as more moisture enters intothe package but it cannot diffuse into the silicone gel due tomoisture saturation in the silicone [3], and thus it can onlyfill the gap between the encapsulant and the dice surface andresults in more severe delamination of the encapsulant polymermaterial from the dice surface.


Fig. 16. EDS spectrum of Part B at 10 KV, showing that it is likely to be thephosphor coating.

TABLE VIIIFORWARD VOLTAGE DROP (V) OF LEDs AT 300 mA

FOR THE DEGRADED UNITS

The presence of the water layer between the dice and theencapsulant caused more scattering of the light from the dice,and may also cause internal reflection of the light from the dicedue to the change in the refractive index of the water layerand encapsulant, both results in further reduction in the lumenoutput as observed from 10%–15% degradation under 95% RHas shown in Table I.

Another possible degradation mechanism from 10% degra-dation level onward is likely the delamination of the die attachdue to the increases in von Mises stress at the interface betweenthe die attach and the dice as the result of the evolution ofhygroscopic stress induced by moisture absorption with testtime as indicated by Tan et al. [15]. In fact, Hu et al. [23]showed that conditions of 85 ◦C/85%RH for 24 hours can ini-tiate delamination at the die attach and a subsequent increasesin the thermal resistance from chip to package. This rendersan increase in their junction temperatures, and decreases theirlumen outputs.

As the forward voltage (Vf) of the LEDs at a given forwardcurrent decreases with increasing junction temperature, wemeasured the Vf at 300 mA for the degraded units that weexamined in detail in this work as shown in Table VIII. Themeasurements are done with the ambient temperature keep at30 ◦C with the use of thermo-electric cooler. The Vf values inTable VIII are the measured Vf at 300 mA minus the voltagedrop due to the series resistance as obtained in our earlier cal-

culations, and thus they represent the actual Vf of the junctions.Values at time zero is not included as the initial measurementwas not done with keeping the ambient temperature constant.

We can indeed see that, for units under 95% RH and 85 ◦C,their Vf are decreasing from 10% degradation onward (as inthe shaded cells), correspond to the increasing in their junctiontemperatures, indicate an increase in the thermal resistancethat is likely due to the die attach delamination. However, theincrease in Vf from 5% to 10% and even to 15% for the unitsunder 70%RH remain to be explored.

IV. CONCLUSION

In this paper, we presented a method to identify the time evo-lution of degradation physics in LEDs under high temperature-humidity condition as their lumen degrade, using OsramGolden Dragon Plus LEDs. At 95% RH and 85 ◦C, we believedthat the degradation proceeds from kirkendall voids formationand bondpad contamination, galvanic dissolution of Ag tophosphor dissolution, encapsulant delamination and die-attachdelamination. More concrete experimental evidences such asthe use of scanning acoustic microscope are needed to con-firm the latter delamination. Unfortunately, this was not donebefore the decapsulation.

A clear conclusion from our analysis is that the degradationmechanisms of LEDs at 95%, 85% and 70% are completelydifferent under temperature-humidity tests, and this rendersthe inappropriate use of the standard 85%RH at 85 ◦C datafor lifetime extrapolation, as in the case of integrated circuit.The degradation mechanisms of the LEDs under the hightemperature-humidity tests are also very different from that ofintegrated circuit. Therefore, there is a strong need for a newhumidity model in order to estimate the lifetime of high powerLEDs under humid operating condition.

As the temperature will also affect the kirkendall voidsformation and thus the weakening of the bond, temperature-humidity tests at different temperatures might result in differentdegradation mechanisms which will be reported in our futurework.

ACKNOWLEDGMENT

The authors would like to thank Luminous Research Centrein EEE, NTU for the provision of the optical characterizationequipment. Thank is also extended to Shuai Zhang for hiscontribution in helping up in the SEM and EDS analysis.Thanks also to Amity University, India for providing the secondauthor to come to Singapore to perform the work.

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Cher Ming Tan (M’84–SM’99) received the Ph.D.degree in electrical engineering from the Universityof Toronto, Toronto, ON, Canada, in 1992. He hasten years of working experiences in reliability in theelectronic industry (both in Singapore and Taiwan)before joining Nanyang Technological University(NTU), Singapore, where he has been a FacultyMember since in 1996. He has published more than200 international journal and conference papers andholds eight patents and one copyright for reliabilitysoftware. He has given two keynote talks and many

invited talks in international conferences. He has written three books and threebook chapters in the field of reliability. His research interests include reliabilityand failure physics modeling of electronic components and systems, finite-element modeling of material degradation, statistical modeling of engineeringsystems, nanomaterial and device reliability, and prognosis and health manage-ment of engineering system. He was the Chair of IEEE Singapore Section, aSenior Member of the American Society for Quality, a Distinguished Lecturerof the IEEE Electron Devices Society on reliability, the Founding Chair of IEEENanotechnology Chapter—Singapore Section, a Fellow of The Institution ofEngineers Singapore, a Fellow and an Executive Council Member of SingaporeQuality Institute, the Director of SIMTech-NTU Reliability Laboratory, and aSenior Scientist with SIMTech. He is an Editor of the IEEE TRANSACTIONS

ON DEVICE AND MATERIALS RELIABILITY; an Associate Editor of theInternational Journal on Computing; and a Guest Editor of the InternationalJournal of Nanotechnology, Nanoscale Research Letters, and MicroelectronicReliability. He is also the Series Editor of SpringerBriefs in Reliability.

Preetpal Singh was born in Jammu and Kashmir,India, in 1989. He received the B.S. degree fromGuru Nanak Engineering College, Hyderabad, India,in 2007, and the M.S. degree from Amity Univer-sity, Noida, India, in 2013. He is currently work-ing toward the Ph.D. degree in the Department ofElectronic Engineering and the Semiconductor Lab-oratory, Chang Gung University, Taoyuan, Taiwan.His research interests include graphene-based high-power LEDs.

time evolution degradation physics in high power white leds under high temperature-humidity...

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