Light-emitting Diode Encapsulation Research Articles

Optimization of Al2O3/TiO2 nanolaminate thin films prepared with different oxide ratios, for use in organic light-emitting diode encapsulation, via plasma-enhanced atomic layer deposition.

Encapsulation is essential for protecting the air-sensitive components of organic light-emitting diodes (OLEDs), such as the active layers and cathode electrodes. Thin film encapsulation approaches based on an oxide layer are suitable for flexible electronics, including OLEDs, because they provide mechanical flexibility, the layers are thin, and they are easy to prepare. This study examined the effects of the oxide ratio on the water permeation barrier properties of Al2O3/TiO2 nanolaminate films prepared by plasma-enhanced atomic layer deposition. We found that the Al2O3/TiO2 nanolaminate film exhibited optimal properties for a 1 : 1 atomic ratio of Al2O3/TiO2 with the lowest water vapor transmission rate of 9.16 × 10(-5) g m(-2) day(-1) at 60 °C and 90% RH. OLED devices that incorporated Al2O3/TiO2 nanolaminate films prepared with a 1 : 1 atomic ratio showed the longest shelf-life, in excess of 2000 hours under 60 °C and 90% RH conditions, without forming dark spots or displaying edge shrinkage.

A quality photocatalytic activity of an amorphous titanium oxide film achieved using the selectively photochemical etching and its application for the organic light emitting diode encapsulation

In this study, a rapid, cost-effective, and room-temperature achieved selectively photochemical etching (SPCE) process was developed to improve the photocatalytic activity of a PECVD-deposited amorphous titanium oxide (a-TiOx) film. The a-TiOx film treated by the SPCE process showed a photocatalytic activity comparable to that of a high-temperature prepared TiOx film. As examined from the surface roughness and morphologies, optical transmittance, and chemical bond configurations, the mechanism responsible for the significant enhancement on the photocatalytic activity was ascribed to the formation of the nano-textures with the increase in the surface acidity due to the incorporating of fluorine ions. The SPCE-treated a-TiOx film with the quality photocatalytic activity was then applied to encapsulate the blue organic light emitting diode (OLED) to result in the surface with self-cleaning function. A super-hydrophilic surface was available using the internal light irradiation. In addition, the OLED encapsulated by the SPCE-treated a-TiOx film also was beneficial for enhancing the light-extraction efficiency as a consequence of its notably nano-textured surface.

Performance enhancement of white light-emitting diodes using an encapsulant semi-solidification method

High-optical-quality white light-emitting diodes (LEDs) are leading candidates for next-generation lighting and numerous other potential applications. In this work, a simple and innovative strategy based on semi-solidification was used to prepare white LEDs with a highly uniform illumination distribution. Semi-solidification facilitates uniform dispersion of fluorescent phosphors throughout the LED encapsulant by adopting a B-stage intermediate step, thereby enabling superior color uniformity in the LED. This approach is based on the control of the curing process and offers a reasonable and effective solution for problems associated with conventional LEDs, including the yellow ring phenomenon that arises from non-uniform illumination. Our novel and simple approach provides a facile route for the fabrication of novel, high-performance LED encapsulants. This method is effective for the enhancement of luminous efficacy, long-term stability, and color uniformity of LEDs, indicating their potential use in industrial and practical applications.

Separation Characteristics of an LMPA-Polymer Mixture for the Encapsulation of Organic Light-Emitting Diode

An encapsulation method using the separation of low melting point alloy (LMPA) and a polymer from an LMPA-polymer mixture is proposed. Sn-Bi alloy and epoxy were used for the mixture. Separation was conducted using a two-step heat treatment of 160 °C for 5 min and 180 °C for 5 min. We found that the flux contained in the Sn-Bi solder played an important role in the separation process. A Cu sacrificial layer was introduced using the sputtering method under the Sn-Bi layer with thickness variations of 1500, 3000, and 6000 A. Using the 6000-A-thick Cu layer, the LMPA formed a well-separated and continuous sealing line. This double-line structure shows good potential for use as a sealing line for organic light-emitting diode encapsulation.

Synthesis of an adhesion‐enhancing polysiloxane containing epoxy groups for addition‐cure silicone light emitting diodes encapsulant

Addition‐cure silicone resin is considered as a good choice for light emitting diodes (LEDs); however, it has very poor adhesion to the substrate, which limits its practical application. A novel polysiloxane with self‐adhesion ability and higher refractive index for the encapsulating of high‐power LEDs is prepared and characterized. This polysiloxane containing vinyl groups, phenyl groups, and epoxy groups was synthesized by a sol‐gel condensation process from methacryloxy propyl trimethoxyl silane, γ‐(2,3‐epoxypropoxy)propytrimethoxysilane, and diphenylsilanediol under the catalysis of an anion exchange resin. Then, the resin‐type encapsulation material was prepared by hydrosilylation of methylphenyl hydrogen‐containing silicone resin and the newly synthesized polysiloxane material. The novel polysiloxane was characterized by 1H‐NMR and Fourier transform infrared spectroscopy. On the basis of higher refractive index, higher transparency, excellent thermal stability, and appropriate hardness, as well as good adhesive strength between the encapsulating material and the LED lead frame (polyphthalamide), the curable silicone resin‐type encapsulation material can be used as an encapsulant for LEDs. Copyright © 2014 John Wiley & Sons, Ltd.

An Optimal Cure Process to Minimize Residual Void and Optical Birefringence for a LED Silicone Encapsulant.

Silicone resin has recently attracted great attention as a high-power Light Emitting Diode (LED) encapsulant material due to its good thermal stability and optical properties. In general, the abrupt curing reaction of the silicone resin for the LED encapsulant during the curing process induces reduction in the mechanical and optical properties of the LED product due to the generation of residual void and moisture, birefringence, and residual stress in the final formation. In order to prevent such an abrupt curing reaction, the reduction of residual void and birefringence of the silicone resin was observed through experimentation by introducing the multi-step cure processes, while the residual stress was calculated by conducting finite element analysis that coupled the heat of cure reaction and cure shrinkage. The results of experiment and analysis showed that it was during the three-step curing process that the residual void, birefringence, and residual stress reduced the most in similar tendency. Through such experimentation and finite element analysis, the study was able to confirm that the optimization of the LED encapsulant packaging process was possible.

Enhanced Light Extraction Using Blue LED Package Consisting of <inline-formula> <tex-math notation="TeX">${\rm TiO}_{2}$ </tex-math></inline-formula>-Doped Silicone Layer and Silicone Lens

A novel, blue light-emitting diode (LED) packaging structure with improved light and thermal characteristics is presented. The package includes a TiO2-doped silicone layer and silicone lens. This dual-layer graded refractive index structure used for LED encapsulation was fabricated using dispensing technology. In the LEDs doped with TiO2 nanoparticles and a silicone lens, the light scattering capability and average refraction index of silicone were enhanced, thus reducing reflection losses. The LED achieved a light extraction efficiency increase of 24.5% and a junction temperature reduction of 12.1 °C compared with a conventional single-layer encapsulated LED. Consequently, the proposed structure not only improved the LED performance, but also reduced thermal accumulation in the packaging.

Comparison of silicone and spin-on glass packaging materials for light-emitting diode encapsulation

Traditional white light light-emitting diode (LED) encapsulation is performed by mixed phosphors and silicone coating on LED die. However, this encapsulation with silicone coating incurs overheated temperatures and yellowing problem. Therefore, this work attempts to replace silicone paste by using spin-on-glass (SOG) materials. Experimental results indicate that although initial brightness of SOG-based packaging is lower than that of silicone packaging, its light attenuation is significantly lower than that of silicone for a long lighting time. After the LED power is turned on for 12h, the brightness of LED with silicone and SOG material packaging decreases from 84 to 48lm and 73 to 59lm, respectively. Therefore, SOG material provides an alternative packaging solution for high power LED lighting applications.

Silicone hybrid materials useful for the encapsulation of light-emitting diodes

Novel ZrO2/silicone hybrid materials useful for the encapsulation of light-emitting diodes (LEDs) are synthesized by in situ sol–gel reaction of zirconium propoxide directly in a commercial-grade silicone resin. The domain size of the produced ZrO2 particles in the hybrid materials is controlled to be smaller than 350 nm based on SEM pictures to have a high optical transparency greater than 95% at 400 nm. The refractive index of the hybrid material increases from 1.52 to 1.62 when its ZrO2 content is increased from 1 to 15 wt.%. In addition, these hybrid materials are stable enough to tolerate the treatment of soldering at 260 °C for 2 min. For the hybrid materials with the ZrO2 content up to 7 wt.%, their weight-loss values are less than 0.25%. Finally, these hybrid materials are applied to encapsulate LEDs and their luminous flux is measured. The results showed that all the hybrid materials have higher luminous flux than the pristine silicone material. The luminous flux increased to a plateau value of 4.35 lm as the ZrO2 content in the hybrid material was increased to 3.0 wt.%. For all the hybrid materials studied here, the ZrO2/AB3 hybrid is considered to be the most suitable candidate as the encapsulating material for the LED used in this study.

FE Analysis of Cure-Process-Induced Residual Stress in Light Emitting Diode Encapsulant Silicone Resin Considering Mechanical Property Evolution

FE Analysis of Cure-Process-Induced Residual Stress in Light Emitting Diode Encapsulant Silicone Resin Considering Mechanical Property Evolution

Photoinitiated Polymerization for Active Packaging of Light-Emitting Diodes

The kinetics of photoinitiated polymerization for active packaging of light-emitting diodes (LEDs) is presented. The LED chip in our model was considered to consist of an upper emission surface and a lateral emission surface. Light emission from the LED chip triggers the photoinitiated polymerization reaction, leading to the formation of a mushroom-shaped encapsulant lens on the LED chip. With our proposed simulation process, the concentrations of both the photoinitiator and monomer, and the photolysis as time evolves can be calculated by the finite-difference method. The time evolution of the lens profile during polymerization can be analyzed as the monomer concentration varies. The proposed technique can depict the shape and precise size of the mushroom-shaped lens for any specific parameters in the polymerization reaction, e.g., LED power, polymerization time, absorptivity of the epoxy, and photolysis. LED encapsulation experiments with different LED power and polymerization time were made to support our simulation results, and the simulation results agree well with the experiment results. Finally, the radiation patterns of the LEDs with the encapsulant lenses are measured to prove the light focusing characteristics of the mushroom-shaped lenses.

Study of a high-efficiency, -precision, one-shot OCA dispensing technique

This study presents a technique for fast precise coating an optically clear adhesive (OCA) layer. A well-sealed loop dispensing system and a small multi-nozzle array are developed. Step-difference depth for obtaining highly reliable adhesive thickness is schemed to facilitate estimation of the total amount of adhesive needed for dispensing, and thus save on subsequent procedures such as shaving and OCA repair. The distribution method for OCA-droplet array is based on a ‘middle-dense, outer-sparse, up-down-bilateral symmetry’ strategy whereby air bubbles are expelled from gaps in the droplet array. Experimental results show that the developed one-shot adhesive dispensing technique is a highly efficient for the bonding task and encapsulation of LED (Light Emitting Diode) arrays with microelectronic chips.

Thermal behavior of epoxy blends for light emitting diode encapsulation

AbstractEpoxy encapsulant with high glass transition temperature (Tg) is required to ensure light emitting diode (LED) functions properly during its service life. Epoxy blends (cycloaliphatic epoxy, diglycidyl ethers of bisphenol-A and novolac resin) were thermally cured using methylhexahydrophthalic anhydride. Effects of the three types epoxy resins on the Tg of the ternary blends were examined. The Tg was mainly controlled by the concentration of cycloaliphatic epoxy. Optimization with constraint was conducted to produce epoxy blends formulation with optimum Tg. It was found that linear mixture model can fit well the experimental Tgof the epoxy ternary blends.

Intelligent Parametric Design for a Robust LED Encapsulation Process

Light-emitting diodes (LEDs) have been adopted in a wide variety of lighting situations. As lighting technology evolves, illumination devices are moving toward miniaturization and high-power applications. Packaging of illumination devices is essential to providing a path for heat dispersion. A full understanding of the viscosity change of the encapsulation material is critical to ensure manufacturability and improved process yield. The typical viscosity model is the initial viscosity plus the change in viscosity over time at a given temperature. The change in viscosity is a function of the reaction rate and the pro-exponential factor. This paper includes the pro-exponential factor in the regression model to improve the prediction accuracy. Through experimental data, the material's viscosity is determined as a function of process temperature, the ratio of the curing agent, and the curing time. A computer-aided parametric design is used to assess the effect of the variation of individual factors on the quality function and proposes a robust LED encapsulation process. An artificial neural network that considers interactions between parameters and establishes fitness functions is used to further enhance the accuracy of predictions and determine the optimal process solution through an iterative training algorithm. The optimal process scenario suggested by this research is a 10% curing agent preheated at 333 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> K for 40 min. This robust process can improve the signal-to-noise ratios by approximately 43%.

Cycloaliphatic epoxy resin modified by two kinds of oligo-fluorosiloxanes for potential application in light-emitting diode (LED) encapsulation

Oligo-fluorosiloxane (DFOS) and epoxy-containing oligo-fluorosiloxane (DFEHOS) were synthesized by the hydrolytic condensation reaction to modify 3, 4-epoxycyclohexylmethyl-3, 4-epoxycyclohexanecarboxylate (ERL-4221) for potential application in LED packaging. The chemical structures of DFOS and DFEHOS were characterized by Fourier transform infrared (FT-IR), 29Si nuclear magnetic resonance (29Si NMR), and gel permeation chromatography (GPC). The thermal behavior, mechanical properties, morphologies of impact fracture surfaces, surface wettability and absorbency of the modified epoxy resins were examined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), tensile and impact testing, scanning electron microscopy (SEM), and contact angle measurement, respectively. The experimental results indicated that the contact angles, surface energies and water absorption ratios of the modified epoxy resins were effectively improved by the introduction of oligo-fluorosiloxanes. Compared to neat epoxy resin, the thermal stabilities of DFEHOS-modified epoxy resins were basically kept, and that of DFOS-modified epoxy resins were slightly depressed with the increasing content of modifiers. As the additive quantity of modifiers was about 5pph to 15pph relative to ERL-4221, good thermal stability, fracture toughness and surface hydrophobicity of the modified epoxy resin was exhibited, and the cured DFEHOS-10 that embraced the relatively optimum comprehensive property was possible for LED encapsulation. Moreover, the reactable groups formed during hydrolytic condensation in DFOS and DFEHOS made good compatibilities between the modifiers and the epoxy matrix.

Effect of thermal aging on the optical, dynamic mechanical, and morphological properties of phenylmethylsiloxane-modified epoxy for use as an LED encapsulant

We investigated the thermal stability of the diglycidyl ether of bisphenol A (DGEBA)–methylhexahydrophtalic anhydride (MHHPA) modified with phenylmethylsiloxane-modified epoxy (PMSE) and the effects of this hybrid on the performance of light emitting diodes (LEDs). The optical and dynamic mechanical properties of the materials, and the effects of these properties on the light output from the encapsulated LEDs, were studied following long-term thermal aging at 150°C. The DGEBA–PMSE hybrids were highly transparent and its optical stability was shown in the stable light output of the LED, which was 19% higher than that of LEDs encapsulated by DGEBA–MHHPA after 30 days. Molecular rearrangement occurred in the nanoscale domain of the hybrids after thermal aging, which caused the dynamic mechanical effect on the LED performance, as demonstrated by TEM. We observed the hybrid modified using 0.2 equivalents of PMSE possessed both optical and dynamic mechanical stability, and resulted in the most stable LED performance.

Thermally Stable Transparent Sol−Gel Based Siloxane Hybrid Material with High Refractive Index for Light Emitting Diode (LED) Encapsulation

Nonhydrolytic sol−gel condensation for the synthesis of nanosized inorganic−organic hybrid resin and hydrosilylation reaction under a Pt catalyst for the fabrication of oligosiloxane-based hybrid material (phenyl hybrimer) were used in this research. This combination of two chemical reactions results in material with useful properties for light emitting diode (LED) encapsulation. Until now, no polymers and phenyl polysiloxane materials have been reported with a high refractive index (over 1.52) and high transparency accompanied by resistance to yellowing induced by thermal degradation above 150 °C aging in air. In this article, we focused on the fabrication of a transparent and thermally stable phenyl hybrimer with high refractive index for LED encapsulation through siloxane networking by a hydrosilylation reaction with cross-linker and Pt caltalyst. The phenyl hybrimer has a high refractive index about 1.56. In particular, it has good thermal stability against discoloration to yellow by aging even at 200...

Light-emitting-diode inspection using a flatbed scanner

There is a need to ensure that light-emitting diodes (LEDs) are performing well as they are increasingly used in demanding applications. In this work, we present an adapted usage of flatbed scanners to inspect LEDs. This offers the benefits of lower cost and higher measurement throughput than specialized equipment like goniophotometers. The technique is demonstrated to detect LED encapsulation defects, to identify low illumination and misaligned LEDs in arrays, and to evaluate angular color nonuniformity of white LEDs.

Cationic copolymerization of epoxy siloxane monomer with liquid poly‐butadiene and its light emitting diode encapsulation

AbstractA novel epoxy siloxane hybrid was prepared using epoxy siloxane monomers of 1,3‐bis[2‐(3‐{7‐oxabicyclo[4.1.0]heptyl})ethyl]‐tetramethyldisiloxane (BEPDS) with hydroxyl terminated hydrogenated polybutadiene (GI‐1000), in different proportions. Apparent polymerization reactivity was decreased with increasing GI‐1000 concentration but the normalized reactivity per epoxy group was slightly increased due to reaction between hydroxyl group and epoxy group. Increasing GI‐1000 concentration showed significant flexibility improvement in epoxy siloxane hybrid. At 30 wt % of GI‐1000 addition, glass transition temperature was decreased from 116 to 21°C and shore D hardness was decreased from 75 to 46.5% weight loss temperature of these epoxy siloxane hybrid was decreased with increasing GI‐1000 concentration, whereas thermal discoloration was increased. LED encapsulation with this epoxy siloxane demonstrated no crack when GI‐1000 was 30 wt % or more. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

High-refractive-index TiO2-nanoparticle-loaded encapsulants for light-emitting diodes

A high-refractive-index (high-n) encapsulant is highly desirable because it can result in enhancement of light-extraction efficiency from high-n semiconductor light-emitting diode (LED) chips. A uniform dispersion of TiO2 nanoparticles in epoxy for LED encapsulation is demonstrated for surfactant-coated TiO2 nanoparticles by drying, mixing with a solvent, refluxing, centrifuging, and mixing with epoxy. The refractive index of surfactant-coated TiO2-nanoparticle-loaded epoxy is 1.67 at 500nm, significantly higher than that of conventional epoxy (n=1.53). Theoretical analysis of optical scattering in nanoparticle-loaded encapsulants reveals that the diameter of nanoparticles and the volume loading fraction of nanoparticles are of critical importance for optical scattering. Quasispecular transparency of the encapsulant film can be achieved if the thickness of the film is kept below the optical scattering length. A graded-refractive-index multilayer encapsulation structure with the thickness of each layer being less than the mean optical scattering length is proposed in order to reduce optical losses from scattering and Fresnel reflection. Furthermore, three-dimensional optical ray-tracing simulations demonstrate that encapsulants with an optimized scattering coefficient, ks, benefit from optical scattering by extracting deterministic trapped modes. Theoretical light-extraction enhancements larger than 50% are found when comparing scattering-free to scattering encapsulation materials.