Light Emitting Diode Package Research Articles

Impact of thermal interface material on luminous flux curve of InGaAlP low-power light-emitting diodes

Purpose One major problem in the lighting industry is the thermal management of the devices. Handling of thermal resistance from solder point to the ambiance of the light-emitting diode (LED) package is linked to the external thermal management that includes a selection of the cooling mode, design of heatsink/substrate and thermal interface material (TIM). Among the significant factors that increase the light output of the of the LED system are efficient substrate and TIM. In this work, the influence of TIM on the luminous flux performance of commercial indium gallium aluminium phosphide (InGaAlP) low-power (LP) LEDs was investigated. Design/methodology/approach One batch of LEDs was mounted directly onto substrates which were glass-reinforced epoxy (FR4) and aluminium-based metal-core printed circuit boards (MCPCBs) with a dielectric layer of different thermal conductivities. Another batch of LEDs was prepared in a similar way, but a layer of TIM was embedded between the LED package and substrate. The TIMs were thermally conductive epoxy (TCE) and thermally conductive adhesive (TCA). The LED parameters were measured by using the integrated system of thermal transient tester (T3Ster) and thermal-radiometric characterization of LEDs at various input currents. Findings With the employment of TIM, the authors found that the LED’s maximum luminous flux was significantly higher than the value mentioned in the LED datasheet, and that a significant reduction in thermal resistance and junction temperature was revealed. The results showed that for a system with low thermal resistance, the maximum luminous flux appeared to occur at a higher power level. It was found that the maximum luminous flux was 24.10, 28.40 and 36.00 lm for the LEDs mounted on the FR4 and two MCPCBs, respectively. After TCA application on the LEDs, the maximum luminous flux values were 32.70, 36.60 and 37.60 lm for the FR4 and MCPCBs, respectively. Moreover, the findings demonstrated that the performance of the LED mounted on the FR4 substrate was more affected by the employment of the TIM than that of MCPCBs. Research limitations/implications One of the major problems in the lighting industry is the thermal management of the device. In many low-power LED applications, the air gap between the two solder pads is not filled up. Heat flow is restricted by the air gap leading to thermal build-up and higher thermal resistance resulting in lower maximum luminous flux. Among the significant factors that increase the light output of the LED system are efficient substrate and TIM. Practical implications The findings in this work can be used as a method to improve thermal management of LP LEDs by applying thermal interface materials that can offer more efficient and brighter LP LEDs. Using aluminium-based substrates can also offer similar benefits. Social implications Users of LP LEDs can benefit from the findings in this work. Brighter automotive lighting (signalling and backlighting) can be achieved, and better automotive lighting can offer better safety for the people on the street, especially during raining and foggy weather. User can also use a lower LED power rating to achieve similar brightness level with LED with higher power rating. Originality/value Better thermal management of commercial LP LEDs was achieved with the employment of thermal interface materials resulting in lower thermal resistance, lower junction temperature and brighter LEDs.

Performance of Spherical Quantum Well Down Converters in Solid State Lighting.

We report the color conversion performance of amber and red emitting quantum dots (QDs) on InGaN solid-state lighting (SSL) light emitting diode (LED) packages. Spherical quantum well (SQW) architectures (CdS/CdSe1-xSx/CdS) were prepared using a library of thio- and selenourea synthesis reagents and high throughput synthesis robotics. CdS/CdSe1-xSx QDs with narrow luminescence bands were coated with thick CdS shells (thickness = 1.6-7.5 nm) to achieve photoluminescence quantum yields (PLQY) up to 88% at amber and red emission wavelengths (λmax = 600-642 nm, FWHM < 45 nm). The photoluminescence from SQWs encapsulated in silicone and deposited on LED packages was monitored under accelerated aging conditions (oven temperature = 85 °C, relative humidity = 5-85%, blue optical power density = 3-45 W/cm2) by monitoring the red photon output over several hundred hours of continuous operation. The growth of a ZnS shell on the SQW surface increases the stability under long-term operation but also reduces the PLQY, especially of SQWs with thick CdS shells. The results illustrate that the outer ZnS shell layer is key to optimizing the PLQY and the long-term stability of QDs during operation on SSL packages.

Wearable Devices Made of a Wireless Vertical-Type Light-Emitting Diode Package on a Flexible Polyimide Substrate with a Conductive Layer

In this study, wearable devices are made using wireless vertical-type light-emitting diode (LED) packages of a transparent conductive film coated on flexible colorless polyimide (PI) with 50 μm thi...

Highly Efficient Liquid-Quantum Dot/Melamine- Modified Urea-Formaldehyde Microcapsules for White Light-Emitting Diodes

A liquid matrix is beneficial for improving the luminescence performance of quantum dots (QDs), although it is difficult to be packaged with light-emitting diode (LED) chips. In this study, liquid-QDs with chlorobenzene solution and melamine-modified urea-formaldehyde (MUF) were used as the core and shell structures, respectively, successfully yielding liquid-QD/MUF microcapsules for white LED packaging. The concentration of the liquid-QDs in the microcapsules was optimized, demonstrating an increase of 15.9% in the luminous efficiency, as compared to the traditional solid-QDs at a specific QD energy proportion. The efficiency roll-off can be attributed to the increased reabsorption and back-scattering loss at higher liquid-QDs and microcapsules concentrations. It was also confirmed that the liquid-QD/MUF microcapsules have an excellent thermal stability below 200 °C, owing to the protection of the MUF shells. Finally, white LEDs compounding the red liquid-QD/MUF microcapsules and YAG phosphor particles were fabricated, achieving a high luminous efficiency of 149.2 lm/W at 20 mA, correlated color temperature of 3301 K, and color rendering index of 88.8. This study will open new research avenues for liquid-QD packaging in white LEDs.

A Gamma process-based degradation testing of silicone encapsulant used in LED packaging

Silicone encapsulant is widely used in light-emitting diode (LED) packaging because it offers high light transmittance, high refractive index, high thermal stability, and long lifetimes. However, it is extremely sensitive to moisture when the LED operates under high temperature and high humidity. In this study, constant moisture stress-accelerated degradation tests (CSADT) are designed with three different thermal stresses on silicone encapsulant, and its real lumen decay and color shift lifetimes are predicted by integrating the accelerated lifetime model with the Gamma process model considering the random effects of the degradation data. The results show that: (1) the lumen decay path after natural logarithmic transformation as well as the color shift degradation path can be well fitted with the linear model. In addition, the degradation rate is related to the thermal stress under the same humidity stress condition; (2) the Gamma process model has a high prediction accuracy when the lumen decay lifetime is used for estimation. On the contrary, the least squares regression (LSR) model has shown superior prediction accuracy versus the Gamma process model when color shift degradation data are used.

Controlling Phosphor Particle Distribution for High-Angular-Color-Uniformity and Low-Cost LEDs Based on Thermalcapillary Flow

Low cost and high performance are the pursuing goals of the light-emitting diode (LED) packaging. The phosphor layer configuration of LEDs significantly influences optical performances. In this study, thermalcapillary flow was introduced in the phosphor coating by the blue laser local irradiation. It has been demonstrated to successfully manipulate the phosphor particle distributions. In the phosphor dispensing coating, a centripetally concentrated particle distribution was realized and resulted in high angular color uniformity (ACU) and low-cost manufacture. Its angular correlated color temperature (CCT) deviation was reduced by more than 65%, compared to the traditional phosphor dispensing coating. Besides, the phosphor amount is saved by about 12%. The mechanism of ACU enhancement is analyzed. The realized phosphor particle distribution can inspire further development of the optimized phosphor layer configuration.

A three-dimensional thermal analysis and optimization of square light edding diode subcomponents

The purpose of this paper is to assess thermal management and optimization of several parameters such as: materials, size and thickness of all subcomponents of single-chip Light Emitting Diode (LED) in order to minimize its junction temperature and thereby maximizing light output and device reliability.For this, we developed a three-dimensional code using Comsol Multiphysics that solves the systems of equations for the mass, momentum and energy. After validation of this numerical 3D code, the thermal Characterization of Subcomponents of LED system is studied numerically. This system is divided into 6 parts such as: metallization, die, die-attach, substrate, Thermal Interface Material and heat sink. Several materials which can be used in each part of the LED package from metallization to the heat sink are studied. By moving to one part to another, it was selected the best materials always allowing a minimum junction temperature.

Thermal management of high-power LED module with single-phase liquid jet array

Next generation high-power Light–Emitting Diodes (LED) require specialized cooling systems for ensuring performance and reliability. The ability of a multi-jet single-phase liquid cooling system (jet diameter 400 µm) for thermal management of the high-power LED based luminaire is investigated and successfully demonstrated for a 300 W nominal power module. Operating pressure of the jet system is varied in the range of 0–8 bar, leading to coolant water flow rates varying from 65 ml/min to 782 ml/min (Re = 3833–46119). With the proposed multi-jet cooling system, the experiments show the possibility of maintaining the module surface temperature well below 70 °C for substrate level heat flux up to ~125 W/cm2. We show that the system can still maintain safe operating temperature, without loss of luminous efficiency for input power up to 130% of the nominal design power. Supporting detailed three-dimensional numerical simulations of the conjugate module heat transfer inside the LED package volume are provided. It is concluded that single-phase array of water-based jet flow is an excellent potential option for thermal management of high-power LED luminaire.

Controlling the Surface Properties of TiO2 for Improvement of the Photo-performance and Color Uniformity of the Light-emitting Diode Devices

The photo-performance and color uniformity of the light-emitting diodes (LEDs) were enhanced by controlling the surface properties of the titanium dioxide (TiO2) particles as light scatting materials using a sol-gel-based surface modification method. Silane coupling agents of different carbon chain lengths (C3–C16) were introduced onto the TiO2 surface for investigating the light scattering effect depending on the TiO2 dispersibility in the encapsulant. The dispersibility of the modified TiO2 particles was greater than that of the as-prepared TiO2 particles (1.6–2.5 times). The dispersibility of TiO2 improved with the increasing carbon chain length of silane (C3–C6). However, the dispersibility did not increase but reduced when the carbon chain length was considerably long (C16) because the TiO2 surface was more hydrophobic than the encapsulant. After the fabrication of the LED devices, the TiO2-loaded LED device showed high photometric flux and low correlated color temperature despite using the same concentration of phosphor (3.0wt.% with respect to the added silicone resin). Furthermore, the photo-performance and color uniformity improved when the TiO2 dispersibility was increased. This can be attributed to the enhancement of light scattering in the LED package when the interfacial affinity of the TiO2/encapsulant was improved.

Failure-analysis method of soldering interfaces in light-emitting diode packages based on time-domain transient thermal response

New soldering-interface failure-analysis method based on the transient thermal response of the LED packages proposed. This method is based on the physical properties that the operating voltage of the LED packages is inversely proportional to the junction temperature and the change of the junction temperature, which is monitored by voltage, represents the change of thermal gradient in the LED packages. Fast time-domain algorithm for failure-sensitivity enhancement was developed. And the feasibility of this algorithm was verified using a two-point measurement method during a thermal shock test.

An Optimized Ag–5Pd–3.5Au Bonding Wire for the Resistance of Ag Ion Migration in LED Packages

The bondability and reliability of Ag–5Pd–3.5Au alloy wire was verified previously for application in light-emitting diode (LED) packages. This Ag-alloy bonding wire has an electrical resistivity of 4.6 $\mu \Omega \cdot $ cm, lower than those of the ternary Ag–3Pd–20Au (8.6 $\mu \Omega \cdot $ cm) and Ag–3Pd–8Au wires (5.0 $\mu \Omega \cdot $ cm). Further evaluation of the resistance of ion migration in Ag-alloy bonding wires using water drop tests indicated that a declining tendency of ion migration rates existed for various binary Ag–Pd and ternary Ag–Pd–Au alloy wires with increases in Pd and Au contents. Among the all Ag-alloy bonding wires in this study, the Ag–5Pd–3.5Au wire exhibited the lowest ion migration rate of 4.87 $\mu \text{m}$ /s. The measurements of anodic dissolution rates were consistent with the tendency of ion migration rates, which were dependent on Pd and Au contents, resulting in the low value of Ag–5Pd–3.5Au alloy wire (0.017 $\mu \text{m}$ /s).

Precise measurement of junction temperature by thermal analysis of light-emitting diode operated at high environmental temperature

Light-emitting diodes (LEDs) have become a promising solid-state light source in numerous applications. During operation, they are often exposed to high environmental temperatures, resulting in performance and reliability degradation. It is thus important to monitor the temperature at the junction, which is a primary lighting and heat source, especially for an LED operated under a high-temperature environment. In this study, the temperatures at the junction, phosphor, and thermal pad of the LED operated at high temperatures (up to 107.2 °C) are measured and analyzed. The junction temperature is measured by a transient thermal tester at known environmental temperatures and input currents. Moreover, the temperatures at the phosphor and thermal pad are simultaneously measured by using an infrared (IR) thermometer and a thermocouple, respectively. A single highly linear temperature correlation (R2 = 0.997) is found between the junction and phosphor temperatures regardless of the operating conditions. This could aid in precisely estimating the junction temperature by simply measuring the surface temperature of phosphor. On the other hand, the correlation between the junction and thermal pad temperature that is often treated as a single correlation exhibits dependence on the input current over the tested temperature range. The structural thermal analysis on the thermal resistance of the LED package supports the experimental results along with the numerical analysis. The obtained results in this study could provide a set of guidelines for the precise prediction of junction temperature and thermal management of LEDs, which are operated under high-temperature environments.

Multi-Site Cation Control of Ultra-Broadband Near-Infrared Phosphors for Application in Light-Emitting Diodes.

Near-infrared (NIR) phosphors are fascinating materials that have numerous applications in diverse fields. In this study, a series of La3Ga5GeO14:Cr3+ phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was successfully synthesized using solid-state reaction to explore every cationic site comprehensively. The crystal structures were well resolved by combining synchrotron X-ray diffraction and neutron powder diffraction through joint Rietveld refinements. The trapping of free electrons induced by charge unbalances and lattice vacancies changes the magnetic properties, which was well explained by a Dyson curve in electron paramagnetic resonance. Temperature and pressure-dependent photoluminescence spectra reveal various luminescent properties between strong and weak fields in different dopant centers. The phosphor-converted NIR light-emitting diode (pc-NIR LED) package demonstrates a superior broadband emission that covers the near-infrared (NIR) region of 650-1050 nm. This study can provide researchers with new insight into the control mechanism of multiple-cation-site phosphors and reveal a potential phosphor candidate for practical NIR LED application.

Y2O3:Ho3+ and ZnO:Bi3+: a selection for enhancing color quality and luminous flux of WLEDs

As the luminescence industry develops, the white light light-emitting diode (LED) package with a single chip and a single phosphor although produces good luminous flux but has a poor color rendering index (CRI) can no longer fulfill the requirements of modern lighting applications. Therefore, this research is conducted to response to the urgent demands of improving other lighting qualities of WLED while maintaining high luminous efficiency. To achieve this target, we applied the new WLED package, which contains multi-chips and multi-phosphor layers, and have obtained outstanding results in both CRI and luminous efficacy. Two types of phosphor used in the WLED package are Y2O3:Ho3+ and ZnO:Bi3+. A color configuration model is also developed to adjust the shading of the white-light LED module. The results of this research show that the triple-layer phosphorhas the best performance when applied in a white-light LED package, which is demonstrated through better color quality, CRI and luminous efficacy, The manufacturers can rely on this research to produce the optimal-quality WLED, or WLED that is appropriate to their quality demands.

The Improvement of Bonding Metal Layers in a GaAs Vertical Structure Light-Emitting Diode Package

This paper presents an investigation into the reliability of substrate bonding between metal layers and gallium arsenide vertical-injection light-emitting diodes (GaAs VLEDs). In this study, molybdenum (Mo) acts as a buffer layer between the GaAs VLED and the aluminum-based metal core printed circuit board (MCPCB). The thermal expansion coefficient (CTE) value of GaAs is approximately equal to that of the Mo layer under the substrate. This is beneficial for slowing the decay of the adhesive bond due to temperature cycling from − 40°C to 125°C for 50 cycles. The experimental results show that the use of the Mo buffer layer greatly improved the electrical and optical performance of the LED packages. The average leakage current was reduced from 33.63 mA to 2.37 mA at the reversed voltage of − 3 V and the increase in light output power was 130% at an injection current of 0.7 A. The results also show that a Mo buffer layer can reduce the risk of cracking on bonding wires after the thermal shock test (TST). These results are useful for improving the aging of GaAs VLEDs.

Influence of structural and surface properties of MgO thin film as a heat spreader on high power LED performance

Thermal management difficulties in light emitting diodes (LEDs) incredibly impacts LEDs performance, reliability, and lifespan. To find a lasting solution to LEDs thermal management difficulties, MgO thin film was deposited on Cu substrate and used as heat spreader for LED package. Influence of structural and surface properties on thermal performance of LED was investigated by XRD, FESEM and AFM. Optimum LED thermal performance was recorded from the LED mounted to MgO heat spreader with low microstrain of 7.3 × 10–3, uniform particle size (34 nm), surface roughness (4.5 nm) and minimum peak-valley distance (1.90 nm). Thermal conductivity and thermal transient characterization illustrated that 0.6 M MgO thin film showed 15.73 W/mK, higher junction temperature difference (18.06 °C) with higher total thermal resistance difference (3.35 K/W). Improved illumination and reduction in LED surface temperature were recorded using MgO heat spreader from optical and thermal infrared analysis. Therefore, magnesium oxide thin film would be suggested as a heat spreader for enhancing LED’s thermal management.

Micro-rolling Forming of Light Extraction Structure on Substrate for LED Chip-on-Board Package

The light extraction structure (LES) on substrate for chip-on-board (COB) packaging light-emitting diodes is an effective means to address the intrinsic problem—the total internal reflection occurred within a COB packaging device. This work develops a new manufacturing method—micro-rolling forming to machine inverted pyramid LESs with high efficiency and low cost. In this method, a roller with closely arrayed micro pyramid teeth is designed and rolled on a copper substrate. A finite element model is established to analyze the forming process of the inverted pyramids and the forming process can be divided into four stages: elastic stage, yielding stage, hardening stage, full plastic deformation stage. The stress and strain mainly concentrate on the contact zone with the micro tooth abdomen. In this contact zone, the stress plateaus in the full plastic deformation stage are high and long, which drives the inverted pyramid formation. The side length of the inverted pyramid increases, while the cone angle decreases with the rolling depth increasing. Whereas the side length and cone angle vary slightly with the rolling speed increasing. The better rolling depth and speed for LES forming are 0.3–0.4 mm and 10–20 mm/s, respectively.

Effects of silicone lens aging on degradation kinetics of light-emitting diode package in various accelerated testing

Silicone is commonly used in light-emitting diode (LED) package as lens and/or encapsulation owing to its high light transmittance, large refractive index, and good thermal stability. However, the effect of silicone lens aging on the lumen decay and color shift of LED package is a considerable concern for package level reliability. This study empirically investigates the effects of silicone lens aging on lumen decay and color shift of LED package under three accelerated tests, namely, the constant stress-accelerated degradation test, step-up stress-accelerated degradation test, and step-down stress-accelerated degradation test (SDSADT). Results reveal that silicone lens aging substantially contributed to lumen decay and color shift of LEDs and considerably influenced the degradation kinetics of LED package. The color coordinates of LED package covered with aged silicone specimen in three testing methods exhibit a steady yellowing shift law at the same 45-degree direction. Silicone lens aging primarily reduces its transmittance to blue light as accumulated thermal oxidation and hydrolysis reaction, and its effect on the blue peak intensity of LED is independent of the stress-loaded order of step stress-accelerated degradation testing (SSADT). In addition, the effect law of silicone lens aging on LED lumen and color shift could be presented with the exponential and power function models, respectively. Lumen decay and color shift are mostly independent of the historical stress and loaded order in SSADT. The SDSADT testing manner more effectively distinguishes the color shift effect of thermal stress level compared with other accelerated tests. These findings are assumed to benefit the design and reliability assessment of LED products.

Effect of (Ni, Au)3Sn4 growth on the thermal resistance of Au-20 wt% Sn solder/ENIG joint in flip-chip LED packages

In this study, the effect of growth of intermetallic compound (IMC) layer between Au-20 wt% Sn solder and electroless Ni/immersion Au (ENIG) on the thermal resistance of a flip-chip (FC) light emitting diode (LED) package was investigated. Two IMC layers were formed at the interface of FC LED package, including Au5Sn and (Ni, Au)3Sn4 that were identified using a transmission electron microscopy. A thermal aging test was carried out at 200 °C for 1000 h to observe IMCs growth. After 1000 h, the (Ni, Au)3Sn4 thickness increased to about 1.5 μm and the thermal resistance of the FC LED package increased by 3.5 times compared to the initial thermal resistance of 2 K W−1. As the (Ni, Au)3Sn4 grew, a calculation of thermal resistance also increased comparing to the initial aging test, thus, the growth of (Ni, Au)3Sn4 layer can be effective factor on the increase of thermal resistances of FC LED package.

Improving color rendering index of WLEDs with convex-dual-layer remote phosphor geometry using red-emitting CaGa2S4:Mn2+ phosphor

The white-light light-emitting diode (LED), a conventional illumination solution, usually consists of one chip and one phosphor layer, which leads to the insufficient color rendering index (CRI) in this configuration. To optimize the efficiency of WLED, a new LED package with 2 chips and one phosphor was proposed, this innovative configuration can yield more lights and achieve high CRI. Thus, this study aims at perfecting the color performance with the two chips and dual phosphor layers package with the proportions and densities of phosphor in the silicone constantly changed to find the best option. The white-light LED module is adjusted using a specialized color design model. The comparison results between the measured and the simulation from the color design model CIE 1931 color coordinates suggest that the highest discrepancy is about 0.0063 and is achieved at around 5600K correlated color temperature (CCT). This study’s results lay a firm path in customizing white-light LED modules that guarantee CRI and lumen output qualities.