Light-emitting diodes (LED) chip scale packages (CSPs) have been promoted as a new light source with many advantages in smaller package size, lower material and process cost, and better heat dissipation effect. However, as it is exposed in harsh environments such as high temperature, high humidity, and high blue light irradiation, silicone material used in LED CSPs always suffers deterioration, which will seriously affect the LED's reliability and working life. Thus, the preparation of high reliable silicone has practical significance to promote the application of LED CSPs in lighting. In this research, titanium was introduced into the molecular chain of phenyl silicone by using the hydrolysis condensation method. A high temperature aging test was then performed to the prepared silicone before and after modification, and their optical, thermomechanical, and dielectric properties were characterized to evaluate their reliabilities. The results show that: (1) the Arrhenius function with the dielectric property as an aging characterization can be used as a temperature accelerated life model to predict the service life of the prepared silicone and (2) the titanium modified silicone can advance the high temperature stability on optical properties, thermomechanical, and dielectric properties and enhance the life expectancy. The major contributions of this study are to support the improvement of the novel LED CSP packaging materials and processes, and also to provide the technical guidance on the fast, accurate, and cost-effective reliability assessment for high-quality LED light sources.

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Better mechanical, thermal properties and longer lifetimes are needed for the die attach layer in high-power electronic packaging. As traditional Sn-Ag-Cu (SAC) solders have many limitations, the sintered nanosilver materials are becoming one of the substitutes for high-power electronic packaging. However, the high performance of sintered nanosilver materials is only achieved when its fine sintering densification is formed. This article investigates the sintering densification process of nanosilver particles based on the design of orthogonal experiments and sintering kinetics modeling in which both the macroproperties and micromorphology are linked and analyzed. The results lead to several conclusions, such as: 1) the orthogonal experiments consider the effects of sintering temperature, dwell time, and sample preparation pressure on the sintering relative shrinkage and relative density - the results show that the most critical impact factor on sintering densification is the sintering temperature. (2) In the sintering kinetic experiments, the sintering densification rates obtained by fitting the relative density versus dwell time curves during 175 °C-250 °C follow the Arrhenius model, and the apparent activation energy of sintering kinetics is calculated to be 36 kJ/mol, while it is calculated from the particle size is 38.1 kJ/mol. 3) Through modeling the relationship between particle size, line shrinkage, and porosity, the line shrinkage and porosity first increase at the initial stage, while the particle size increases, and the macroscopic volume decreases at the end of sintering, the porosity decreases.

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The inherent luminous characteristics and stability of LED packages during the operation period are highly dependent on their junction temperatures and driving currents. In this paper, the luminous flux of LED packages operated under a wide range of driving currents and junction temperatures are investigated to develop a luminous flux response surface model. The coefficients of the proposed model are further extracted to compare the luminous efficacy decay mechanisms of LED packages with different packaging structures. Furthermore, a spectral power distribution (SPD) method modeled by the Gaussian function is proposed to analyze the long-term degradation mechanisms of all selected LED packages. The results of this study show that: (1) The luminous flux of phosphor converted white LED decreases to accompany with the increase of junction temperature, while that of bare blue LED die keeps relatively stable; (2) The proposed general luminous flux response surface model can be used to predict the luminous flux of LEDs with different packaging technologies accurately, and it can be known from the proposed model that the influences of driving current and temperature on LED chip and phosphor vary with different packaging structures; and (3) The driving current and temperature dependent sensitivities and degradation mechanisms of LED packages can be investigated by using both the luminous flux response surface model and the spectral power distribution method.

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With the popularity and widespread application of high-power light-emitting diode (LED) in lighting industry, its reliability has gradually become one of research focuses.The failure of gold bonding wires in the traditional LED package has been a critical bottleneck that restricts its reliability. In this paper, the failure mechanism of LED under cyclically electrical loading is firstly identified through both gold bonding wire mechanical simulation and power cycling test experiment, which is the fatigue fracture of gold bonding wire. Then, two lifetime prediction methods, the acceleration factor extraction method based on current acceleration model and the strain-based Coffin-Manson analytical method, are established and verified with experimental results. The results show that the lifetime prediction accuracy of the proposed methods is high and they can achieve a fast and accurate reliability assessment for high-power LEDs with wire-bonding packaging technology.

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High-power light-emitting diode (LED) chip-scale packages (CSPs) prepared by the flip-chip technology have become one of the most promising light sources. The die attach solder layer always plays an important role in heat dissipation, mechanical support, and electronic conductivity. Among different types of solder materials, Sn-3.0Ag-0.5Cu (SAC305) solder alloy shows its great competitiveness on solderability and mechanical properties for the interconnection of high-power LED CSPs. However, reliability problems caused by voids in the SAC305 solder limit its wide application in the high-power LED chip-scale packaging process. Existence of the voids has been considered as one of the major issues causing chip-on-substrate level reliability problems in microelectronic and optoelectronic devices. In this paper, mechanical and thermal properties of SAC305 solder layers with arbitrary voids used in high-power LED CSPs are studied with both finite-element simulations and experiments. The results show that void size and void position within the solder layer are the two most critical issues on the shear strength of interconnection and the chip-on-substrate level thermal distribution in high-power LED CSPs.

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with the increasing requirements on guaranteeing the color uniformity and improving the luminous efficacy and manufacturing efficiency, a wafer level chip scale packaging (WLCSP) technology has been developed by thermally impressing a thin multiple phosphor film on a LED wafer, then being segmented into individual LED chips. In this paper, a high power white LED Chip-on-Board (COB) module with high color rendering index (CRI, Ra > 93) and tunable correlated color temperatures (CCTs) is prepared by the flip chip technology. In this COB module, the tunable color temperatures are achieved by using two types of white LED CSPs with different target CCTs of 3000 K and 5000 K. The thermal and photochromatic properties and the photochromatic stability of the COB module are studied through both experiments and simulations. The results show that: 1) The measured spectral power distribution (SPD) intensity of the prepared COB module is smaller than the arithmetic sum of SPD intensities of its each series connected CSPs, which may be attribute to the light absorption happened among CSPs; 2) The junction temperature and driven current have the different effects on the photochromatic properties of COB module (i.e. luminous flux, CCT and CRI); 3) The nonlinearity of luminous flux as a function of driven current and junction temperature should be considered in its modeling.

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As a widespread application of high power phosphor-converted white light emitting diodes (pc-WLEDs) with long lifetime and color-consistence, the highly reliable phosphor/silicone composite, one of core materials used for light-conversion, working under severe operation conditions has become increasingly necessary. This paper selects three widely used monochromatic phosphors (Aluminates, Silicates and Nitrides based) as well as a pristine silicone to determine the potential interaction effects in their phosphor/silicone composites operated under different environments. Firstly, the transient thermal quenching and long-term high temperature ageing tests are used to investigate the thermal stabilities of both monochromatic phosphor powders and phosphor/silicone composites. Furthermore, the degradation mechanisms of phosphor/silicone composites aged under the high temperature & blue light exposure and high temperature & high humidity conditions are studied by considering the interaction effect. The results show that: (1) the thermal stabilities of different phosphors are different and the phosphor/silicone composites have more severe thermal quenching effects than those of their corresponding phosphor powders, but with the similar degradation trends; (2) the blue light irradiation can deteriorate silicone which is related to the photolysis effect; (3) the moisture can accelerate the degradation of phosphor/silicone composites because the change of pH condition, owing to the dissolution of phosphor powders in moisture, can degrade both phosphors and silicone.@en

This paper compared the fatigue damage accumulation of the gold-tin eutectic die attach layer in different high electron mobility transistor (HEMT) packages with various types of die attach layers and substrates under thermal cyclic loading. The fatigue damage per cycle used in this study was characterized by the accumulation of plastic work, which was derived by the finite element analysis (FEA). The effects of die attach layer's standoff height and thickness of substrate on fatigue damage accumulation was discussed. The results show that increasing the standoff height of the die attach layer is an effective way to prevent the early crack initiation in gold-tin die attach layer especially for a gallium nitride (GaN) die. It is also indicated that for a GaN die, a thicker die attach layer and a thinner substrate are preferable in order to retain a comparable lifetime with silicon (Si) die and Cu substrate system.@en

In this work, a physics-of-failure (PoF) reliability prediction methodology is combined with statistical models to consider the interaction between the lumen depreciation and catastrophic failures of LEDs. The current in each LED may redistribute when the catastrophic failure occurs in one of LEDs in an array, thus affecting the operation conditions of the entire LED array. A physics-of-failure based reliability prediction methodology is combined with statistical models to consider the interaction between the lumen depreciation and the catastrophic failure. Electronic-thermal simulations are utilized to obtain operation conditions, including temperature and current. Meanwhile, statistical models are applied to calculate possibilities of the catastrophic failure in different operation conditions.

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In this study, an electro-optical simulation method is developed to predict the light intensity distribution and luminous flux of an in-house fabricated GaN based blue LED chip. The entire modeling process links an electrical simulation with ANSYS and optical simulation with LightTools, by assuming a proportional relation between the distributed current density and light emission energy on the multiple quantum well (MQW) layer. Experimental results show that the proposed simulation method can give a good prediction on the light intensity distribution for a semi-packaged GaN based blue LED chip. Further analysis on the simulation results reveals that an increase of at most 8% of the luminous flux can be achieved when the current density is controlled to evenly distribute on the MQW layer whereas the chip structure and electro pattern remains the same.

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By solving the problem of very long test time on reliability qualification for Light-emitting Diode (LED) products, the accelerated degradation test with a thermal overstress at a proper range is regarded as a promising and effective approach. For a comprehensive survey of the application of step-stress accelerated degradation test (SSADT) in LEDs, the thermal, photometric, and colorimetric properties of two types of LED chip scale packages (CSPs), i.e., 4000 °K and 5000 °K samples each of which was driven by two different levels of currents (i.e., 120 mA and 350 mA, respectively), were investigated under an increasing temperature from 55 °C to 150 °C and a systemic study of driving current effect on the SSADT results were also reported in this paper. During SSADT, junction temperatures of the test samples have a positive relationship with their driving currents. However, the temperature-voltage curve, which represents the thermal resistance property of the test samples, does not show significant variance as long as the driving current is no more than the sample's rated current. But when the test sample is tested under an overdrive current, its temperature-voltage curve is observed as obviously shifted to the left when compared to that before SSADT. Similar overdrive current affected the degradation scenario is also found in the attenuation of Spectral Power Distributions (SPDs) of the test samples. As used in the reliability qualification, SSADT provides explicit scenes on color shift and correlated color temperature (CCT) depreciation of the test samples, but not on lumen maintenance depreciation. It is also proved that the varying rates of the color shift and CCT depreciation failures can be effectively accelerated with an increase of the driving current, for instance, from 120 mA to 350 mA. For these reasons, SSADT is considered as a suitable accelerated test method for qualifying these two failure modes of LED CSPs.@en

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.@en

With the expanding application of light-emitting diodes (LEDs), the color quality of white LEDs has attracted much attention in several color-sensitive application fields, such as museum lighting, healthcare lighting and displays. Reliability concerns for white LEDs are changing from the luminous efficiency to color quality. However, most of the current available research on the reliability of LEDs is still focused on luminous flux depreciation rather than color shift failure. The spectral power distribution (SPD), defined as the radiant power distribution emitted by a light source at a range of visible wavelength, contains the most fundamental luminescence mechanisms of a light source. SPD is used as the quantitative inference of an LED's optical characteristics, including color coordinates that are widely used to represent the color shift process. Thus, to model the color shift failure of white LEDs during aging, this paper first extracts the features of an SPD, representing the characteristics of blue LED chips and phosphors, by multi-peak curve-fitting and modeling them with statistical functions. Then, because the shift processes of extracted features in aged LEDs are always nonlinear, a nonlinear state-space model is then developed to predict the color shift failure time within a self-adaptive particle filter framework. The results show that: (1) the failure mechanisms of LEDs can be identified by analyzing the extracted features of SPD with statistical curve-fitting and (2) the developed method can dynamically and accurately predict the color coordinates, correlated color temperatures (CCTs), and color rendering indexes (CRIs) of phosphor-converted (pc)-white LEDs, and also can estimate the residual color life@en

The spectral power distribution (SPD) is considered as the figureprint of a light emitting diode (LED). Based on the analysis on its SPD, a method to predict both lumen depreciation and color shift for the phosphor converted white LEDs (pc-LEDs) is proposed in this paper. First, the entire SPD of a pc-LED is predicted by superimposing two asymmetric double sigmoidal (Asym2sig) models, which represent the decomposed blue light and phosphor converted light peaks, respectively. For a better understanding of how the SPD model affects the photometric and colorimetric characteristics of a pc-LED, a sensitivity study of the SPD parameters is then performed on its luminous flux Φ, color coordinates CIE1976( u′, v′). Second, the evolutionary process of the SPD is predicted for a pc-LED with the color temperature as 3000 K under degradation testing. And based on these predicted SPDs, the drift curves of Φ, u′, v′, and du′ v′are further predicted. Finally, lifetimes of the pc-LED due to lumen depreciation and color shift are estimated simultaneously from the predicted Φ and du′ v′ drift curves.

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In this paper, an integrated LED lamp with an electrolytic capacitor-free driver is considered to study the coupling effects of both LED and driver's degradations on lamp's lifetime. An electrolytic capacitor-less buck-boost driver is used. The physics of failure (PoF) based electronic thermal simulation is carried out to simulate the lamp's lifetime in three different scenarios: Scenario 1 considers LED degradation only, Scenario 2 considers the driver degradation only, and Scenario 3 considers both degradations from LED and driver simultaneously. When these two degradations are both considered, the lamp's lifetime is reduced by about 22% compared to the initial target of 25,000 h. The results of Scenario 1 and 3 are close to each other. Scenario 2 gives erroneous results in terms of luminous flux as the LED's degradation over time is not taken into consideration. This implies that LED's degradation must be taken into considerations when LED and driver's lifetimes are comparable.

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In this paper, the heat transfer performance of the multi-chip (MC) LED module is investigated numerically by using a general analytical solution. The configuration of the module is optimized with genetic algorithm (GA) combined with a response surface methodology. The space between chips, the thickness of the metal core printed circuit board (MCPCB), and the thickness of the base plate are considered as three optimal parameters, while the total thermal resistance (R_tot) is considered as a single objective function. After optimizing objectives with GA, the optimal design parameters of three types of MC LED modules are determined. The results show that the thickness of MCPCB has a stronger influence on the total thermal resistance than other parameters. In addition, the sensitivity analysis is performed based on the optimum data. It reveals thatR_tot increases with the increased thickness of MCPCB, and reduces as the space between chips increases. The effect of the thickness of base plate is far less than that of the thickness of MCPCB. After optimization, three types of MC LED modules obtain lower T_j andR_tot. Moreover, the optimized modules can emit large luminous energy under high-power input conditions. Therefore, the optimization results are of great significance in the selection of configuration parameters to improve the performance of the MC LED module.

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Light Emitting Diode (LED) luminaires and lamps are energy-saving and environmental friendly alternatives to traditional lighting products. However, current luminous flux depreciation test at luminaire and lamp level requires a minimum of 6000 h testing, which is even longer than the product development cycle time. This paper develops an accelerated test method for luminous flux depreciation to reduce the test time within 2000 h at an elevated temperature. The method is based on lumen maintenance boundary curve, obtained from a collection of LED source lumen depreciation data, known as LM-80 data. The exponential decay model and Arrhenius acceleration relationship are used to determine the new threshold of lumen maintenance and acceleration factor. The proposed method has been verified by a number of simulation studies and experimental data for a wide range of LED luminaire and lamp types from both internal and external experiments. The qualification results obtained by the accelerated test method agree well with traditional 6000 h tests.@en

The temperature of electrolytic capacitor in light-emitting diode (LED) drivers continuously increases under operation conditions, thus the capacitors degrade faster than that with constant temperature assumption. In this paper, a physics-of-failure (PoF)-based reliability prediction methodology is developed for LED drivers to consider the temperature change of electrolytic capacitor. SPICE simulation, compact thermal modeling, and Monte Carlo simulation are integrated to predict the failure rate distribution of an electrolytic capacitor of given LED driver systems. The simulation results agree well with the accelerated test results for an RC linear AC-DC converter. Furthermore, a single inductor buck-boost DC-DC converter is simulated to understand the degradation behavior of electrolytic capacitor. It has been found that the temperature of an output stage capacitor increases significantly during operation time. The capacitor's performance without taking temperature change into account results in an overestimated driver lifetime by more than 38% for the selected case study.@en

PMMA material is widely used in LED-based luminaires due to several advantages such as excellent optical transparency, durability against radiation, surface hardness (scratch free), rigidity and strength and can be completely recycled. However, few studies have been reported on the colour shift and failure mechanisms caused by this type of material. This paper experimentally investigated PMMA materials with different aging conditions. The following conclusions could be drawn. (1) Discolouration was not observed for any sample subjected to aging of 85 °C for 5000 h, or with additional blue light irradiation for 5000 h, or with additional humidity of 85%RH for 5000 h, or even with aging of 100 °C for 3000 h. (2) The specimen subjected to aging of 150 °C for 360 h has a surface discoloration and has a significant wavelength dependent degradation in the transmission spectrum caused by oxidation. The specimen with aging of 100 °C for 3000 h has a less oxidation, although no significant transmission spectrum reduction was observed. (3) Using such aged specimen as a diffuser mounted on a LED-based luminaire, the radiant flux peak intensity in the blue light area has a more severe reduction than that in the yellow light area, which results in a reduction of the radiant flux intensity ratio of blue light to yellow light and hence induces the colour shift to yellow. The colour shift investigated is 0.005, very close to the general failure criterion of 0.007, while the lumen decay is 10.2%, far less than the failure criterion of 30%.@en

The drive to increase electrical currents input, and to achieve high lumen output for High Brightness Light Emitting Diode (HB LED) has led to a series of thermal problems. Hence, thermal management is one of key design parameters as the temperature directly affecting the maximum light output, quality, reliability and life time. For LED arrays, the arrangement of chips will affect the thermal performance and junction temperature is very important in the design. However, because direct junction temperature determination is hard to detect due to the encapsulation, indirect methods are more preferred. Among them, the diode forward-voltage method is the most accurate one. Therefore, we used the diode forward-voltage method to optimize thermal design of LED arrays in this work.@en