Bofeng Shang

Effect of Packaging Method on Performance of Light-Emitting Diodes With Quantum Dot Phosphor

In this letter, remote quantum dot phosphor-converted light-emitting diodes (QD-LEDs) with air encapsulation, silicone lens, and silicone encapsulation were fabricated. The effects of different packaging methods on the optical and thermal performances of QD-LEDs were evaluated based on the experimental tests and simulation. Optical efficiency and spectral stability were tested by experiment, and the temperature was assessed by finite-element simulation and infrared thermal imager tests. It was found that the silicone encapsulation type could convert more blue light into QDs emission light due to the reabsorption of backward reflected blue light. The silicone encapsulation type showed only a 6.2% decrease in QDs emission peak intensity when the driving current varied from 50 to 500 mA, while the silicone lens type dropped by 20.4% and the air encapsulation dropped by 36.8%. It was also confirmed that the QDs temperature in silicone encapsulation was 24 °C lower than those in the air encapsulation type and the silicone lens type at driving current of 300 mA.

A Cylindrical Tuber Encapsulant Geometry for Enhancing Optical Performance of Chip-on-Board Packaging Light-Emitting Diodes

Low light efficiency and poor angular color uniformity (ACU) are the key challenges of chip-on-board packaging light-emitting diodes (LEDs). In this paper, we demonstrate a phosphor geometry, and its controlling method for enhancing the optical performance of chip-on-board packaging LEDs, its fabrication flexibility, and availability are validated by experiments, and its effect on the optical performance is analyzed by optical simulations and experiments. The simulation results show that compared with the conventional flat geometry, the cylindrical tuber geometry can effectively improve the light efficiency and ACU, and the light efficiency enhancement increases with correlated color temperature. Experimental results show that for the case that the encapsulation layer only consists of silicone, the proposed geometry enhances the light efficiency up to 63.1%. In addition, for another case, in which the encapsulation layer consists of silicone and phosphor particles, when the average correlated color temperature (CCT) is about 5000 K, the proposed geometry increases the light efficiency by 11.7%, and the angular CCT deviations for the flat geometry and the proposed geometry are 1805 and 475 K, respectively.

Numerical and Experimental Study on the Transferred Volume in Phosphor Dip-Transfer Coating Process of Light-Emitting Diodes Packaging

The phosphor dip-transfer coating method is simple and flexible for transferring a pre-analyzed volume of phosphor gel, which can be beneficial to the high angular color uniformity (ACU) of white light-emitting diodes (LEDs). The crux of this method is the volume control of the phosphor gel; however, the critical factors which influence the volume control remain unrevealed. In this paper, we concentrate on investigating the transferred volume in terms of three parameters: withdrawal speed, post radius, and dipping depth. Numerical simulations were carried out utilizing the volume of fluid (VOF) model combined with the dynamic mesh model. The experiments were also conducted on an optical platform equipped with a high-speed camera. The simulation results coincide well with the experimental results, with the maximum relative difference within 15%. The results show that the transferred volume increases with the increasing withdrawal speed and remains stable when the speed is greater than 1 mm/s, and it shows a linear relationship with the cube of post radius. And the transferred volume will increase with the dipping depth. Based on the experimental and numerically work, it is concluded that the volume of the pre-analyzed phosphor gel can be precisely obtained.

Spreading behaviors of silicone droplet impact on flat solid surface: Experiments and VOF simulations

Silicone, as the carrier of phosphor powder in the phosphor coating process of white light emitting diode (LED) packaging, its spreading behaviors will influence the morphology of phosphor layer and thus affect the optical and thermal performances of light emitting diodes (LEDs). In this paper, the spreading behaviors of a silicone droplet impact on a flat silicon surface was experimentally and computationally studied. Droplets with the same volume (R=1.1±1%mm) were deposited on a flat silicon substrate at a range of weber number from 5 to 20, morphology change of silicone droplets was captured using a high-speed digital camera. A computational fluid dynamics (CFD) model, based on the volume of fluid (VOF) approach, was used to simulate spreading behaviors of a silicone droplet using the same boundary condition getting from experiments. Time evolution of dynamic radius R(t) and dynamic contact angel θ(t) were analyzed. The CFD simulation results were compared with the experiment results, and the simulation results showed good agreement with the experimental data which indicated that a VOF-based computational model was able to capture key features of the interaction of a silicone droplet with flat solid surfaces.

A Facile Approach to Fabricate Patterned Surfaces for Enhancing Light Efficiency of COB-LEDs

Light efficiency of chip-on-board light-emitting diodes (COB-LEDs) is much lower than the single-chip packaging LEDs due to its flat phosphor layer, and hemispherical phosphor layer realization is a great challenge in COB-LEDs packaging due to the low surface tension of the phosphor gel. In this paper, we demonstrated a facile method to fabricate patterned surfaces to deal with this challenge. First, nanosilica (NS) particles with average diameter of 70 nm were fabricated by hydrolyzing the tetraethoxysilane and further modified by 1H,1H,2H,2H-perfluorooctyl-trichlorosilane, then patterned surfaces were fabricated by introducing a tailored template into the NS coating process. The results show that the NS coated surfaces display repellency to the water and phosphor gel with porous lotus leaf-like hierarchical structure, when the particle deposition density (PDD) of the NS particles increases from 0 to 6 g/m2, the contact angle (CA) of water increases from 34° to 161°, and the CA of phosphor gel increases from 22° to 145°. Hemispherical phosphor layer was achieved with the patterned surfaces when the PDD is 1.5 g/m2. Compared to the conventional flat phosphor layer, the hemispherical phosphor layer enhances the light efficiency by 11.74% and 14.52% for 4000 and 5000 K COB-LEDs.

Investigation on dip-transfer phosphor coating for light-emitting diodes: Experiments and VOF simulations

Dip-transfer phosphor coating method and its benefit on enhancing angular color uniformity (ACU) of white light-emitting diodes (LEDs) were previously reported, however, for applying this method in mass production, its fluid transfer mechanism and packaging consistency needs to be further investigated. The dip-transfer process is divided into two process, they are dipping process and transfer process. In our previous study, the dipping process were studied with experiments and simulations. In this study, we further studied the transfer process with numerical simulations based on combination of the volume of fluid (VOF) method and the dynamic mesh model, four parameters include post radius, withdrawal velocity, transfer height and phosphor gel viscosity were investigated. Besides, the packaging consistency of the dip-transfer phosphor coating method was studied with experiments. The simulated results show that the transfer volume decreases with the post radius, phosphor withdrawal velocity and phosphor gel viscosity, while keep the same with the transfer height. The experimental results show that the packaging consistency is highly rely on the transfer volume, with transfer volume varies from 0.71 μl to 6.12 ul, the maximum transfer volume deviation (MTVD) changes from 6.98% to 2.31%.

Structural design of LED packaging in terms of lumen reliability by a statistical method

The structural design for LED packaging is of great importance. S o far, the methods for LED packaging are mostly from the perspective of enhancing the emitted lights optical performance. In this paper, we proposed a statistical method for the structural design of LED packaging in terms of lumen reliability. Orthogonal experimental design was used in the determination of the experimental scheme. Optical simulations were conducted to calculate the optical power of LED package according to the scheme. Two traditional structural models were selected as objectives. Range and variance analyses were applied to investigate the significance of structural factors on the LEDs light output before guiding the structural design. According to the analyses, it is suggested that the lens and silicone encapsulant can be combined as one and the quality of the heat slugs top surface should be enhanced pointedly. It is concluded that this method is useful for the development and design of new packaging structures from the perspective of reliability.

Experimental study of chip offset on the packaging consistency of high power light-emitting diodes

In this study, we experimentally investigated the effect of chip offset on the packaging consistency of high power LED. A series of LED modules with different chip offset distances and phosphor mass concentration were packaged, and two comparative experiments were conducted. Compared with the conventional modules, the maximum CCT deviation of off-centered modules increased from 128K to 667K. The results show that chip offset contributes significantly to the consistency of LED packaging.

An improved substrate structure for high angular color uniformity of white light-emitting diodes

Angular color uniformity (ACU) is a key optical property of white light-emitting diodes (LEDs) and high ACU is strongly demanded in illumination applications. In this paper, we proposed an improved substrate structure to enhance the ACU of phosphor-converted white LEDs. Compared with the conventional substrate, the improved structure has an inverted trapezoid on the top of the substrate. We adopted the volume-of-fluid (VOF) model to simulate the phosphor gel spreading processes. Optical simulations were also carried out by the Monte Carlo ray-tracing method. The results show that a hemispherical geometry could be obtained with present structure. Compared with the traditional substrate, the ACU of LED module with improved substrate increased from 1148K to 196K at the average CCT of about 4000K.