Power-type White Led Vacant Phosphor Coating Process

Since the incandescent light bulb was invented in the late 1800s, human have never given up the research on light sources. In the late 20th century, a new kind of light source named white light-emitting diode (LED) came out. Compared with conventional illumination sources, white LEDs have many outstanding characteristics in terms of longer life, higher brightness, lower power consumption, and less harmfulness. They are expected to have a considerable impact on issues such as energy consumption, environment and even the health of individuals. During several decades of development, luminous efficiency of white LEDs is improving at very fast speed. Especially, phosphor converted white LEDs are becoming more and more attractive for general lighting applications because of the steadily increasing luminous efficiency reported by LED-suppliers. Now white LEDs of Cree Company have broken 200lm/w efficacy barrier.Highly efficient packaging for power LED is crucial for the development of solid state lighting. Different optical structures with various states of phosphor will greatly influence the packaging performance such as luminous efficiency, quality of white light and color uniformity. Now, traditional process that phosphor-containing silica gel is dropped on the surface of a blue LED chip directly is adopted by most LED package manufacturers, however there are some disadvantages with this method. The primary problem is that the thickness of phosphor layer is uneven, which highly impacts restrain the spatial homogeneity of the white light emitted.To solve those issues encountered in the traditional process, a new phosphor coating process is put forward in this paper. By combining water-soluble photoresist with Remote phosphor structure, white LEDs which are of high luminous efficiency and good color uniformity are made.Some interesting conclusions from our work are listed as follows:1. With the method of Monte Carlo, different package structures of white LED are modeled and the factors that impact on the performance of white LEDs such as packaging materials, phosphor layer, light emitting angles, various packaging technology etc are analyzed. The calculating results show that Remote phosphor module can be of up to 26% better optical efficacy than conventional process module; Because the output of LED light source are of a Lambertian radiation profile , the module with even phosphor layer can not gain uniform white light spot. Only if the thickness of phosphor layer matches with radiation profile of the LED chip, perfect color uniformity will be realized.2. Based on the conclusions of simulation, a novel phosphor coating method is put forward, which adopts a kind of water-soluble photoresist consisting of polyvinyl alcohol(PVA) and ammonium dichromate(ADC) instead of conventional silicon resin, and self-exposure of LED chip was employed as an optimized self-adaptive exposing way to make the photoresisit crosslinking. Eventually, ideal phosphor layer comes into being, which matches the intensity distribution of LED chip. So the ratio of yellow and blue will be keeping consistent at every angle. The light spot of white LED will be more uniform spatially. The problem of color uniformity is solved perfectly. At the same time Conformal coating and Remote phosphor coating are achieved successfully with this newly-developed coating method.3. Luminous efficiency, color uniformity, spectrum of light output is measured. The measurement revealed that the devices with new phosphor coating process introduced in this paper shows better performances than the ones of traditional process and other Conformal coating processes in the terms of color uniformity, and the luminous efficiency of different package processes are almost equivalent. The Remote phosphor coating white LEDs deliver better homogenized white light. They are ideal way for packaging of white LEDs for general illumination purposes.