The Study of Thermal Resistance Deviation of High ... - IEEE Xplore

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 61, NO. 8, AUGUST 2014

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The Study of Thermal Resistance Deviation of High-Power LEDs Shang-Pin Ying, Han-Kuei Fu, Wei-Feng Tang, 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 lighting and automotive lighting. However, there is more than 70% of electrical power converted into heat due to the low electrical–optical conversion efficiency. Because of the characteristics of the semiconductor, the electrical property of LEDs is sensitively varying with 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 deviation of thermal resistance in different electrical and thermal conditions. The consistent trends of simulations and experiments represent that the design of the heat dissipating path and geometry affects the measurement of thermal resistance. Index Terms— Light-emitting diode (LED), thermal engineering, thermal measurement, thermal resistance.

I. I NTRODUCTION

L

IGHT-EMITTING DIODES (LEDs) have been widely used in general lighting due to its advantages in high efficiency and long life. With the advantages of energy saving, high efficiency, long lifetime, small size, and mass production, LED lighting is expected to replace the commonly used incandescent and low-pressure mercury discharge lamps in the future. To satisfy the requirement of high luminous flux for general lighting, the increasing input power used to drive the LEDs leads to high heat generated from LED chips. The junction temperature affects the electro-optical properties of LEDs in terms of light output, spectral shift, and light output degradation over time [1], [2]. Thermal management for the solid-state lighting applications is an important concern for both package and system levels. The main issue of thermal design engineering in LED package is the reduction of junction temperatures. The primary heat conduction for the LED package is the leadframe in the package. Thus, the material, size, and geometry of isolated thermal pad of leadframe that are used in the package of LEDs play a major role in thermal dissipation. It is the major challenge for LED manufactures.

Manuscript received December 16, 2013; revised June 3, 2014; accepted June 9, 2014. Date of publication June 27, 2014; date of current version July 21, 2014. The review of this paper was arranged by Editor R. Venkatasubramanian. (Corresponding author: Han-Kuei Fu.) S.-P. Ying, W.-F. Tang, 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]; [email protected]; [email protected]). H.-K. Fu is with the 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.2014.2330340

As the level of popularity for using LEDs rises, accurate and reliable techniques for measuring the junction temperature and thermal resistance of LEDs are needed for the further applications of high-power LEDs [3], [4]. It is very difficult to measure the junction temperature of LED directly, so the electrical test method is the most widely used for junction temperature measurement. It is a direct, noncontact technique since it utilizes the LED chip itself as the temperature sensor [5], [6]. However, the steady-state dc measurement approach is time consuming and not suitable for the needs of both LED and lighting manufacturers. In present researches, researchers at the National Institute of Standards and Technology (NIST) proposed a new method to obtain accurate, reproducible, and comparable measurements to measure the LED junction temperature and thermal resistance [7]. In this report, thermal resistances obtained from the method proposed by NIST for different heat sink temperature and input current were conducted. The thermal resistance intuitively obtained from the measurement was a constant for a given condition, but there were few studies explaining about how it was affected by parameters, such as desired heat sink temperature and the input current [8]–[12]. The geometry and isolated thermal pad of the leadframes in these commercial packages are different to find the relation between heat dissipation and isolated thermal pad of the leadframe. In the meantime, different parameters, such as heat sink temperature and input current, are varied in the thermal resistance measurement obtained from the method proposed by NIST. The simulations of the measured samples were realized by the commercial software, CFDesign. Comparing the simulations with experiments, the same trends were found and showed that the design of heat dissipating path and geometry varied the measured thermal resistance. These results provided the information of thermal design of LED module. The LED module was the integrated LEDs that needed to consider the design of large heat management. To design the optimal thermal management of LED module, the design of heat dissipating path and geometry should be carefully considered. 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

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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 higher value of thermal resistance. Although the concept of thermal resistance can be greatly simplified by the calculation of the 1-D heat transfer simulation, the heat transfer simulation of LED thermal distribution is a 3-D heat transfer condition. To obtain the accurate simulation of LED thermal distribution of 3-D heat transfer condition, we use the finite element method by commercial software, CFDesign, to do the thermal characteristics of simulation. The new method to obtain accurate, reproducible, and comparable measurements to measure the LED junction temperature and thermal resistance were proposed by NIST [7]. For brief description, the LED mounted on a temperaturecontrolled 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 measuring the voltage flowed across the junction, the LED was operated at the specified dc and the temperature of the heat sink was set to ensure the voltage remains constant. Therefore, the junction temperature and thermal resistance could be obtained. To detail and explain the procedure of calculations and experiments, the objective of this paper was to verify the thermal resistances obtained from the method proposed by NIST with different heat sink temperature and input current. The thermal resistance Rth j c from the junction to case of a high-power LED could be estimated by measuring the junction temperature T j and the case temperature Tc for a given power dissipation Pe . The definition of Rth j c was Rth j c = (T j − Tc )/Pe .

(1)

If we consider the power that was not turned into radiant energy, (1) became Rth j c = (T j − Tc )/(Pe − Po )

(2)

where Po was the radiant optical power emitted by the LED. The LED package attached on 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 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 and being heated up, we adjusted (lowered) the heat sink temperature Tc (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) − Tc (t). Therefore, the thermal resistance Rth j c could be obtained. The test processes were schematic representation, as shown in Fig. 2 [7].

Fig. 1. Photograph of experimental setup is with the temperature-controllable heat sink on the left hand of the integrating sphere.

Fig. 2.

Scheme of test processes.

The LEDs used in this paper were the commercial surface mounted devices packages of 5050, 5630, and 5074. The name of 5050 was defined as the plastic sizes of length, 5 mm, and width, 5 mm. The name of 5630 was defined as the plastic sizes of length, 5.6 mm, and width, 3 mm. The name of 5074 was defined as the plastic sizes of length, 5 mm, and width, 7.4 mm. The small heat dissipating path of 5050 conducted by electric pad was compared with the large heat dissipating path of 5630 conducted by isolated thermal pad with area of 1.5 mm × 2.1 mm and the largest heat dissipating path of 5074 conducted by isolated thermal pad with area of 2.5 mm × 2.5 mm to study the dissipating size and geometry effect of thermal resistances. The commercial 1-W high-power LED chip with 1 mm × 1 mm attached on the leadframe was assumed by the wire bonding process, and the die-attached material was epoxy. 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 hour and at 150 °C for 1 h to cure the silicone. Finally, the test samples were obtained, as shown in Fig. 3. To understand the physical phenomena of deviation of thermal resistance in different 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 that then

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Fig. 3. Front and back photographs of test samples. (a) 5050. (b) 5630. (c) 5074.

sent to CFDesign to do the simulations and analysis. The simulation models of samples 5050, 5630, and 5074 were as shown in Fig. 4. The encapsulant was composed of several materials, as shown in Table I from the built data base of CFDesign. For the setting parameters of boundary condition, the bottom of aluminum base layer was set as the desired heat sink temperature. Other LED surfaces were set as convection coefficient, 15 W/m2 ·K, for air-conditioned surrounding. The input heat source of chip was the electrical power subtracting the optical power, Pe − Po . III. R ESULTS AND D ISCUSSION To verify the thermal resistances obtained from the method proposed by NIST, the different input currents of 50, 100, 150, 200, 250, 300, and 350 mA were used with different heat sink temperatures of 35 °C, 45 °C, 55 °C, 65 °C, and 75 °C. The experimental results and simulation results of 5630 and 5074 were as shown in Fig. 5. The thermal resistances were defined as (2). When using the above equation, 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 power dissipation, ambient temperature, and the amount of external heat sink provided to the LED [9]. The experimental results of Fig. 5(a) showed the variation of thermal resistances of 5630 with different input currents at five different heat sink temperatures. The results showed that the thermal resistances were constant only if the conditions, such as heat sink temperature and input current were given. The increased thermal resistance as a function of input current could be attributed mostly to current crowding phenomenon and some to the conductivity changes of GaN and thermal interface materials (TIMs) caused by heat rise [9]. For given

Fig. 4. Material components of models of samples. (a) 5050. (b) 5630. (c) 5074.

input current, the thermal resistances of 5630 were not a constant and increased when the heat sink temperature decreased. For comparison, Fig. 5(b) showed the simulation results of 5630 that the thermal resistances increased when the heat sink temperature decreased. These showed the same trend between experiment and simulation. This was because of the

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TABLE I T HERMAL C ONDUCTIVITIES OF D IFFERENT M ATERIALS U SED IN F IG . 4

NIST method that measured the difference of given heat sink temperature and cooling temperature. The given heat sink temperature might not completely represent the junction temperature especially at which the given heat sink temperature was much higher than ambient temperature. In other words, the ambient cooling effect of LED was obvious at high given heat sink temperature. Therefore, the temperature difference that was proportional to thermal resistance decreased when the heat sink temperature increased. The thermal resistances in the same condition of 5074 were lower than 5630, as shown in Fig. 5. The only difference was that the heat dissipating path of aluminum base layer of 5074 was larger than 5630. Therefore, the thermal resistances of 5074 were lower than 5630. In Fig. 5, the thermal resistances without optical power were higher than the thermal resistance with optical power. This was because of the definition of thermal resistance that the optical power affected the denominator of definition. The simulation result considered the thermal resistances without optical power, so it had the same trend and similar value with experimental result. There was one possible reason to cause the deviation of simulation and experimental results. In the simulation, the ambient cooling effect of LED was hard to be quantified in the different conditions. Therefore, the simulation and experiment did not have the same value. The obvious phenomena of the conduction of copper layer and convection of air contributing other paths of heat dissipating were the sample of 5050. The experimental result and simulation result of 5050 were as shown in Fig. 6. The small heat dissipating path of 5050 conducted by copper layer without aluminum base layer caused the junction of 5050 under operating current, 150 mA, reaching the same high temperature of 5630 and 5074 under operating current, 350 mA. In case the over temperature of 5050 happened, the highest operating current of 5050 was 150 mA in this experiment. Therefore, the heat dissipating path could not be considered as 1-D dissipating path. The measurement of thermal resistance did not obviously depend on input current. Besides, the experimental thermal resistances were higher than simulating thermal resistances. This was different from 5630 and 5074 that the experimental thermal resistances were similar to simulating thermal resistances. The reason of mentioned phenomenon was that the heat dissipating path was not one main path. This demonstrated that (1) and (2)

Fig. 5. (a) Experimental result of the calculated thermal resistance without and with optical power of 5630. (b) Simulation result of 5630. (c) Experimental result of the calculated thermal resistance without and with optical power of 5074. (d) Simulation result of 5074.

were not suitable for small heat dissipating path encapsulant. The thermal resistances were multidimensions and should be obtained from complex equations.

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are still existence. In this paper, we do the experiments and simulations with varying operating conditions of different encapsulants and try to understand more detailed phenomena of thermal resistance. A well conduction path in the encapsulant decides the accuracy of measured thermal resistance. The experiments represent that thermal resistance increases as a function of input current. For given input current, the ambient cooling effect of LED causes that the thermal resistance increases when the heat sink temperature decreases. The results show that the thermal resistances are constant only if the conditions, such as heat sink temperature and input current are given. This thermal resistance measurement method can be used accurately with a well conduction path in the encapsulant. In the same type of LED encapsulants, the measured thermal resistances can be compared when the operating conditions, such as heat sink temperature and input current are given. In the different types of LED encapsulants, the measured thermal resistances may vary with different structures. The LED module is the integrated LEDs that need the design of heat management. Nowadays, the thermal resistance of LED module is measured and evaluated in the similar method. The ambient cooling effect and dissipating paths may be complex and should be carefully studied. The accurate thermal resistance measurement can effectively evaluate the thermal management of LED module. To design the optimal thermal management of LED module, the design of heat dissipating path and geometry should be carefully considered. This subject will be continuously studied in our group in the future work. Fig. 6. (a) Experimental result of 5050 has no trend and is much higher than the simulation result. (b) Simulation result of 5050 has the same trend similar to Fig. 5.

In summary, the thermal resistances were not constant in different operating conditions. The increased thermal resistance as a function of input current could be attributed mostly to current crowding phenomenon and some to the conductivity changes of GaN and TIMs caused by heat rise [9]. There was one possible reason to cause the deviation of simulation and experimental results. In the simulation, the ambient cooling effect of LED was hard to be quantified in the different conditions. The thermal resistance accuracy of NIST method might depend on the main heat dissipating through the conduction of aluminum base layer for considering thermal resistance as 1-D dissipating path. If the conduction of aluminum base layer could not be mainly considered as 1-D dissipating path, the experimental thermal resistance did not obviously depend on input current. The results showed that the thermal resistances were constant only if the conditions, such as heat sink temperature and input current were given. IV. C ONCLUSION 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 the academia and industry have different viewpoints on the subject of thermal resistance, the ambiguous phenomena and argument of thermal resistance

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Shang-Pin Ying received the Ph.D. degree with the Institute of ElectroOptical Engineering, National Chiao Tung University, Hsinchu, Taiwan. He is currently an Assistant Professor with the Department of Opto-Electronic System Engineering, Minghsin University of Science and Technology, Hsinchu.

Wei-Feng Tang is currently pursuing the bachelor’s degree 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 the 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.