Prediction of Lumen Depreciation and Color Shift for Phosphor-Converted White Light-Emitting Diodes Based on A Spectral Power Distribution Analysis Method

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.Electronic Components, Technology and Material

A PoF and statistics combined reliability prediction for LED arrays in lamps

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.Electronic Components, Technology and Material

Efficient Li-Metal Plating/Stripping in Carbonate Electrolytes Using a LiNO₃-Gel Polymer Electrolyte, Monitored by Operando Neutron Depth Profiling

The development of safe and high-performance Li-metal anodes is crucial to meet the demanded increase in energy density of batteries. However, severe reactivity of Li metal with typical electrolytes and dendrite formation leads to a poor cycle life and safety concerns. Therefore, it is essential to develop electrolytes that passivate the reactivity toward Li metal and suppress dendrite formation. Carbonate electrolytes display severe reactivity toward Li metal; however, they are preferred above the more volatile ether-based electrolytes. Here, a carbonate electrolyte gel polymer approach is combined with LiNO3 as an additive to stabilize Li-metal plating. This electrolyte design strategy is systematically monitored by operando neutron depth profiling (NDP) to follow the evolution of the plated Li-metal density and the inactive lithium in the solid electrolyte interface (SEI) during cycling. Individually, the application of the LiNO3 electrolyte additive and the gel polymer approach are shown to be effective. Moreover, when used in conjunction, the effects are complementary in increasing the plated Li density, reducing inactive Li species, and reducing the overpotentials. The LiNO3 additive leads to more compact plating; however, it results in a significant buildup of inactive Li species in a double-layer SEI structure, which challenges the cell performance over longer cycling. In contrast, the gel polymer strongly suppresses the buildup of inactive Li species by immobilizing the carbonate electrolyte species; however, the plating is less dense and occurs with a significant overpotential. Combining the LiNO3 additive with the gel polymer approach results in a thin and homogeneous SEI with a high conductivity through the presence of Li3N and a limited buildup of inactive Li species over cycling. Through this approach, even high plating capacities, reaching 7 mAh/cm2, can be maintained at a high efficiency. The rational design strategy, empowered by monitoring the Li-density evolution, demonstrates the possibilities of achieving stable operation of Li metal in carbonate-based electrolytes.RST/Storage of Electrochemical Energ

Reliability Assessment of Light-Emitting Diode Packages with Both Luminous Flux Response Surface Model and Spectral Power Distribution Method

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.Electronic Components, Technology and Material

Study on Sintering Mechanism and Mechanical Properties of Nano-Cu based on Molecular Dynamics Simulation

Nano-metal materials sintering has received increasing attention in recent years for its promising performance in the wide bandgap semiconductor packaging. In this paper, molecular dynamics (MD) simulation method were applied to simulate the nano-Cu sintering mechanism and the subsequent mechanical behavior. Hybrid sintering, comprising nanosphere (NS) and nanoflake (NF), was carried out at temperatures ranging from 500K to 650K. Furthermore, shearing simulations were conducted with constant strain rates on the sintered structure at multiple temperatures, and subsequently correlated the extracted mechanical properties with the sintering behavior. The results indicated that the mechanical properties of nano-Cu sintered structure were improved by tuning material composition and increasing the sintering temperature. We established a relationship between the sintered microstructure and mechanical response, the shear modulus and shear strength of the sintered structure with NF particles increased to 41.2GPa and 3.51GPa respectively. It offers valuable insights into the preparation phase of nano Cu paste for sintering technology.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic Components, Technology and Material

Color Shift Failure Prediction for Phosphor-Converted White LEDs by Modeling Features of Spectral Power Distribution with a Nonlinear Filter Approach

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 lifeElectronic Components, Technology and Material

Photometric and colorimetric assessment of LED chip scale packages by using a step-stress accelerated degradation test (SSADT) method

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.Electronic Components, Technology and Material

Tensile characterization and constitutive modeling of sintered nano-silver particles over a range of strain rates and temperatures

Sintered nano-silver die-attach materials have been widely used in high-power electronics packaging because of their high thermal and electrical conductivities. In this study, we characterized the tensile properties of sintered nano-silver particles over a range of strain rates and temperatures, and established the constitutive models. First, 50 nm nano-silver particles were sintered at 275 °C for 50 min as test samples, and their tensile tests were conducted under a dynamic thermomechanical analyzer (DMA Q800) and an IBTC 300SL in-situ mechanical test system respectively with different strain rates and ambient temperatures. Then, both Anand and variable-order fractional models (VoFM) were adopted to analyze the obtained stress-strain data and we studied their fitting accuracy and applicability. The results showed that: (1) The Young's modulus of the sintered nano-silver particles decreased with increasing temperature. In addition, the tensile strengths declined under lower strain rates and higher temperature conditions; (2) both the Anand model and VoFM characterized the tensile stress-strain properties of the sintered nano-silver material well. Compared to the Anand model, the VoFM utilized a simpler formula with fewer parameters and higher precision.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic Components, Technology and Material

A novel lifetime prediction for integrated LED lamps by electronic-thermal simulation

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.Accepted author manuscriptElectronic Components, Technology and Material

Numerical thermal analysis and optimization of multi-chip LED module using response surface methodology and genetic algorithm

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 (Rtot) 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 thatRtot 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 Tj andRtot. 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.Electronic Components, Technology and Material