Research On Electrophoretic Deposition Method For Wight PC-LED

With the development of science and technology, white light-emitting diodes(WLEDs) play a more and more principal role in lighting area. Nowadays blue LED with YAG: Ce3+yellow phosphor coated on is the main approach to emit white light.Within several methods to coat phosphors on blue LEDs, the LED wafer level packaging technology attracts more and more interests since this approach has many advantages such as high process efficiency, low cost and short process time, which are appropriate for large scale preparation. Among wafer packaging, EPD has several advantages: one, phosphor layers are uniform; two, the layer thickness can be easily controlled; three, it could be used to prepare different kinds of chips arrays. However, in traditional EPD, phosphors in suspension precursors deposit easily, and the concentration of anions is difficult to control. These two disadvantages lead to the fall of the luminous efficiency. This paper utilized butyl acetate as the suspension solvent, and replace magnesium nitrate with chromium salts. With the EPD process improved, the suspension time of phosphors were significantly lengthened.(1) Antimony tin oxide(ATO) nanoparticles were coated uniformly on the surface of LED chip arrays by spraying. The prepared transparent conducting layer were used as electrodes. The transmission of ATO in 450 nm layer can reach 85%, while the standard deviation is 1.71% in an area of 25.4*76.2mm2. In the following preparation of phosphor suspensions, after analyzing the EPD equation and various parameters of traditional electrophoretic solvent, butyl acetate which has low permittivity and moderate viscosity was chosen as the solvent. In addition, the concentration of anions and uniformity of phosphors layer thickness were also observed and analyzed, leading to the appropriate adding concentration 0.052% for phosphor anions. Finally, poly(alkyl methacrylate) were also added as dispersants to improve the stability.(2) Based on traditional direct current EPD, inverting current was added to remove the phosphor particles which did not react yet with chip matrix. The change of correlated color temperature(CCT) under different voltage and the uniformity of the phosphor layer thickness were tested. it was found that the phosphor layer became thinner after adding inverting current and unconformal phosphor layers didn’t appear. In addition, after adding the invert voltage to direct current voltage, the standard deviationof CCT about 5000 K was 224.34 K in the surface of 25.4*76.2mm2. Moreover, the relation between time and CCT was showed as a negative exponent.(3) The measurement confirmed that the coated LED chips prepared by improved phosphor EPD method had a dense and uniform phosphor layer. The CCT was about5500 K, and the standard deviation of CCT and luminous efficiency are 256.03 K and3.06lm/W respectively. The chip performance also kept stable in the followed life test of2500 hours.