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A Study on the Self-adaptive Conformal Coating for White LED Packaging

Time:2013-12-24 07:38:45  From:CNKI  Author:Wang Wei

ABSTRACT
         Growing increase in the level of science and the worsening of environmental issues,energy saving and environmental protection has become a new important theme of development with all the countries over the world. LED (light emitting diode) has showed its superiorities on application as a new energy-efficient lighting source. More and more attention of governments and companies has been paid in semiconductor lighting and related fields. The rapid improvement of the LED luminous efficiency also means the fourth-generation light source is evolving.
         In the manufacturing process of LED, the phosphor coating is a crucial step. The main way currently popular approach to fabricate white LEDs is coating yellow YAG phosphor on the blue LED chip, YAG phosphor absorb the blue light( λ = 460nm) and emit yellow light( λ = 550nm), the transimitted blue LED emitting light with yellow phosphor emitting light mixed in the space to realize the white light illumination. In the traditional process, the mixture of yellow phosphor particles and a transparent curable resin(such as silicone, epoxy, etc.) is coating surrounding the LED chip surfaces through dispensing or spraying, however, the difficult to achieve uniform layer thickness with those traditional coating results in non-uniformity of LED light. Meanwhile, the lack of control on the phosphor layer thickness leads to low level of consistency between the LEDs of the same batch. The research on the advanced phosphor coating technology are expected The white LED self-adaptive conformal coating technology based on water-soluble photoresist is an optimized method of LED conformal coating technology. The polyvinyl alcohol (PVA) and ammonium bichromate (ADC) work together as the photosensitive system. Wherein the polyvinyl alcohol as film-forming agent, and ammonium dichromate as photosensitive agent, both of them were uniformly mixed by a certain proportion with phosphor to make slurry. Then dispensing methods which is similar to the conventional process is applied to coat the slurry on the blue LED chip, a layer of phosphor dispersived in photosensitive is realized surrounding the LED chip surfaces.
          The photosensitive system is sensitive to light emitted from the blue LED chip (wavelength at about 460nm), so the blue light from LED chip itself could be used to exposure, i.e. the process of crosslinking polymerization of PVA could be induced by blue light from LED chip. The self-adaptive exposure could fabricate the controlled morphology phosphor layer with high matching degree. The exposure time and current parameters in the process are the key factors whitch determine the wavelength of emitting light to satisfy different white LED applications.
          The LED packaged in the way of Self-adaptive conformal coating technology always has a promotion in spatial chroma consistency due to the high matching degree between the phosphor layer and chip. Compared to traditional silicone dispensing process and silicone phosphor conformal coating process, self-adaptive conformal coating technology owns the minimal fluctuation of color coordinates, which means the most evenly emittting light.
          Self-adaptive conformal coating process can also be applied to the packaging of multi-chip integrated LED module. More phosphor will be coated because of the larger module. Therefore, the traditional silicone dispensing process is quite difficult to fabricate a uniform layer of powder concentration and morphology, so the space chromaticity consistency has become one of the main challenges of the packaging process. Due to the presence of the self-adaptive exposure step in self-adaptive conformal coating process, the entire module chip deemed an overall self-adaptive exposure, which could achieve a higher degree of coupling between the powder layer and the chip.
          The junction temperature will increase a lot while the LED chip in the operating state, the phosphor powder layer will receive heat because of the direct contact with the chip. The color spectrophotometer and the light extraction efficiency will be affected after the excitation wavelength changed. In addition, about 40% of the LED chip light will be backscattered by phosphor powder, and a large part of which will be re-absorbed by the chip, resulting in the loss of light efficiency. Remote phosphor coating based on self-adaptive conformal coating method used the independent patent technology of silicone diluted to form the silicone isolation layer and separate the chip and the phosphor layer; surface activation with plasma activation process on the surface of the chip and silicone isolation layer surface to ensure that the stability of the device. The high degree of match phosphor powder layer self-adaptive conformal coating process also means the excellent spatial chroma consistency and stable optical efficiency improvements. The package structure of the remote phosphor coating greatly reduces the LED light backscatter loss, the luminous efficiency has been improved significantly.

Keywords: white LED, self-adaptive phosphor coating technology, multi-chip integrated LED module, remote phosphor coating

 


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