Junction Temperature Of Light-emitting Diodes Research Articles

Effect of solder void on mechanical and thermal properties of flip-chip light-emitting diode: Statistical analysis based on finite element modeling

With the increasing demand for highly efficient lighting in the automotive industry, flip-chip light-emitting diodes (LEDs) have become widely used for both interior and exterior lighting. Solder, serving as a crucial interconnecting material, often develops voids during the reflow process, compromising the integrity and reliability of the connections. Thus, understanding the impact of these voids on the mechanical and thermal properties of the product is vital for improving reliability accuracy. This work employs computational methods alongside experimental approaches to address the challenges of replicating solder voids and controlling the solder void fraction. A comprehensive study investigates the effects of solder voids on shearing properties and thermal conductance. Random voids were introduced into the solder pads of an LED assembly within a finite element model (FEM), leading to predictions of maximum shear stress and LED junction temperature. The findings correlate well with the experimental data, validating the FEM's applicability. Furthermore, a statistical analysis was conducted to explore the relationship between solder void fraction, position, and size, aiming to provide objective guidelines for analyzing soldered assembly tomography in reliability assessments.

LED Junction Temperature Measurement: From Steady State to Transient State.

In this review, we meticulously analyze and consolidate various techniques used for measuring the junction temperature of light-emitting diodes (LEDs) by examining recent advancements in the field as reported in the literature. We initiate our exploration by delineating the evolution of LED technology and underscore the criticality of junction temperature detection. Subsequently, we delve into two key facets of LED junction temperature assessment: steady-state and transient measurements. Beginning with an examination of innovations in steady-state junction temperature detection, we cover a spectrum of approaches ranging from traditional one-dimensional methods to more advanced three-dimensional techniques. These include micro-thermocouple, liquid crystal thermography (LCT), temperature sensitive optical parameters (TSOPs), and infrared (IR) thermography methods. We provide a comprehensive summary of the contributions made by researchers in this domain, while also elucidating the merits and demerits of each method. Transitioning to transient detection, we offer a detailed overview of various techniques such as the improved T3ster method, an enhanced one-dimensional continuous rectangular wave method (CRWM), and thermal reflection imaging. Additionally, we introduce novel methods leveraging high-speed camera technology and reflected light intensity (h-SCRLI), as well as micro high-speed transient imaging based on reflected light (μ_HSTI). Finally, we provide a critical appraisal of the advantages and limitations inherent in several transient detection methods and offer prognostications on future developments in this burgeoning field.

Isı Emici-Isı Borulu İnovatif Bir Yüksek Güçlü LED Termal Yönetim Sisteminin Deneysel ve Nümerik Analizi

The efficiency and lifespan of light emitting diodes (LEDs) are adversely affected by the junction temperature. Therefore, it is very important to operate a LED at a low junction temperature. In this study, it is aimed to minimize the junction temperature of high power LEDs so that reliability and light output of the device can be maximized.
 In the study, a heat pipe-heat sink cooler was designed for the high power LEDs. The study was carried out experimentally and the results obtained from the experimental study were also verified numerically in the ANSYS Fluent software. Total power inputs ranging between 40 W and 100 W were applied to the LEDs and the performance of the cooler in the current design was examined. To observe the effect of the heat pipe on the LED junction temperature, a heat sink without heat pipe was designed and analyzed both experimentally and numerically. The results show that, the heat sink with fin is sufficient at low LED input powers, while at high LED input powers, the heat pipe-heat sink provides much more effective cooling. At the same time, the effect of different thermal interface materials on LED junction temperature was observed, by using with materials with thermal conductivities of 1.8 W/m.K, 8.5 W/m.K and 11 W/m.K, for each power input. As the coefficient of thermal conductivity of the thermal interface materials increased, the temperature of the LED solder point decreased.

Enhanced Optical–Thermal Performances of High-Power LED by Plated Copper on Thick Film Ceramic Substrate

Thick printed ceramic substrate (TPC) is a promising technique for the packaging of high-power light-emitting diode (LED). Nevertheless, the incorporation of glass phase and voids in the sintered Ag layer decrease the electrical conductivity and heat dissipation of TPC substrate, which contributes to an increase in the junction temperature of LED. This article demonstrates a novel packaging design to plate copper on the sintered Ag layer of TPC substrate, which was applied to enhance the optical–thermal performances of high-power LED. After plating, the copper layers are well bonded with the Ag layer and the sheet resistance decreases rapidly. With the plated copper on thick film (PCTF) substrate package, the total thermal resistance is 17.45% lower than that with TPC package, while the junction temperature change of LED samples is 8.04 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C and 9.92 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C, respectively. Moreover, the surface temperature of LED sample packaged with PCTF is 11.16% lower than that with TPC at the working current of 1600 mA, and the optical power is 4.79% and 3.2% higher at the working current of 400 and 1200 mA, respectively. The results demonstrate that the optical–thermal performances of LED are enhanced by using the PCTF substrate due to its excellent heat dissipation.

Optimization of heat-dissipated structures with cuprous oxide deposits to improve thermal management of high-brightness LED arrays

This project reports on improving the thermal performance of light-emitting diode (LED) packages based on an optimized thermal structure with a sprayed cuprous oxide coating. Eighteen orthogonal arrays of LED aluminum plates were used and the influence of the parameters of thermal dissipation structure on the LED junction temperature was established. The junction temperature of the LED assembly is developed by various heat dissipation structures based on Taguchi methods. The experimental results show that by measuring the temperature of the 12W LED module, the best results were compared with the lowest values in the overall experiment, where the junction temperature dropped from 67°C to 51°C Also, the contribution of the error to the total variance is about 0.8%, indicating that the experiment is quite successful and robust results. It was found that lower junction temperature values were obtained with optimized heat sink structures with Cu2O-coated Al substrates, and the thermal performance of the LEDs was further improved.

Arrangement of LEDs and Their Impact on Thermal Operating Conditions in High-Power Luminaires

Solid-state light sources are currently the fastest-growing group of light sources, replacing the previously used discharge and incandescent light sources. Thermal operating conditions of LEDs (Light Emitting Diode) play an important role in t maintaining long service life and constancy of luminous-electrical parameters. In the field of illumination, the service life parameter of light sources is important for the costs of maintenance of the illumination system, while the maintenance of the value of certain light parameters over time, such as luminous flux, color temperature and color rendering index, is related to the aesthetic effect of the illumination. In addition, limiting the junction temperature of solid-state light sources is particularly important in high-power luminaires dedicated to flood illumination. One of the elements shaping the thermal operating conditions of multi-source LED luminaires is the number of luminaires used, their arrangement, and the distance between LEDs installed on the MCPCB (Metal Core Printed Circuit Board) substrate. This article presents the results of simulation studies, realized using CFD (Computational Fluid Dynamics) software, where the temperature distribution and the junction temperature of the LED panel were determined for different configurations and distances between the LEDs. The results obtained were analyzed and conclusions were drawn based on them. Thermal tests performed and presented in the article cover scientific issues related to shaping the temperature distribution of the LED panel. They make it possible to determine the influence of thermal couplings between the sources, related to their number, distance and the value of the forward current, on the final temperature of the LED junction temperature. The presented research results may constitute auxiliary materials for designers of lighting luminaires, especially high-power luminaires, where a large number of high-power LED sources are installed in close proximity.

Defect Dynamics and Internal Quantum Efficiency Decay Resultant Ultraviolet C‐Band Light‐Emitting Diode Lifetime Performance

The internal quantum efficiency (IQE) decay resultant lifetime performance of ultraviolet C‐band (UVC) light‐emitting diodes (LEDs) is investigated, and an LED lifetime model is deduced assuming that defect generation is the sole cause for IQE decay and the defect generation process is to maximize the entropy of the defects. The proposed model reveals that the lifetime of an LED is proportional to its radiative recombination coefficient and inversely proportional to the product of its initial nonradiative recombination coefficient and defect growth interest rate, thus providing explicit dependence of the lifetime on the LED junction temperature and current density. The model fits excellently well to full sets of lifetime measurement data, which were collected from UVC LEDs stressed at different currents and different ambient temperatures for 2500 up to ≈10 000 h. The sensitivity of the lifetime to the junction temperature and current density is revealed for some UVC LEDs. Using the model, a lifetime of ≈96 000 h is projected to maintain 70% of the initial optical output power for a UVC LED based on a stress test for ≈10 000 h.

Development of Radiator with Thermoplastic Polymer and Insert-Molded Aluminum Alloy Parts for Light-Emitting Diode Headlights

The increasing popularity of electric vehicles has increased the demand for lightweight auto parts. However, the excessive weight of traditional metal heat sinks has remained a concern. Metal has excellent thermal conductivity but low radiation efficiency. Conversely, thermoplastic polymers have excellent heat radiation efficiency but poor thermal conductivity. In this study, we propose a radiator constructed using thermoplastic polymer and insert-molded aluminum alloy parts to maintain the low junction temperature of light-emitting diodes (LEDs); the radiator’s weight is reduced through a combination of aluminum alloy and a thermally conductive polymer designed for automotive headlights. At an LED thermal load of 11.48 W, the measured temperature on the LED pad is 60.8 °C. The weight of the proposed radiator is 23.37% lighter than that of a pure metal radiator. When the lightweight radiator is used in high-power LED headlights, it effectively dissipates heat within a limited space.

Toward Class AAA LED Large Scale Solar Simulator With Active Cooling System for PV Module Tests

Solar simulators are significant in testing solar cells and photovoltaic (PV) modules since they provide spectrum and optic characteristics close to natural sunlight. This research introduces two fundamental novelties to the literature on light-emitting diode (LED) light source solar simulators. One of these innovations is designing and developing a solar simulator to achieve <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> characterization on a PV module basis instead of cell-based contrary to the previous studies. For this purpose, a unique board with a modular structure is designed on which the LEDs are positioned. In the research, LEDs of at six different wavelengths are used. Spectral match, spatial nonuniformity, and temporal instability performance criteria tests conducted on the illuminated area is performed based on IEC 60904-9 and ASTM 927E-10 standards. Class AAA was obtained according to the large-scale solar simulator standard specified in ASTM 927E-10. The second novelty the study introduces is developing an active cooling system that can prevent an increase in LED junction temperature when the number of LEDs increases in an LED solar simulator with a wide illuminated area. In addition to the fans, this solution is suggested to prevent any damage to the LEDs during the test, as well as high temperatures from affecting the wavelengths and at the same time achieve 25 °C, one of the standard test conditions. Also, the device dimensions are optimized ergonomically for ease of use. With this system designed and devised, it becomes possible to test commercially available PV modules successfully. The active cooling system minimizes the heating problem and the impact of wavelengths that might invalidate performance criteria classes.

Experimental and numerical investigation on the thermal performance of linear LED housings under forced convection

This study aims to experimentally and numerically test the thermal performances of three different linear housings (Model 1-without fin, Model 2-with inner fin, and Model 3-with inner and outer fin) with medium power light-emitting diode (LED) arrays at 30 °C and 40 °C ambient temperatures under forced convection for 25 W and 35 W. Although the mass of Model-3 was reduced by 54.2% and 34.67% compared to Model-1 and Model-2, its surface area was increased by 46.3% and 9.11%, resulting in an improvement in junction temperature (Tj) up to 4.4 °C and 7.75 °C, respectively. Thermal resistance (Rth) for Model-3 is approximately 1.38–3.1% and 7.85–9.72% smaller than that of Model-1 for 25 W at ambient air temperature (Taa = 30 °C and Taa = 40 °C), respectively. The Rth for Model-3 is approximately 1.39–2.6% and 8.65–6.07% smaller than that of Model-1 for 35 W at the same Taa, respectively. The Model-3 offers lower junction temperature, lower cost, and safe operating temperature range by inner and outer fins compared to the others. The results demonstrated that all of the temperatures reduced with the increase in air mass flow rate; however, it was found that as the LED power and ambient temperature increased, the junction temperature increased. Experiment and simulation results show that adding a fan to the luminaire significantly lowers the LED junction temperature, in which case it provides a safe operating range and protects the luminaire against thermal shocks. The forced application allows using of comparatively smaller housing as it provides a lower junction temperature.

A Technique for Improving the Precision of the Direct Measurement of Junction Temperature in Power Light-Emitting Diodes.

Extending the lifetime of power light-emitting diodes (LEDs) is achievable if proper control methods are implemented to reduce the side effects of an excessive junction temperature, TJ. The accuracy of state-of-the-art LED junction temperature monitoring techniques is negatively affected by several factors, such as the use of external sensors, calibration procedures, devices aging, and technological diversity among samples with the same part number. Here, a novel method is proposed, indeed based on the well-known technique consisting in tracking the LED forward voltage drop when a fixed forward current is imposed but exploiting the voltage variation with respect to room temperature. This method, which limits the effects of sample heterogeneity, is applied to a set of ten commercial devices. The method led to an effective reduction of the measurement error, which was below 1 °C.

Design and Thermal Analysis of High Power LED Light

In the development of lighting technologies, light emitting diode (LED) technology plays an important role due to its high efficiency. LED lightening is the future of indoor and outdoor illumination solutions that all around of the world there is a widespread transition to save energy and have better quality illumination via LED-based solutions. On other hand, cooling of high power LED is crucial factor to utilize LED in high performance. In addition, finned heat sink could not be efficient based on the its geometry and usage areas due to the powder, the rain and the muds effects. In this study, an un-finned heat sink LED armature consisting of aluminum sheet metal was designed using a computer-aided-design (CAD) program. Furthermore, manufacturing of the heat sinks from sheet metal plates with bending machines enables to increase variety of LED armatures cheaply according to design requirements. Then, the thermal analysis of the system was conducted for the case of natural heat convection under different electrical powers and ambient temperatures by a commercial finite element analysis (FEA) program: SolidWorks Simulation. As a result of the thermal analysis, the ultimate heat sink sizes were obtained to minimize junction temperature of LED. Finally, the experimental studies were carried out to verify the feasibility of numerical solutions that they were in good agreement.

A Continuous Rectangular-Wave Method for Junction Temperature Measurement of Light-Emitting Diodes

Junction temperature is an important parameter for evaluating thermal property of light-emitting diodes (LEDs). How to accurately measure the junction temperature still remains controversial. In this paper, we propose a continuous rectangular-wave method (CRWM) for measuring the junction temperature of LEDs. This method uses a continuous rectangular wave to drive LED under either voltage mode or current mode, and acquires the transient voltage response, i.e., the heating curve, by a high-definition oscilloscope at the rising edge of each cycle. Compared with the conventional forward voltage method (FVM), where a switch is necessary to transit the heating state to the sensing state and a transient cooling curve is usually acquired, the switch-free-periodic-acquisition CRWM avoids the acquisition time delay caused by switch and decreases charging/discharging effect of LEDs effectively with rising time of the rectangular wave less than 40 μs. Besides, periodic acquisition of the heating curve further enhances the signal-to-noise ratio. Comparative experiments by FVM, CRWM, and micro-thermocouple show coincidence and accuracy of CRWM with relative error less than 3% in current mode and 1.5% in voltage mode. COMSOL simulation is applied to verify the validity of CRWM and discuss the impact of rising time of the rectangular wave.

A Comparative Study for the Junction Temperature of Green Light-Emitting Diodes

Solid-state lighting devices offer a wide variety of color options applicable for general and automotive lighting, various display systems, and a number of niche applications. As they get smaller, generated heat fluxes become more intense and induce serious lifetime and performance issues. Although the light output from light-emitting diodes (LEDs) is the most efficient at a narrow optical spectrum compared with conventional lighting sources, they are still not adequate to satisfy consumer demands due to considerable amounts of lost energy and emerging thermal issues. On the other hand, it is possible to achieve effective thermal solutions if the junction temperature of LEDs is precisely determined. A number of techniques have been proposed for the junction temperature measurement of LEDs such as forward voltage change and infrared (IR) thermal imaging. In this study, green LEDs were studied to observe optothermal interactions using a number of proposed junction temperature measurement techniques. The effect of an LED lens on junction temperature and optical extraction was investigated by examining the change in the thermal and optical properties of an LED chip after the LED lens was removed. In addition, the results of the green LED were compared with a 450-nm blue LED and verified with numerical findings. As a result, it has been determined that the thermal behavior of LEDs is significantly affected by electrical conditions, since the junction temperature of green and blue LEDs has risen by around 45% after the operating current has been increased from 200 to 500 mA.

Thermal Effect on Junction Temperature of Light-emitting Diodes of AlGaN

This paper discusses the heat effect on the junction temperature of light-emitting diodes (LEDs) of AlGaN. The junction temperature of AlGaN LED based on heat balance is analyzed and agrees with the available experimental data. The junction temperature decreases with the decreasing ambient temperature and the increasing the effective thermal conductivity and effective heat-conduction area.

A Comparative Study on the Junction Temperature Measurements of LEDs With Raman Spectroscopy, Microinfrared (IR) Imaging, and Forward Voltage Methods

Energy efficiency, long life, exceptional color, and performance of solid-state light sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. Besides, light-emitting diodes (LEDs) have thermal limitations that are vital for device quality and lifetime. Specifically, to improve the heat dissipation, one major parameter used to evaluate the LED performance is thermal resistance ( $R$ ). Reducing the resistance can improve the heat flow from the p-n junction to ambient during operation. To quantify this parameter, the LED junction temperature ( $T_{J}$ ) must be determined. In this paper, the junction temperatures are first measured with forward voltage method (FVM), Raman spectroscopy, and infrared (IR) imaging for a 465-nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-particles added epoxy mixture (%13, 4300 CCT) to convert blue to white light, and the junction temperatures were measured again experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman method and FVM were in reasonably good agreement on measuring the junction temperature for 465-nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with IR imaging and Raman methods, while FVM has shown consistent results for coated chips.

Thermal and Optical Performance of LEDs Mounted on Substrates With Converging–Diverging and Diverging–Converging Channels

Though light-emitting diodes (LEDs) are gaining attention for implementation in various applications, thermal management remains a major concern in solid-state lighting devices. Thermal resistance and junction temperature are the critical parameters of LEDs, which should be maintained at nominal levels for its reliable and robust operation. In this paper, novel substrates were proposed to enhance the thermal performance of LEDs. Aluminum substrates with converging–diverging (CD) and diverging–converging (DC) geometries were fabricated using high-precision milling machine. Three different converge–diverge angles such as 45°, 60°, and 90° were used in CD and DC substrates. Numerical simulations were done using computational fluid dynamics software to gain more insights on heat distribution and flow dynamics in the proposed substrates. Thermal characterization was done using thermal transient method by mounting the LED on CD and DC substrates. The thermal performances of the proposed substrates were compared with bare aluminum substrate by measuring the thermal resistance and junction temperature of LED. Optical analysis on LED was done by utilizing still air chamber and spectrometer. From the analysis, DC substrates showed lower thermal resistance and junction temperature compared to the other two substrates due to the conjugate heat transfer effect. Precisely, DC substrate with 90° showed higher thermal and optical performance than any other substrates considered.

Design Methodology for LED Lighting Systems Based on Photo-Electro-Thermal Interrelationships

This paper presents a design methodology for LED (Light-Emitting Diode) lighting systems based on photo-electro-thermal (PET) interrelationships. The proposed methodology uses only LED datasheet information, which makes experimental tests unnecessary to obtain the design parameters. The methodology allows identifying several design specifications, such as, luminous efficacy, heatsink thermal resistance, LED junction temperature and forward current, essential aspects to produce a satisfactory lighting system. Thus, it is possible to define the lighting system features based on standards requirements to obtain the desired system results. Initially, a review of several PET theories is presented, and a new mathematical analysis is performed, in order to highlight the main contributions of the methodology. An LED bulb lamp design is presented to exemplify the methodology. Finally, experimental tests with the proposed LED lamp resulted in a luminous flux of 1271 lm, with a luminous efficacy of 112 lm/W, and LED junction temperature of 79.67 ºC. The errors between calculated and measured luminous flux, luminous efficacy and LED junction temperature were 3.70%, 1.88%, and 3.85%, respectively. These results validate the proposed methodology.

Study of the Light Emitting Diode as a photoreceptor: Spectral and electrical characterization as function of temperature and lighting source

In this study, the temperature influence on the spectral responsivity of a Light Emitting Diode (LED) used as a photoreceptor, combined to light source spectrum is correlated to electrical characteristics in order to propose an alternative method to estimate LED junction temperature, regardless of the absolute illumination intensity and based on the direct correlation between the integral of the product of two optical spectra and the photo-generated currents. A laboratory test bench for experimental optical measurements has been set in order to enable any characterizing of photoelectric devices in terms of spectral behaviour, in a wavelength range placed between 400–1000 nm, and of current-voltage characteristics as function of temperature by using two different illumination sources. The temperature is analysed in a range from 5 °C up to 85 °C, so as to evaluate thermal variation effects on the sensor performance. The photo-generated current of two LEDs with different peak wavelengths has been studied. Research has observed and mathematically analysed what follows: since the photo-generated current strictly depends on the combination between the spectral response of the photoreceptor and the lighting source response, it becomes possible to estimate indirectly the junction temperature of the LEDs by considering the ratio between the photogenerated currents obtained by using two different illumination sources. Such results may for one thing increase knowledge in the fields where LEDs are used as photo-detectors for many applications and for another, they could be extended to generic photodetectors, thus providing useful information in photovoltaic field, for instance.

Thermal Characterization of LEDs Mounted on Substrates With Converging–Diverging and Diverging–Converging Channels

Thermal management stances a massive threat in solid-state lighting devices. With light-emitting diodes (LEDs) gaining attention for implementation in various applications, it is important to address its thermal issues. Thermal resistance and junction temperature are the critical parameters of LEDs which should be maintained at optimum ranges for its reliable operation. In this paper, novel substrates were proposed to minimize the thermal parameters of LEDs. Aluminum substrates with converging–diverging (CD) and Diverging–converging (DC) geometries were micromilled. Thermal characterization was done using thermal transient method by mounting LED on CD and DC substrates. The thermal performance of proposed substrates was compared with bare aluminum substrate by measuring the thermal resistance and junction temperature of LED. On analysis, DC substrates showed lower thermal resistance and junction temperature compared with the other two substrates due to conjugate heat transfer effect within the channels.