Multi-chip Light-emitting Diode Research Articles

Natural Convection Immersion Cooling With Enhanced Optical Performance of Light-Emitting Diode Systems

Electronics driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. This common problem occurs at light-emitting diode (LED) chips and it is not easily observed by end-users. Driving over 700 mA over a 1 mm2 chip is expected to generate local temperature gradients. In addition, bonding failures at manufacturing or during operation (cracks, delamination, etc.) may also lead to local hot spots. Therefore, possible hot spots over an LED chip have turned attention to direct cooling with dielectric liquids comprises the current study. Computational and experimental studies have been performed to understand the impact of conduction and alternatively convection with various dielectric fluids to abate local hot spots in a multichip LED light engine. To capture the local temperature distributions over the LED light engine with a dome in the domain especially over the LED chip; first, computational models have been built with a commercial computational fluid dynamics (CFD) software. Later, attention has been turned into experimental validation by using a multichip high brightness LED (HB LED) light engine. An optothermal evaluation has been made at single and multiphase heat transfer modes with dielectric fluids (LS5252, HFE7000, and silicone oil, etc.) to compare with a series of CFD models and experimental studies. While multiphase liquid-cooled LED system has a better cooling performance but lower optical extraction, single-phase liquid-cooled LED system has shown a reasonable thermal performance with a 15% enhancement at light extraction.

The Study of Thermal Resistance Measurement of Multichip LED

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.

Thermal Impedance Measurement on Different Chip Arrangements for Various Multichip LEDs Application

The effect of thermal impedance of three chip arrangements in a multichip light-emitting diode (LED) package has been discussed with each chip attached to a heat sink with thermal interface material. Each orientation represents different existing lighting applications such as T8 tube, MR 16 down light, and spotlight. In this paper, the emitter and sensor current are constantly maintained. One LED of each orientation is driven, while other LEDs are sensed until all LEDs are driven with the emission current of all the orientations. Comparisons are made between the thermal impedance matrix pairs of each LED arrangement to illustrate the difference in the thermal dissipation. Thermal impedance curves show the fluctuation in the thermal flow inside the package, implying the reciprocal behavior of each orientation. This paper further explains the reciprocal behavior of the chips and the importance of the orientation based on junction temperature. Moreover, the principle of linear superposition is implemented to estimate the junction temperature matrix. The theoretical and the experimental results are compared using thermal transient tester. Among three orientations, the square-shaped LED arrangement shows an average junction temperature of 116 °C within the package while exerting the highest change of junction temperature of 4 °C, comparing with the linear superposition method.

Thermal Analysis of High-Power Multichip COB Light-Emitting Diodes With Different Chip Sizes

This paper demonstrates the thermal analysis of high-power light-emitting diodes (LEDs) with various chip sizes, in the same chip-on-board (COB) package and with the same total chip size and input power. The 4-, 9-, 25-, and 100-chip multichip COB LEDs with chip-side lengths of 1500, 1000, 600, and 300 $\mu $ m were used. The simulation results indicate that the maximal junction temperature of the multichip COB LEDs increased as the number of chips in the multichip COB LED decreased. In addition, with the same input power, the difference between the maximal and minimal junction temperatures of the multichip COB LED increased as the number of chips in multichip COB LEDs increased. That is, if the multichip COB LED contains a few number of large chips, the output power of each chip will drop but uniform owing to the high junction temperature and low temperature difference. Oppositely, if the multichip COB LED contains a lot of small chips, the output power of the chips at the corners will be higher than other chips, but the output power of each chip will be nonuniform owing to the nonuniform junction temperature distribution.

Thermal Analysis and Optimization of 6.4 W Si-Based Multichip LED Packaged Module

Multichip packaging was achieved the best solution to significantly reduce thermal resistance at the same time, to increase luminance intensity in LEDs packaging application. For the packaging, thermal spreading resistance is an important parameter to get influence the total thermal performance of LEDs. In this study, silicon-based multichip light emitting diodes (LEDs) packaged module has been examined for thermal characteristics in several parameters. Compared to the general conventional single LED packaged chip module, multichip LED packaged module has many advantages of low cost, low density, small size, and low thermal resistance. This analyzed module is comprised of multichip LED array, which consists of 32 LED packaged chips with supplement power of 0.2 W at every single chip. To realize the extent of thermal distribution, the computer-aided design model of 6.4 W Si-based multichip LED module was designed and was performed by the simulation basis of actual fabrication flow. The impact of thermal distribution is analyzed in alternative ways both optimizing numbers of fins and the thickness of that heatsink. In addition, a thermal resistance model was designed and derived from analytical theory. The optimum simulation results satisfies the expectations of the design goal and the measurement of IR camera results. tart after striking space key 2 times.

High-luminance and high-efficiency multi-chip light-emitting diode array packaging platform with nanoscale anodized aluminum oxide on silicon substrate

A silicon-based packaging platform is developed for the packaging component of a high-luminosity and high-efficiency multi-chip light-emitting diode (LED) module, which is patterned on an insulating layer that consists of a nanoporous anodized aluminum oxide (AAO) layer and silicon dioxide (SiO2) deposited by plasma-enhanced chemical vapor deposition on a doped silicon substrate. Compared to a single thick layer of SiO2 on a conductive silicon substrate, the proposed substrate (SiO2/AAO/Si) was proven to be effective in terms of its abilities to reduce the insertion loss and to increase thermal conduction. The proposed structure can be used to effectively improve the reliability and reduce the thermal fatigue of high-luminance and high-efficiency LED array modules. We demonstrate an 8W, cool-white (5700K) LED array with a chip size of 570μm×550μm, exhibiting a luminous intensity of 100lm/W and a color rendering index of more than 83 at the forward current and voltage of 610mA and 16.1V, respectively.

Thermal Stability of High-Power LEDs Analyzed With Efficient Nondestructive Methodology

With the continuing advancement in light-emitting efficiency of high-power light-emitting diodes (LEDs), a nondestructive evaluation on thermal reliability, especially regarding the operating temperature and thermal resistance, which directly affect their lifetime and luminous characteristics, is needed. In this paper, we develop a methodology of using a 3-D numerical simulation for transient thermal-conduction analysis, which is combined with the electrical test method to evaluate the junction temperature of a variety of LEDs. As a result, the simulated temperature for LEDs was found to increase with time, whereas the slug temperature showed the same trend for the simulation and the experiment. The measured results indicate that the thermal resistance of AlGaInP-material-based red and amber LEDs reveals higher variability than that of InGaN-material-based white and green LEDs. Via the luminous flux experiment, we have found that the thermal resistance of the LEDs is not the major determination factor of their luminous flux decay. In contrast, the luminous flux decay of the LED is dependent on the chip material being used. Notably, the slug temperature, observed from the pulsed-driven experiment, of the LED appeared to be lower than that observed in the normal steady operation. It is demonstrated that the lifetime and reliability of high-power LEDs can be improved by decreasing the junction and slug temperatures of the devices. This is important as the slug temperature can be controlled by external cooling techniques. In practice, the developed methodology is not restricted to analyze particular LED packaging forms and can be used to investigate various kinds of multichip LED modules as well.

Estimation of heat dissipation through the interchip structure for a multichip light-emitting diode package via simulation

The heat dissipation through interchip structures was investigated by introducing 4-partition and 16-partition ceramic interchip structures for a 16-chip light-emitting diode (LED) package. Using the finite volume method simulation with ICEPAK V13.0, the temperature distribution inside the LED package and the junction temperature were estimated, from which the effect of the interchip structures on the heat dissipation characteristics of the LED package was investigated. The results showed that the ceramic interchip structures provided a more effective upward heat dissipation path for the LED package and contributed to a 23°C decrease in the junction temperature for the 4-partition interchip structure and a 51°C decrease for the 16-partition interchip structure at the 16 W operation.

Comparison of the Thermal Performance of the Multichip LED Packages

Single chip, 4-chip, and 16-chip light emitting diode (LED) packages were prepared to compare the thermal performance of the LED packages. Vertical type, GaN blue LED chips with alumina submount were used, and thermal transient analysis was conducted to investigate the thermal characteristics of the LED packages. Temperature sensitive parameter value was measured and the junction temperature was predicted. Thermal resistance between the p-n junction and the ambient was obtained from the structure function with the junction temperature evolution during the cooling period of LED. Junction temperature and thermal resistance were investigated with the change of input current level and compared for 1-chip, 4-chip, and 16-chip LED packages. The results showed that the junction temperature and the thermal resistance are strongly related to the number of the chips and the chip arrangement.

Cup-shaped copper heat spreader in multi-chip high-power LEDs application

In this study, cup-shaped copper sheets were developed to improve heat dispassion for high-power light emitting diodes (LEDs) array module (3 × 3, 4 × 4, and 5 × 5) using an electroplating technique. The cup-shaped copper sheets were directly contacted with sapphire to enhance the heat dissipation of the chip itself. The lateral emitting light extraction and heat dissipation of high-power LEDs were enhanced and efficient. The surface temperature was not only decreasing but also uniform for each LED chip with the cup-shaped copper heat spreader adoption. The high thermal transmitting performance of cup-shaped copper heat spreader allows thermal resistance reducing 0.7, 0.6, and 0.7 K/W of 3 × 3, 4 × 4, and 5 × 5 LED array module, respectively. In addition, the light output power was increased of 14, 13, and 12% with 3 × 3, 4 × 4, and 5 × 5 LEDs array module using cup-shaped copper sheet at high current injection. High heat dissipation performance and light extraction were obtained by cup-shaped copper sheet with copper bulk and silver mirror.

A Study on the Working Performance of Dimming Methods for Single- and Multichip Power LEDs

Light emitting diodes (LEDs) have many positive features, such as long life, low power consumption, and high luminous efficacy. Consequently, they are very attractive for use in many applications. Also, the use of LEDs ensures energy savings. Power LEDs have increasingly been used for general lighting via multichip module technology. The output of power LEDs should be controlled to obtain life and illuminance values in catalog. In this study, the effects of different dimming methods are investigated for single-and multichip LEDs. To achieve this aim, pulse width modulation (PWM) and current variation (CV) dimming methods are applied to single and multichip power LEDs during ten stages of dimming. A current-regulated DC source is used for current variation (CV) dimming, and a driver designed by the researcher is used for pulse width modulation (PWM) dimming. Moreover, all of the applications are made for active and passive cooling. Finally, the results are compared.

A Novel Silicon-Based Packaging Platform for High-Efficiency LED Modules

A novel silicon-based packaging platform with the electroplated-based reflector and the electrode- guided interconnections is developed for the packaging component of a high-luminosity and high-efficiency multi-chip light-emitting diode (LED) module, which is patterned on a new type of insulating layer that consists of nanoporous anodized aluminum oxide (AAO) layer and plasma- enhanced chemical vapor deposition (PECVD) deposited silicon dioxide (SiO2) on a doped silicon substrate. The reflector and the electrical interconnections are successfully fabricated by using the electroplating method in the same body. In order to obtain the benefits of high efficiency LED modules, the requirements concerning thermal management and photomechanical layout have to be met. In this paper, we will discuss a novel fabrication method in LED module packaging platform, and then describe the thin layer of electroplated Cu/Ni/Au in order to reduce thermal resistance and to increase thermal diffusion efficiency. The heat generated by the LED chips is dissipated directly to the silicon body through the metal-plated platform, and truly excellent heat dissipation characteristics are observed. We demonstrate 987 lm 8 W-level cool-white light (5000 K, 16 V, 110 lm/W, CRI = 77) emission for 570 µm × 230 µm-chip LEDs at 600 mA operation.

Numerical Analysis for Thermal Performance of High Power Multi-Chip LED Packaging

Heat dissipation and cost are the key issues for light-emitting diode (LED) packaging. In this paper, based on the thermal resistance network model of LED packaging, three-dimensional heat dissipation model of high power multi-chip LED packaging is developed and analyzed with the application of finite element method. Temperature distributions of the current multi-chip LED packaging model are investigated systematically under the different materials of the chip substrate, die attach, and/or different structures of the heat sink and fin. The results show that the junction temperature can be decreased effectively by increasing the height of the heat sink, the width of the fin, and the thermal conductivity of the chip substrate and die attach materials. The lower cost and higher reliability for LED source can be obtained through reasonable selection of materials and structure parameters of the LED lighting system.

Eliminating hotspots in a multi-chip LED array direct backlight system with optimal patterned reflectors for uniform illuminance and minimal system thickness

We propose an optical design process that significantly reduces the time and costs in direct backlight unit (BLU) development. In it, the basic system specifications are derived from the optical characteristics of RGB light-emitting diodes (LEDs) comprising the BLU. The driving currents are estimated to determine the theoretical RGB flux ratio for a desired white point. The number of LEDs needed to produce the target luminance is then calculated from the combined optical efficiencies of the components. Last, an appropriate array configuration is sought based on the illuminance distribution function for meeting the target uniformity. To showcase the design process we built two 42-inch triangular cluster arrays of 40 x 16 LED elements. When a flat reflective sheet was used, the minimum thickness required of the system to satisfy the target uniformity was 30 mm. Introducing a patterned reflective sheet removed hotspots that resulted from reducing the system thickness without the aid of additional optical components. Using an optimized patterned reflective sheet, reduction in system thickness as much as 5 mm was possible.

Thermal Resistance Measurement of LED Package with Multichips

Thermal transient measurements of high power GaN-based light-emitting diodes (LEDs) with multichip designs are presented and discussed in the paper. Once transient cooling curve was obtained, the structure function theory was applied to determine the thermal resistance of packages. The total thermal resistance from junction to ambient considering optical power is 19.87 K/W, 10.78 K/W, 6.77 K/W for the one-chip, two-chip and four-chip packages, respectively. The contribution of each component to the total thermal resistance of the package can be determined from the cumulative structure function and differential structure function. The total thermal resistance of multichip packages is found to decrease with the number of chips due to parallel heat dissipation. However, the effect of the number of chips on thermal resistance of package strongly depends on the ratio of partial thermal resistance of chip and that of slug. Therefore, an important thermal design rule for packaging of high power multichip LEDs has been analogized.

Rectangular Microlens Array Having High Sag for Multi Chip LED Packaging

A novel rectangular shape microlens array having high sag for solid-state lighting is presented. The rectangular shape of proposed microlens can maximize the fill factor of silicon based light-emitting-diode (LED) packaging and minimize the optical loss through the reduction of unnecessary reflection at the same time. Microlens, which has high sag, over 3 75 μm and large diameter, over 3 mm can enormously enhance output optical extraction eff iciency. Moreover wafer level packaging technology is adopted to improve the aligning accu racy and mass production of LED packaging. This wafer level microlens array can be direc tly fabricated on LED packaging using replication method. It has many advantages in optica l properties, low cost, high aligning accuracy, and mass production.

Modelling of multichip LED packages for illumination

Modelling of the spatial uniformity of the light output from multicolour light emitting diode (LED)-based lamps can be used to evaluate and optimize LED and reflector configurations. The percentage of the output beam that meets or exceeds a user-selected colour accuracy threshold is used as a figure of merit to compare lamp configurations. Results are presented for various distributions of red, green and blue (RGB) or red, amber, green and blue (RAGB) LEDs in a 4 × 4 square matrix. In general, the distributions with highest (4-fold rotational) symmetry have the most uniform output beams. However, those with least uniformity also have 4- fold symmetry, which indicates the importance of choosing the chip layout carefully. The colour uniformity for RAGB configurations is, in general, higher than the colour uniformity for RGB configurations. The colour uniformity at a correlated colour temperature (CCT) of 7000 K is generally higher than the colour uniformity at 3500 K for the LED spectral characteristics investigated.