A Study On The Self-adaptive Conformal Coating For White LED Packaging

Growing increase in the level of science and the worsening of environmental issues,energy saving and environmental protection has become a new important theme ofdevelopment with all the countries over the world. LED (light emitting diode) hasshowed its superiorities on application as a new energy-efficient lighting source. Moreand more attention of governments and companies has been paid in semiconductorlighting and related fields. The rapid improvement of the LED luminous efficiency alsomeans the fourth-generation light source is evolving.In the manufacturing process of LED, the phosphor coating is a crucial step. Themain way currently popular approach to fabricate white LEDs is coating yellow YAGphosphor on the blue LED chip, YAG phosphor absorb the blue light(λ=460nm) andemit yellow light(λ=550nm), the transimitted blue LED emitting light with yellowphosphor emitting light mixed in the space to realize the white light illumination. In thetraditional process, the mixture of yellow phosphor particles and a transparent curableresin(such as silicone, epoxy, etc.) is coating surrounding the LED chip surfaces throughdispensing or spraying, however, the difficult to achieve uniform layer thickness withthose traditional coating results in non-uniformity of LED light. Meanwhile, the lack ofcontrol on the phosphor layer thickness leads to low level of consistency between theLEDs of the same batch. The research on the advanced phosphor coating technology areexpectedThe white LED self-adaptive conformal coating technology based on water-solublephotoresist is an optimized method of LED conformal coating technology. Thepolyvinyl alcohol (PVA) and ammonium bichromate (ADC) work together as thephotosensitive system. Wherein the polyvinyl alcohol as film-forming agent, andammonium dichromate as photosensitive agent, both of them were uniformly mixed bya certain proportion with phosphor to make slurry. Then dispensing methods which issimilar to the conventional process is applied to coat the slurry on the blue LED chip, alayer of phosphor dispersived in photosensitive is realized surrounding the LED chipsurfaces. The photosensitive system is sensitive to light emitted from the blue LED chip(wavelength at about460nm), so the blue light from LED chip itself could be used toexposure, i.e. the process of crosslinking polymerization of PVA could be induced byblue light from LED chip. The self-adaptive exposure could fabricate the controlledmorphology phosphor layer with high matching degree. The exposure time and currentparameters in the process are the key factors whitch determine the wavelength ofemitting light to satisfy different white LED applications.The LED packaged in the way of Self-adaptive conformal coating technologyalways has a promotion in spatial chroma consistency due to the high matching degreebetween the phosphor layer and chip. Compared to traditional silicone dispensingprocess and silicone phosphor conformal coating process, self-adaptive conformalcoating technology owns the minimal fluctuation of color coordinates, which means themost evenly emittting light.Self-adaptive conformal coating process can also be applied to the packaging ofmulti-chip integrated LED module. More phosphor will be coated because of the largermodule. Therefore, the traditional silicone dispensing process is quite difficult tofabricate a uniform layer of powder concentration and morphology, so the spacechromaticity consistency has become one of the main challenges of the packagingprocess. Due to the presence of the self-adaptive exposure step in self-adaptiveconformal coating process, the entire module chip deemed an overall self-adaptiveexposure, which could achieve a higher degree of coupling between the powder layerand the chip.The junction temperature will increase a lot while the LED chip in the operatingstate, the phosphor powder layer will receive heat because of the direct contact with thechip. The color spectrophotometer and the light extraction efficiency will be affectedafter the excitation wavelength changed. In addition, about40%of the LED chip lightwill be backscattered by phosphor powder, and a large part of which will be re-absorbedby the chip, resulting in the loss of light efficiency. Remote phosphor coating based onself-adaptive conformal coating method used the independent patent technology ofsilicone diluted to form the silicone isolation layer and separate the chip and thephosphor layer; surface activation with plasma activation process on the surface of thechip 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 processalso means the excellent spatial chroma consistency and stable optical efficiencyimprovements. The package structure of the remote phosphor coating greatly reducesthe LED light backscatter loss, the luminous efficiency has been improved significantly.