Microelectronics Reliability 52 (2012) 698–703

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Microelectronics Reliability

journal homepage: www.elsevier .com/locate /microrel

Analysis of thermal characteristics and mechanism of degradation of flip-chiphigh power LEDs

Chien-Ping Wang a, Tzung-Te Chen a, Han-Kuei Fu a, Tien-Li Chang b,⇑, Pei-Ting Chou a, Mu-Tao Chu a

a Electronics and Optoelectronics Research Laboratories, ITRI, Chutung, Hsinchu, Taiwanb Department of Mechatronic Technology, National Taiwan Normal University, Taipei, Taiwan

a r t i c l e i n f o

Article history:Received 9 February 2011Received in revised form 26 October 2011Accepted 14 November 2011Available online 26 December 2011

0026-2714/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.microrel.2011.11.009

⇑ Corresponding author. Tel.: +886 2 77343518; faxE-mail address: tlchang@ntnu.edu.tw (T.-L. Chang

a b s t r a c t

The purpose of this study is to investigate the thermal behavior at the die-attached interfaces of flip-chipGaN high-power light emitting diodes (LEDs) using a combination of theoretical and experimental anal-yses. The results indicate that contact thermal resistance increased dramatically at the die-attached inter-faces with aging time and stress, degrading the luminous flux. The junction temperature and thermaluniformity of the flip-chip structure both strongly depend on the arrangement of gold bumps. Localhot spots effectively reduce light output under high electric and thermal stress, influencing the long-termperformance of the LED device. The results were validated using finite element analysis and in experi-ments using an infrared and an emission microscope. A two-step thermal transient degradation modewas identified under various aging stresses. A simulation further optimized the bump configuration thatwas associated to yield a low junction temperature and high temperature uniformity of the LED chip.Accordingly, the results are helpful in enhancing the performance and reliability of high-power LEDs.

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1. Introduction

GaN-based high-power LEDs have attracted much interest asthey have a wide range of illumination applications, including in-door lighting, automobile lighting and street lighting. The limita-tions on their internal and external quantum efficiency can causeheat to be generated in a LED chip. Both light output performanceand the lifetime of LEDs are well known both to depend on thejunction temperature. However, most heat is dissipated by heatconduction. Therefore, thermal conduction is a severe problemfor high-power LEDs. The thermal resistance characteristics ofLEDs have been studied [1–3]. The thermal resistance coefficientof LED varies as a function of ambient temperature, orientation, in-put power and moisture content. The thermal design of LED pack-ages has also attracted much interest. Various related works havesought to reduce the operating temperature of the LED chip usinga heat pipe [4,5]. The die-attached characteristics of a high-powerLED package have been examined by structure function analysis[6]. The package with Au/Sn bonding has a lower thermal resis-tance than both solder paste and Ag paste. Voids may be formedor delamination may occur at the die-attached interface. An R–Cnetwork approach was adopted to formulate the thermal conduc-tion behavior and thermal impedance of the LEDs. These resultsindicate that the package with Au/Sn bonding has a lower thermalresistance than solder and Ag paste. A more comprehensive review

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: +886 23583074.).

about the relationship between failure causes and mechanisms ofLEDs was also discussed [7].

In this investigation, thermal measurements and a theoreticalanalysis were utilized to design a highly reliable package for flip-chip high-power LEDs. The transient degradation of thermalcontact at die-attached interfaces under various aging stresseswas investigated. Additionally, optical, electric, and thermal analy-ses were conducted to verify the complicated thermal degradationmechanisms. Local hot spots that formed under a poor bump con-figuration easily damaged the LED chip during long-term agingtests. In this study, the reliability of high-power LEDs wasimproved by achieving high thermal uniformity and a low junctiontemperature.

2. Experiments and simulation

A GaN-based blue LED chip with an area of 1 mm2 was placedon a silicon substrate with 4 � 4 gold bumps at the first die-at-tached layer, as shown in Fig. 1. Au–Sn was used as the seconddie-attached material between the silicon substrate and the copperslug in the package. The silicone lenses were used to against atmo-spheric impurities and increase the light extraction efficiency fromthe LED chip. A total of 20 high-power LEDs were aged under var-ious aging stresses of 0.35 A/25 �C, 0.7 A/55 �C, 1 A/55 �C, and 1 A/85 �C. Many commercial high power LEDs could be driven undermaximum rating current 1 A and maximum junction temperature150 �C. In order to avoid thermal overstress on the LED devices un-der 1 A/85 �C and 1 A/55 �C, a big copper slug was designed in the

GaN chip

Au-Sn

Copper slug Thermal grease

Aluminum

Sapphire

Au bump

Silicon

Fig. 1. Schematic diagram of the high power LED package.

Table 1Properties of materials that were used in the present simulation [18].

GaN Gold Sapphire Cu slug

Density (g/cm3) 6.1 19.3 3.98 8.96Specific heat (kJ/kg �C) 0.49 0.129 0.76 0.985Thermal conductivity (W/m K) 130 317 41.9 385

0 200 400 600 800 1000Time (hour)

0.92

0.96

1

Rel

ativ

e lig

ht o

utpu

t

0.35A, 25 oC0.7A, 55 oC1A, 55 oC1A, 85 oC

Fig. 2. Transient degradation of the relative luminous flux of LED samples undervarious aging stresses after 814 h.

-5 -3 -1 1 3Voltage (V)

1E-012

1E-010

1E-008

1E-006

0.0001

0.01

1

Cur

rent

(A)

origin0.35A, 25oC0.7A, 55oC1A, 55oC1A, 85oC

Fig. 3. I–V characteristics under various aging stresses after 814 h under 25 �C.

C.-P. Wang et al. / Microelectronics Reliability 52 (2012) 698–703 699

oven to supplied sufficient heat dissipation capability for the LEDs.The LEDs were placed in a temperature-controlled oven to keepthem at constant temperatures. The thermal transient characteris-tics were measured using a thermal transient tester (MicReD Ltd.).The transient thermal response during cooling was recorded toidentify the heat conduction path. The thermal impedance of eachlayer of the LED package was identified using the structure func-tion, which specifies the correlation between heat capacitanceand thermal resistance. The differential structure function yieldedinformation for identifying the interfaces in the LED package) [8–11].

Kth ¼dCth

dRthð1Þ

where Cth represents the heat capacitance; Rth denotes the thermalresistance, and Kth is the differential structure function.

A numerical simulation was carried out using the finite elementmethod to predict the temperature distribution from the LED chipto the aluminum plate. The simulation was performed using thecommercial software, CF design. The heat transfer inside the LEDdevice is described by the energy equation [12].

qCP@T@tþ~m � rT

� �¼ kr2T þ Q m ð2Þ

where q represents density; ~m is the velocity, and Cp represents theheat capacity under constant pressure. On the right-hand side of Eq.(2), Qm denotes the amount of heat that was produced in the LEDchip. During the aging process, LED devices were placed on a hugecopper plate in the oven. The air flow inside the oven was designedto pass through the channel under the copper plate, such that heatconvection effect of the LED was assumed to be negligible in thesimulation. In the oven, constant temperatures of 25 �C, 55 �C, and85 �C were maintained using three driving currents, 350 mA,700 mA and 1000 mA. In the simulation, the bottom of the alumi-num plate of LED device was considered to be at the temperatureof the environment. Table 1 presents the material properties thatwere used in the finite element analysis.

3. Results and discussion

Various aging stresses were applied to accelerate the aging pro-cess and to investigate the long-term degradation mechanisms of

flip-chip high power LEDs. As presented in Fig. 2, the average lumi-nous flux of the LED samples was normalized to the initial value.The experimental results reveal that the degradation rate clearlyincreased with ambient temperature and driving current. At 1 A/85 �C, 1 A/55 �C and 0.7 A/55 �C, during more than 800 h of aging,the light output of the LED samples decreased by 6.6%, 4.5% and3%, respectively. The degradation of the light output of the LEDscan be treated as a combined effect of electric and thermal stress.Under normal stress, 0.35 A/25 �C, the luminous flux slightly in-creased. Some relevant investigations have shown that one possi-ble mechanism involves a reduction in the defect concentration[13,14]. Fig. 3 demonstrates that the leakage current at forwardand reverse bias were increased with aging stress. After more than800 h of aging stress at 1 A, 85 �C, the leakage current was morethan one and three orders of magnitude greater than that undernormal aging stress at �5 V and 1 V, respectively. The leakage cur-rent can be treated as tunneling current that flows through multi-quantum wells along conduction paths that are formed by defectgeneration [15].

The thermal transient characteristics of the LEDs were mea-sured using a T3Ster apparatus. During the measurement, a ther-moelectric device was used to maintain the temperature of the

0 2 4 6 8 10 12 14 16

1e-4

0.01

1

100

10000

0.6

1.7

Kth

[W]

Rth [K/W]

0.35A, 25

0.7 A, 55

1A, 55 1A, 85

0 2 4 6 8 10 12 14 16

1e-4

0.01

1

100

10000

T3Ster Master: differential structure function (s)

0.6

1.7

Kth

[W2 s/

K2 ]

Rth [K/W]

0.35A, 25ºC

0.7 A, 55ºC

1A, 55ºC 1A, 85ºC

Fig. 4. Thermal resistance variations under different aging stresses after 814 h.

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LEDs at 25 �C. Fig. 4 plots the thermal resistance under variousaging stresses after 814 h. Under the condition 0.35 A, 25 �C, thethermal resistance did not exhibit obvious changes with initialstate. The thermal resistance at the first and second die-attachedlayers increased by 0.6 �C/W and 1.1 �C/W, respectively, as theaging stress was increased from 0.35 A, 25 �C to 1 A, 85 �C. Theincrease in the thermal resistance of the first and second die-attached layer was almost 60% and 21%, respectively. The deterio-ration of thermal conductance substantially increases the junctiontemperature after long-term operation, and reduces the lightefficiency and lifetime of LEDs. A high junction temperature andthermal stress can cause the accumulation of vacancies anddelamination, increasing the contact thermal resistance at thedie-attached interfaces [16]. The scanning electron microscopic(SEM) images in Fig. 5a and b show voids and delaminations inthe first and second die-attached layers after an 814 h-long agingtest under 1 A, 85 �C. In contrast, Fig. 5c and d exhibit perfect bond-ing at these two die-attached interfaces for the initial sample.Fig. 5a and c show the upper and bottom interface betweenAu–Sn and gold layer, respectively. The thermal conductivity ofAu is four orders of magnitude greater than that of air. Therefore,voids can be regarded as thermal insulated regions along the heat

Fig. 5. (a) Delamination and (b) voids were found at first and second die-attached interfainitial sample before aging test.

conduction path, increasing the thermal impedance from the junc-tion to the environment. Results demonstrate that both lifetimeand efficiency of high-power LEDs were strongly related to thestrength of die adhesion.

In the measurement of thermal transient effects, a one-dimen-sional thermal model was assumed. The microscale geometry ofthe bump configuration made the heat transportation very com-plex. An obvious temperature gradient that was caused by signifi-cant lateral heat spreading has been observed [17]. In Fig. 6a and b,images obtained under an infrared microscope reveal a hot regionclose to the sides and corners of the chip after 814 h of aging at0.35 A, 25 �C and 1 A, 55 �C. During the measurement, LEDs wereplaced on the thermoelectric device to maintain the temperatureat 25 �C at a constant driving current of 350 mA. The temperatureuniformity of the LED chip did not obviously change as the drivingcurrent increased from 0.35 A to 0.7 A at a constant temperature of55 �C. As the aging stress increased to 1 A, 85 �C, a hot spot formedat the bottom right-hand corner of the chip, as shown in Fig. 6c.The highest local chip temperature was 63 �C, which was almost26 �C higher than the temperature of the normal region. Fig. 6dpresents two-step thermal degradation under various aging stres-ses. Firstly, the average chip temperature increased from 0.35 A,25 �C to 1 A, 55 �C as the non-radiative recombination increased.As shown in Fig. 2, electric and thermal stress both influencedthe degradation rate and luminous flux. Secondly, a hot spotformed when the LED chip was aged at 1 A, 85 �C owing to the ex-tremely high current density and high junction temperature. Theemission microscopic (EMMI) images further demonstrated thatthe leakage current occurred around the hot region of the chip,as displayed in Fig. 7a–c. A reverse bias �5 V was applied to theLED device and a silicon detector was used to measure infraredphotons. The reverse leakage current was also found to increasewith aging stress because the tunneling current paths increasedthrough the MQW of the LED chip [15]. The results were consistentwith previously obtained current–voltage characteristics and infra-red thermal images.

Fig. 8a and b show optical and electroluminescence (EL) images,respectively, obtained after an 814 h aging under 1 A, 85 �C. Asshown in the EL image, a black burnt region was present at the

ces under SEM cross-section images after 814 h aging test at 1 A, 85 �C, (c and d) are

0 0.5 1 1.5

Distance (mm)

30

40

50

60

70

Tem

pera

ture

(o C)

0.35A/25oC1A/55oC1A/85oC

(d)

Fig. 6. Infrared thermal images on the LED chip after 814 h aging tests: (a) 0.35 A, 25 �C, (b) 1 A, 55 �C, (c) 1 A, 85 �C, and (d) diagonal temperature profiles across the LED chipunder various aging stresses.

Fig. 7. EMMI images at �5 V under various aging stress after 814 h: (a) 0.35 A, 25 �C, (b) 1 A, 55 �C, and (c) 1 A, 85 �C.

C.-P. Wang et al. / Microelectronics Reliability 52 (2012) 698–703 701

bottom right-hand corner of the chip under a driving current of0.5 mA. This location was the same as that of the hot spot and leak-age current, as displayed in Figs. 6c and 7c. Most of the injectedelectron–hole pairs were converted into phonon vibrations andgenerating a local high heat flux on the LED chip. A theoreticalmodel was then used to analyze how to improve further the lateralspreading of heat and to shorten the heat conduction path from thejunction to the silicon submount of the LED.

In a flip-chip structure, heat must be firstly dissipated throughthe contact area of the gold bumps between the junction and thesilicon submount. Thus, both vertical and lateral heat spreadingcapacities significantly affect the propagation of heat. The configu-rations of bumps with various radii and heights were analyzed toreduce the junction temperature and yield a uniform temperaturedistribution of the LED chip. Fig. 9a schematically depicts the bump

configuration at the first die-attached layer. Each cylindrical bumpwas aligned symmetrically between the LED chip and the siliconsubmount and had a height of 24 lm and a radius 30 lm. As dis-played in Fig. 9b, hot regions were clearly present at the sidesand corners on the LED chip. The numerical simulation yielded re-sults very similar to those obtained experimentally using an infra-red microscope.

A simulation was then conducted to optimize the bump config-uration to minimize the junction temperature and obtain a uni-form temperature distribution in the LED chip. Fig. 10a and bplot the highest chip temperature and maximum temperature dif-ference of the chip as functions of bump radius and height. Thehighest chip temperature increased from 38 �C to 45 �C as thebump height increased from 10 lm to 80 lm, with a fixed bumpradius of 30 lm. The thermal impedance was increased by the

Fig. 8. (a) Optical image; (b) EL image of the LED chip at 1 A, 85 �C after 814 h aging stress.

Fig. 9. (a) Schematic diagram of bump configuration. (b) Temperature distributionon the surface of the LED chip.

5 6 7 8Maximum temperature difference (ºC)

0

20

40

60

80

100

Bum

p he

ight

(µm

)

0

20

40

60

80

Bum

p ra

dius

(µm

)

Bump heightBump radius

(a)

(b)

Fig. 10. (a) The highest chip temperature and (b) maximum temperature differenceof the LED chip under various bump height and radius by numerical simulation.

702 C.-P. Wang et al. / Microelectronics Reliability 52 (2012) 698–703

increase in the thermal conductive distance from the junction tothe silicon submount. The maximum temperature difference onthe chip was also increased from 5.7 �C to 7.6 �C because of theinefficiency of the lateral spreading of heat. For a fixed bumpheight of 24 lm, as the bump radius increased from 30 to 60 lm,the highest chip temperature was reduced by more than 6 �C.The maximum temperature difference of the chip was also reducedfrom 6.6 �C to 5.2 �C. The increase in the contact area between thechip and the silicon submount dramatically enhanced the three-dimensional heat propagation and decreased the conductive resis-tance of the LED package. The results demonstrate that changingthe bump radius can change the junction temperature more thancan change the bump height of the flip-chip LEDs.

4. Conclusion

Combined experimental and numerical analyses provide insightinto the thermal characteristics and mechanisms of degradation ofthe flip-chip high power LEDs. The results demonstrate that the

C.-P. Wang et al. / Microelectronics Reliability 52 (2012) 698–703 703

increase in the contact thermal resistance at the die-attachedinterface contributed greatly to the lifetime of the LEDs. A highelectric and thermal stress can cause the accumulation of vacanciesand delamination, increasing the thermal conductive impedancefrom the junction to the environment. The junction temperatureand thermal non-uniformity of a flip-chip structure both stronglydepend on the configuration of gold bumps, which can form localhot spots and easily damage the LED chip during a long-term aging.Simulation results were verified by performing experiments inwhich infrared and emission microscopes were used. A two-stepthermal transient degradation mode was obtained under variousaging stresses. The simulation optimized the bump configurationthat could be used in designing an LED device with a low junctiontemperature and a highly uniform temperature. The present studypresented a favorable solution for the design of highly reliable highpower flip-chip LEDs.

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