thermal analysis and design of gan-based leds for high power applications

phys. stat. sol. (c) 0, No. 7, 2261–2264 (2003) / DOI 10.1002/pssc.200303466

© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Thermal analysis and design of GaN-based LEDs for high power applications

L. Kim1, G. W. Lee1, W. J. Hwang1, J. S. Yang2, and M. W. Shin*, 1 1 Semiconductor Materials/Devices Laboratory, Department of Ceramic Engineering,

Myong Ji University, Korea 38-2 Yongin, Kyunggi, Korea 449-728 2 Electric and Electronics Division, Korea Industries Technology Evaluation & Planning, 701-7 Kangnam,

Yeoksam, Seoul, Korea

Received 5 May 2003, revised 14 June 2003, accepted 13 August 2003 Published online 20 October 2003

PACS 73.61.Ey, 81.70. Pg, 85.60.Jb

In this paper, a thermal analysis was made on blue and white GaN-based LEDs. The thermal analysis con-sists of experimental temperature measurements and finite element calculations on the LED chips and sur-face of epoxy package. The direct on-chip temperature measurement using a nematic liquid crystal re-sulted in a hot spot with a transition boundary of 43 °C in a range of about 450 to 500 µm under a forward voltage of 3.9 V. The surface temperature of the epoxy package was measured as a function of input power and it exhibits a linear relationship. The finite element method was used for the calculation of tem-perature distributions for samples and the simulated data showed good agreement with the experimental results.


1 Introduction Wide-band gap compound semiconductors using III–V materials for blue-green light emitting diodes (LEDs) have been subjected to intensive research in recent years and been commercial-ized for several years. White light for illumination now can be produced from the LEDs either by combi-nation of red, green, and blue emitting chips in one lamp, or by integration of the blue InGaN LED die and the YAG phosphors into a single package [1, 2]. It has been expected that very high power white LEDs can replace standard incandescent bulbs used in many applications due to lower energy consump-tion of LEDs. One of major factors needed for the realization of the white LEDs into such an illumina-tion source is an effective thermal management during the high power operation. Thermal management is critical in the design of LED signal lamps because temperature affects the performance and reliability of LEDs. In particular, despite of the excellent optical properties of GaN, the inherently poor thermal

property of the sapphire used as a substrate material in these devices may lead to thermal degradation of

devices during their high power operation. Therefore, dependable thermal design by an accurate thermal

measurement and modeling for the high power GaN LEDs are necessary in both chip and packaging

scale. There have been several reports of experiments on reliability issues of high power GaN-based LEDs [3, 4], but most of reports are on packaged LED lamps, not on the chip scale. In addition, it is hard to find out reports of LEDs on the thermal modeling of data compared with the results of experimental measurements on the chip scale. In this report, we present a thermal analysis for the GaN-based LEDs. The analysis consists of direct chip thermal measurement using a thermogrphic method that employs a nematic liquid crystal and the finite element method done using ANSYS, a well established commercial simulation program. The experimentally obtained results were compared with the results from the simu-

* Corresponding author: e-mail: [email protected], Phone: +82 31 330 6465, Fax: +82 31 330 6465

2262 L. Kim et al.: Thermal analysis and design of GaN-based LEDs for high power applications


lated results. A proto type flip chip die was fabricated for thermal analysis and was compared with the data obtained from a normal LED chip without a flip chip design. 2 Thermal measurement and simulation The LED chips investigated include blue InGaN LEDs and white LEDs with YAG phosphors onto blue LED chips. Thermal measurement was done by the thermo-graphic method using a nematic liquid crystal that was coated on top of the investigated LED chips. LED chips packaged in baskets and lead frames were carefully separated each other and were coated by epoxy resin so that the wire bonding was protected from the stress during the separation process. Each package was loaded onto a probe station and was connected to a power source. After loading, the nematic liquid crystal with specific order-disorder transition temperature was coated on the LED chip. Linearly polar-ized light is required to examine the clearing point of the liquid crystal by viewing it through a crossed polarizer [5]. Apart from the temperature analysis on the chip, the temperature on the surface of the ep-oxy package was measured by a thermocouple during the DC operation of LEDs. Transient finite ele-ment simulations were performed using ANSYS to model the thermal rise through the LED chip and epoxy package. The heat flow was assumed to be made by conduction between the heat source (chip) and epoxy resin and convection from the package to air. The heat flux due to conduction is given by Fourier’s law of conduction. The heat flow rate per unit area in a specific direction is expressed as

*nn

Tq K

n

∂= −

∂ where nnK is the thermal conductivity in direction n , T is the temperature and

T

n

∂

∂ is the

thermal gradient in direction n . The heat flux due to convection is given by Newton’s law of cooling and more details can be found elsewhere [6]. Convection is applied as a surface boundary condition during the simulation. The effect of heat transfer by convection becomes important when the shape of the heat sink and the method of external cooling are the main design parameters in LEDs. 3 Results and discussion Figure 1 shows the white LED chip under a DC bias of 3.9 V. The chip was fabricated by coating of yellow YAG phosphors on a top blue InGaN LED chip. Note that the chip is housed in a basket and attached to wire bonding and lead frame, but it was not packaged with epoxy resin. Therefore, thermal dissipation from the chip is partially made through the metallic component and air. The sample was painted by a liquid crystal with an order–disorder transition temperature of 43 °C. A hot spot was not observed before polarization (Fig. 1a). But, a distinctive hot spot appeared around the chip after polarization (Fig. 1b). From the size of the chip, the size of the hot spot was estimated to be in a range 450 to 500 µm. The temperatures inside the hot spot are in a range of 43 °C or above, but the boundary temperature is 43 °C. Similar behavior was observed for the blue InGaN chip without coated

a) b)

Fig. 1 (online colour at: www.interscience.wiley.com) White LED with a liquid crystal painted on and under the bias of 3. 9 V and the current is 30 mA. No hot spot with a transition temperature of 43 °C ob-served before polarization (a), but a distinctive hot spot around the chip appeared after polarization (b).

phys. stat. sol. (c) 0, No. 7 (2003) / www.physica-status-solidi.com 2263


0.00 0.05 0 .10 0.1 5 0.2 0 0.25 0 .30

2 90

2 95

3 00

3 05

3 10

3 15

3 20

Top

Sur

face

Tem

pera

ture

(K)

In p u t Po w er(W )

B lu e L ED (N o n -Flip C h ip )B lu e L ED (F lip C h ip )W h ite L ED

a) b)

Fig. 2 (online colour at: www.interscience.wiley.com) Temperature of the top surface of the epoxy package for different LEDs (a) and a photography of the blue LED chip with flip chip design in a structure of InGaN chip/Pb–Sn solder/Al heat sink/Si substrate. YAG phosphors. The coating of phosphors did not make a significant effect on the range of temperature rise in our experimental routes. However, a blue LED with a flip chip design (Fig. 2b) did not show any hot spot exhibiting any region with temperature of or above 43 °C. For a detailed mapping of the temperature distribution on the LEDs more liquid crystals with different transition temperatures are needed and the process is on the way. For a better understanding of the ther-mal behavior of LEDs, the examined sample was packaged by epoxy resin and the temperatures on the top surface of epoxy resin were measured during the operation. Figure 2a shows temperatures on the top surfaces of several LEDs including blue InGaN LED without flip chip design, blue InGaN LED in a flip chip design with a structure of InGaN chip/Pb–Sn solder/Al palte/Si (Fig. 2b), and white LED as de-scribed above. The atmospheric temperature is 290 K. The temperatures on the top surfaces of the epoxy packaging are similar for the three investigated structures. In particular, the temperature difference be-tween the flip chip and non-flip chip design was negligible. The blue LED chip with a flip chip design was inserted into a basket (little bigger than the basket used in the blue LED lamp without flip chip de-sign) and has the same epoxy-packaged structure and a lead frame as the other samples. With the results of direct on-chip thermal measurement, Figure 2a implies that effective thermal dissipation can not be achieved without an employment of an extra heat sink design in lead frame. The chip with the thin Al heat sink on the silicon substrate was thermally efficient when the chip was exposed in air, but they were found out to suffer from self-heating when they were in a closed system by the encapsulated epoxy resin. Our finite element simulation was performed to investigate the temperature profiles of the LED chip and packaged structure. The simulations include the thermal conductivity, one of the most important material properties for thermal analysis and device parameters. Thermal conductivities for GaN, sapphire, silver epoxy, and polymer epoxy were assumed (in W/m K) to be 130, 28, 7.5, and 1.0, respectively. The input parameter was taken as an input power (IF × VF) during the simulation. Figure 3a comares the simulated temperature of the top surface of the epoxy package as a function of the input power with the measured data for the blue LEDs. It is shown that the finite element method is very useful to predict the tempera-ture profile of LEDs. Figure 3b presents the simulated temperature distribution of packaged blue LEDs across the surface of the epoxy package and lead frame (top) and of the blue LED chip inside (bottom) at the input power of 0.1 W. The temperature of the chip itself is calculated to be as high as 333 K (60 °C). Compared to the results from the on-chip thermal measurement as is shown in Fig. 1b, the simulated results agree well with the experimental data. The hot spot with a boundary temperature of 43 °C was

expanded in about 450 to 500 µm and it is clear that the center of the hot spot is warmer than 43 °C.

More quantitative analysis will be performed with various liquid crystals with different transition tem-

peratures.

2264 L. Kim et al.: Thermal analysis and design of GaN-based LEDs for high power applications


0.00 0.05 0.10 0.15 0.20 0.25 0.30

290

295

300

305

310

315

320

Top

Sur

face

Tem

pera

ture

(K)

Input P ow er(W )

E xperim entsim ulation

a) b)

Fig. 3 (online colour at: www.interscience.wiley.com) Comparison between the simulated and measured tempera-ture of the top surface of the epoxy package for the blue LED (a) and the calculated temperature profile across the packaged LED (b-top) and in the chip (b-bottom). 4 Conclusion Thermal analysis including experimental measurement and finite element calculations on blue LEDs (flip chip design and non-flip chip design) and white LEDs were investigated and was demonstrated to be useful for the thermal design of GaN-based LEDs.

Acknowledgment This work was supported by Grant No. (R01-2002-000-00356-0) from the Basic Research Program of the Korea Science and Engineering Program. The work was also partially supported by the grants from the Korea Energy Management Corporation (KEMC) and the Korea Institute of Industry Technology Evaluation & Planning (ITEP).

References

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33, 970 (1997). [4] D. L. Barton, M. Osin’ski, P. Perlin, C. J. Helms, and N. H. Berg, Proc. 5th Ann. IEEE Inter. Reliability Physics

Symposium, Denver, CO, 1977, p. 276. [5] H. Lin, M. Khan, and T. Giao, Proc. of the 20th Int. Symposium. for Testing and Failure Analysis, (1994), p. 81. [6] User’s Manual of ANSYS, Theory of Thermal Modeling, 2001.

thermal analysis and design of gan-based leds for high power applications

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