IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 10, OCTOBER 2015
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The Study of Thermal Resistance Measurement of Multichip LED Shang-Ping Ying, Han-Kuei Fu, and Rong-Ci Hong
Abstract— The high-power light-emitting diode (LED) lighting is especially essential in the high-temperature applications such as the street and automotive lighting. However, >70% of electrical power is converted into heat due to the low electrical-optical conversion efficiency. Because of the characteristics of semiconductor, the electrical property of LEDs sensitively varies with the operating temperature. These deviations affect the measurement of light and electricity. In this paper, the steady-state thermal measurement techniques developed by the National Institute of Standards and Technology are used to study the thermal resistances of multichip LEDs. The experimental results show that the measurements of thermal resistances with different connections are constant at the same power dissipation. The simulation results show that the detail of thermal interaction between powered and unpowered chips and confirm the experiment results. Index Terms— Light-emitting diode (LED), thermal engineering, thermal measurement, thermal resistance.
I. I NTRODUCTION
I
N RECENT years, the demand for high-power brightness light-emitting diode (LED) has increased gradually because of the advantages such as long life time, high reliability, robust to vibration shocks, environmentally friendly, low power consumption, and small size. With the rapid improvement in efficiency and luminance, LEDs have been used as the backlight of liquid crystal display and sign applications, and have potential for applications in automotive and general lighting. However, the increase of the luminous flux and input power would lead to high junction temperature and temperature gradient. The thermal management of LEDs is essential to dissipate heat from the chip and limits junction temperature rise, which causes the degradation of electrical and optical performances as well as the low reliability owing to high thermal stresses and strains [1]–[4]. The precise measurement for the junction temperature in LEDs is critical to the applications of high-power LEDs. In general, it is difficult to measure the junction temperature of an LED chip directly. Several approaches such as electroluminescence, emission peak ratio through spectral measurements Manuscript received February 16, 2015; accepted July 21, 2015. Date of publication August 17, 2015; date of current version September 18, 2015. The review of this paper was arranged by Editor R. Venkatasubramanian. (Corresponding author: Han-Kuei Fu.) S.-P. Ying and R.-C. Hong are with the Department of Opto-Electronic System Engineering, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan (e-mail:
[email protected]; nnn820328@ hotmail.com). H.-K. Fu is with Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2460741
for phosphor converter LEDs, infrared, Raman spectroscopy are available for junction temperature measurement [5]–[8]. The electrical method is typically employed to determine the LED junction temperature because of easy operation and high accuracy [9]. The linear relationship between the forward voltage of the diode and the junction temperature allows achieving the junction temperature of LEDs in practical applications. Moreover, the thermal resistance measurement system can be used to estimate the junction temperature of LED after thermal resistance measurement. The thermal resistance is determined by the difference in the temperature between the junction and the reference point under the power dissipation. Based on the structure function of the LED package, the thermal transient measurement method could be used to capture the voltage change during the process of heating or cooling to derive the thermal resistance and thermal capacitance [10]–[15]. However, it is not easy for the LED manufactories to purchase and maintain the thermal transient measurement instrument owing to its high price. In order to provide an accurate, reproducible, and uncomplicated measurement and estimate the thermal characteristic of LEDs, the researchers at the National Institute of Standards and Technology (NIST) proposed a new method to obtain the LED junction temperature and thermal resistance. The junction temperature and thermal resistance are determined using a temperature-controlled heat sink, a pulse current source, and a fast voltage meter to measure the forward voltage of the LED through simple process [16]. For the general LED illumination system, there are two methods to achieve the high output power. One method is the integration of multiple high-power LEDs into one module or array, and the other one is multiple chips integrated into single package named multichip LED. In general, multichip LED is preferred, owing to the compact size and ease of assembly into lighting fixtures. However, for the multichip LED with high input power, the efficient thermal dissipation is the key challenge for the use of multichip LED. Hence, the estimation of the junction temperature or thermal resistance of the multichip LED would be important. Although a great majority of studies are focused on the thermal management and thermal resistance measurement of LEDs with single chip, only few studies attempt to investigate on the thermal performance, such as the thermal resistance and the junction temperature of the multichip LED. The thermal transient measurement of high-power LEDs with multichip designs has been demonstrated in [17]. The thermal resistances from junction to ambient considering for the one-chip, two-chip, and four-chip packages were derived from the cumulative and differential structure functions. The total thermal resistance of
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multichip packages decreased with the number of chips, due to parallel heat dissipation. The thermal transient measurement was also used to verify the heat dispassion for high-power LEDs array module with cup-shaped copper sheets using an electroplating technique [18]. In this paper, the thermal resistance of multichip LED was obtained from the method proposed by NIST. The thermal resistances of the four-chip LED with different connections were conducted. The thermal resistance between the LED chip and the heat sink surface of the four-chip LED could be determined under the thermal equilibrium conditions. The verification of thermal resistance of the four-chip LED obtained from the new method was carried out with the 3-D finite-element model. The simulation results from the experimental and numerical studies will be presented in this paper. Fig. 1. Photograph of the experimental setup with the temperaturecontrollable heat sink on the left-hand side of integrating sphere.
II. E XPERIMENTS AND S IMULATIONS Thermal resistance is the obstruct effect of heat conduction. The reference comes from the resistance concepts of electronic circuitry. Heat flow corresponds to current. Thermal resistance corresponds to electric resistance. Heat capacity corresponds to capacitor. Therefore, the theoretical calculation of the electronic circuitry can be used to simplify the 1-D heat transfer calculation. Thermal resistance is defined as two of the temperature difference divided by the heat power. Simply under the same heat power, the higher temperature difference corresponds to the higher value of thermal resistance. The thermal resistance of a single-chip package can be defined as Rth j R = (T j − TREF )/P
(1)
where T j is the junction temperature, TREF is a reference temperature, and P is the heat power dissipation from the chip. The reference temperature can be ambient (Tamb ), the package case (Tc ), and the metal-core printed circuit board (TB ). The approach to the evaluation of thermal resistance of multichip LED should be modified when dealing with multichip in single package. There are two different approaches to predict the thermal resistance of multichip device packages [19]. One approach is to consider the average junction-to-ambient thermal resistance Rthja-avg = (T j,avg − Tamb )/P
(2)
where T j,avg is the average junction temperature of multichip package, Tamb is the ambient temperature, and P is the total heat power dissipation of multichip package. However, Rthja-ave can be applied only when the chips with identical geometry and power dissipation. It cannot provide the accurate junction temperature of each chip for different powered chips in the single package. The other approach is the junction-to-ambient thermal resistance on the basis of chip location Rthja-i = (T j,i − Tamb )/Pi
(3)
where T j,i is the junction temperature of i th chip, and Pi is the heat power dissipation of i th chip. Equation (3) is not valid for the unpowered chips because the Pi for the unpowered
chip is zero. The junction temperature of the unpowered chip would still increase, owing to the heating up from the neighbor chips. The thermal resistance, Rthja-i , will be infinite, because the Pi is zero [19]. In this paper, (3) should be modified as Rthja-i = (T j,i − Tamb )/P
(4)
where T j,i is the junction temperature of i th chip and P is the total heat power dissipation of the multichip package. The new method to obtain accurate, reproducible, and comparable measurements of the LED junction temperature and thermal resistance was proposed by NIST [16]. For a brief description, the LED mounted on a temperature-controlled heat sink was set to the desired LED junction temperature between 10 °C and 100 °C. After the multiple short pulses of a specified current were applied to the LED and the voltage flow across the junction was measured, the LED was operated at the specified dc current, and the temperature of the heat sink was set to ensure that the voltage remains constant. Therefore, the junction temperature and the thermal resistance could be obtained. If we consider the power that was not turned into radiant energy, (2) of first approach became Rthja-avg = (T j,avg − Tamb )/(P − Po ).
(5)
Equation (4) of second approach became Rthja-i = (T j,i − Tamb )/(P − Po )
(6)
where Po is the radiant optical power emitted by the LED. The LED package attached to the temperature-controllable heat sink (CDS30012RRA; Wise Life Technology) was placed in the input port of a 30-cm-diameter integrating sphere to get the optical power, Po , as shown in Fig. 1. At first, we set the temperature of the heat sink equal to the desired T j (0) and waited for the LED (not turned ON yet) to stabilize thermally. In the second step, we applied multiple short pulses with pulsewidth, 1 ms, and duty cycle, 50%, of the rated current to measure the VF (0) repeatedly for higher accuracy by a pulse mode sourcemeter (2430; Keithley). As the LED was operated on the specified dc current and being
YING et al.: STUDY OF THERMAL RESISTANCE MEASUREMENT OF MULTICHIP LED
Fig. 2.
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Schematic of test processes.
heated up, we adjusted (lowered) the heat sink temperature, Tamb (t), so that the measured VF (t) was equal to VF (0) and thus the same T j (0) was maintained when the LED reached the thermal equilibrium. Using a programmable temperaturecontrolled heat sink, the entire measurement procedures could be fully automated. The temperature difference was defined as T j (0) −Tamb (t). Therefore, the thermal resistance, Rthja-i , could be obtained. The test processes are schematically represented in Fig. 2 [16]. The LEDs used in this paper were the commercial surfacemounted devices packages of 7070. The name of 7070 was defined as the plastic sizes of length, 7 mm, and width, 7 mm. The emission wavelength of the LED is 459 nm. Four commercial 1-W 45-mil high-power LED chips with the same electronic and optical specifications attached on the leadframe were assumed by the wire bonding process. The die-attached material was epoxy. The gap among chips is 0.9 mm. Then, the silicone was encapsulated in the cavity mold and sealed LED chips. Next, LED with encapsulant was heated in an oven at 50 °C for half an hour and at 150 °C for 1 h to cure the silicone. Finally, the test samples for different experimental designs with different connections were obtained, as shown in Fig. 3. In order to understand the physical phenomena of deviation of thermal resistances with different chip powered conditions, the simulations were realized by commercial software, CFDesign, which analyzed the heat flow in materials. Because of the software lacking the 3-D drafting function, the commercial computer aided design software, SolidWorks, was used to draft the models, which then sent to CFDesign to do the simulations and analysis. The simulation model of the samples was as shown in Fig. 4. The encapsulant was composed of several materials, as shown in Table I, from the built database of CFDesign. For the setting parameters of boundary condition, the bottom of thermal grease was set as the desired heat sink temperature. Other LED surfaces were set as convection coefficient, 8 W/m2 · K, for stable surrounding. The input heat source of chip was the electrical power subtracting the optical power, P − Po . III. R ESULTS AND D ISCUSSION The limitations of thermal resistance measurement method are that the heat sink temperature and input current must be constant in all measurements. Therefore, different heat sink
Fig. 3. (a) Parallel connection of four chips. (b) Series connection of four chips. (c) Separate connection of four chips. (d) Cross-sectional schematic of the LED die.
Fig. 4.
Material components of models of samples.
temperatures and input currents may cause the deviation of thermal resistance. Besides, this thermal resistance measurement method can be used accurately with a well conduction path in the encapsulant [20]. In the past papers, the NIST method did not be verified in the measurement of multichip
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 10, OCTOBER 2015
TABLE I T HERMAL C ONDUCTIVITIES OF D IFFERENT M ATERIALS U SED IN F IG . 4
TABLE III T HERMAL R ESISTANCES W ITH O NE C HIP P OWERED
TABLE IV T EMPERATURE D IFFERENCE OF S IMULATION B ETWEEN D IFFERENT P OWERED C HIP C ONDITIONS TABLE II T HERMAL R ESISTANCES OF D IFFERENT C ONNECTIONS W ITH A LL C HIPS P OWERED M EASURED F ROM F IG . 3
thermal resistances. Since the case temperature affects the measurement value of thermal resistance, the experiment sets a case temperature, 55 °C, in this paper. For other case temperatures, the thermal resistances may change values with the same trend studied in [20]. To verify the thermal resistances of multichips obtained from the method proposed by NIST, the parallel connection, series connection, and separate connection with individual measurements were obtained at the same conditions of each chip. In other words, all chips were powered. The conditions of parallel connection were input current, 0.6 A, and case temperature, 55 °C. The conditions of series connection were input current, 0.15 A, and case temperature, 55 °C. The conditions of separate connection were input current, 0.15 A, and case temperature, 55 °C. The experimental results of different connections are shown in Table II. The thermal resistances were defined as (5) for parallel and series connections. The thermal resistances were defined as (6) for separate connection. When using the above equations, it was assumed that the thermal resistance was a time independent constant, independent of how the LED was driven or where it was used. However, the thermal resistance was not a constant and changed with heat power dissipation, ambient temperature, and the amount of external heat sink provided to the LED [20], [21]. The experimental results showed that the measured equivalent thermal resistances were almost the same in different connections. Therefore, the method proposed by NIST was reliable on the thermal resistance measurement of multichip LED. The small deviation of thermal resistances between chip Nos. 1–4 was due to manual processes of die
bond and wire bond and the intrinsic variability of measurement system. Besides, this study was to confirm the simple NIST method to measure the thermal resistance of multichip LED. In order to eliminate the deviations of chip properties causing the deviation of thermal resistances, the emission wavelength, forward voltage, and operation power of chips had to be the same values. However, there were always a few different properties between different chips even in the same bin. These caused the deviation of thermal resistances with different measured chips. Therefore, the thermal resistances measured with only one chip powered based on (6) were different from with all chips powered. Because the thermal resistances were measured from a single chip, the experiment was based on separate connection. The experimental results were as shown in Table III. The deviation of thermal resistances was because of the change of heat power. The junction temperature of measured chip with all chips powered was higher than with one chip powered. The heat power with all chips powered was four times with one chip powered, but the junction temperature rose about two times. Therefore, the thermal resistance decreased about two times. The NIST method could not measure the temperature difference from unpowered chip. In order to realize the physical phenomena between all chips powered and one chip powered, the simulations were performed based on the experiment conditions. The simulation results were as shown in Table IV. According to the experiment condition, the input heat source of each chip, P − Po , was 0.3 W. The temperature
YING et al.: STUDY OF THERMAL RESISTANCE MEASUREMENT OF MULTICHIP LED
of chip No. 1 was measured in different chip powered conditions. The initial temperature was without any chip powered. The final temperature was the simulation results of steady states with powered chip. When the chip No. 3 was powered, the temperature of chip No. 1 increased to 2.3 °C. When the chip Nos. 3 and 4 were powered, the temperature of chip No. 1 increased to 4.85 °C. Further, when the chip Nos. 2, 3, and 4 were powered, the temperature of chip No. 1 increased to 7.51 °C. When the chip No. 1 was individually powered, the temperature of chip No. 1 increased to 6.13 °C. The addition of 7.51 °C and 6.13 °C was 13.64 °C, which was almost the same with the temperature, 14.05 °C, of all chips powered. The simulation result, 14.05 °C, was almost the same with the experimental result, 15.30 °C. The last row of Table IV shows that the four times input power, 1.2 W, increased four times temperature, 25.78 °C. Besides the increased temperature of self-power, the result showed that the chip No. 1 also received the heat power from other powered chips and increased temperature. In summary, the thermal resistances were not constant in different operating conditions. In this paper, the thermal resistances depend on power dissipation. The simulation and the experiment showed that the method proposed by NIST was reliable and consistent in different connection measurements. IV. C ONCLUSION Because of the characteristics of semiconductor, the electrical property of LEDs is sensitively varying with operating temperature. In academia, to identify the explicit dissipating paths and exact thermal resistance is a hard work in the research of LED encapsulant. For industry, the simple definition and measurement of thermal resistance to identify the quality of thermal management are needed. Because academia and the industry have different viewpoints on the subject of thermal resistance, the ambiguous phenomena and argument of thermal resistance are still existence. In this paper, we perform the experiment with different connections of multichip LED and study the simulation of different power dissipations. The experimental results show that the measurements of thermal resistances with different connections are constant at the same power dissipation. The simulation results confirm that the detail of thermal interaction between powered and unpowered chips. In conclusion, the thermal resistance depends on power dissipation and is constant under the same power. Besides, the NIST method is reliable, accurate, reproducible, and uncomplicated on multichip thermal resistance measurement. The more you understand the thermal resistance, the more precise the thermal management you design. This paper provides and confirms the simple NIST method to measure the thermal resistance of multichip LED. R EFERENCES [1] B.-H. Liou, C.-M. Chen, R.-H. Horng, Y.-C. Chiang, and D.-S. Wuu, “Improvement of thermal management of high-power GaN-based light-emitting diodes,” Microelectron. Rel., vol. 52, no. 5, pp. 861–865, May 2012. [2] J. Hu, L. Yang, and M. W. Shin, “Electrical, optical and thermal degradation of high power GaN/InGaN light-emitting diodes,” J. Phys. D, Appl. Phys., vol. 41, no. 3, p. 035107, Feb. 2008.
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Shang-Ping Ying received the Ph.D. degree from the Institute of ElectroOptical Engineering, National Chiao Tung University, Hsinchu, Taiwan. He is currently an Associate Professor with the Department of Opto-Electronic System Engineering, Minghsin University of Science and Technology, Hsinchu, Taiwan.
Han-Kuei Fu received the Ph.D. degree from the Department of Physics, National Taiwan University, Taipei, Taiwan. He is currently with Electronics and Optoelectronics Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan.
Rong-Ci Hong is currently pursuing the bachelor’s degree with the Department of Opto-Electronic System Engineering, Minghsin University of Science and Technology, Hsinchu, Taiwan.
