Research On Key Issues Of Design And Manufacturing For High-Power Light-Emitting Diode Packaging

It has been widely accepted that solid state lighting, which is based on high power light-emitting diodes (LEDs), will substitute the traditional light sources due to its capability of energy saving. Great attentions have been paid to the solid state lighting to study the key issues affecting its performance. Packaging technology is the critical step that connects the LED chip to the final applications. The packaging technology relates to various areas including the optics, thermal management, electronics, mechanics, materials, processes and equipment. As an optoelectronic device, the optical performance is critical for the evaluation of the packaging technology. It determines whether the LED can save more energy, and whether the LED can show better illumination effects than the traditional light sources. Therefore, great efforts have been made to optimize the packaging structure to enhance the optical performance by using simulation and optical design. The optimization of packaging structure normally relates to the packaging processes. It is very important to solve the key issues in current packaging processes before the application of optimized packaging structure. A combination of the optimized structure and the optimized processes is critical for the realization of high optical performance. This dissertation mainly studies the key issues in the optical simulation, optical design, and manufacturing processes of the high power LED packaging, and achieves some innovative works as following:Optical parameters for different material layers of LED chip have been systematically analyzed and summarized. Surface roughness is considered into the optical model of LED chip to improve the simulation precision. By comparisons between optical simulations and experiments, precise optical modeling of LED chip has been constructed and a useful simulation procedure has been presented. The simulation precision has been improved to be higher than 95%. After that, the optical modeling for LED chip with roughened surface has been studied with the purpose as an exploration.Light scattering and absorption of YAG phosphor are calculated by using the Mie scattering theory. A double-integrating-sphere system is constructed for the measurement of optical properties of YAG phosphor. Phosphor films with uniform thickness have been fabricated and tested by the double-integrating-sphere system. The experiment results and theoretical results are compared to verify the accuracy of Mie theory. Monte Carlo ray-tracing method is used to modify the Mie theory results and a revised Mie theoretical calculation is proposed. Finally, the light scattering and absorption for phosphor with different sizes are studied by using the Mie theory.Based on the precise optical model of LED chip and precise optical properties of YAG phosphor, the optical simulation procedure for the white LEDs is presented. The experiments confirm the correction of the optical simulation method and the simulation precision can be higher than 95%. After that, optical film stacks, which can selectively transmit light with different wavelengths, are used in the optical design of LED packaging. An innovative design with the optical film coating on the surface of glass lens is proposed. Simulation results confirm the strong enhancement of light extraction.As the key step of LED packaging processes, various phosphor coating methods are simulated to investigate the differences of optical performance among them. Results reveal that conformal coating is a good method for the realization of high spatial color uniformity. However, the light extraction and product consistency of this method is poor. Optimizing the phosphor coating with hemispherical remote shape can improve the light extraction, product consistency, and spatial color uniformity simultaneously.Some important issues in the packaging processes are carefully analyzed. The packaging processes are standardized by giving the attentions in the operation. The traditional phosphor coating method is systematically studied to find the optimized parameters of the phosphor shape. Finally, an injection molding method is used to fabricate the hemispherical phosphor film. By fabricating white LEDs with various phosphor coating methods, the comparisons confirm that hemispherical phosphor film can improve the spatial color uniformity significantly. The improvement can be as high as 40% comparing with traditional coating method.