Process development for yellow phosphor coating on blue light emitting diodes (LEDs) for white light illumination

There are several methods to produce a white light emitting diode (LED). The most common commercially available white LED is made by mixing the blue light from a GaN chip and the yellow light from the emission of a yttrium aluminum garnet cerium (YAG:Ce) yellow phosphor coating. The quality of white LED using this approach heavily depends on the optimization of packing density, thickness and uniformity of the phosphor layer. The present study is intended to develop a new methodology for coating a uniform yellow phosphor layer. This newly developed coating method is based on the concept of screen-printing. A matrix array of LEDs is firstly mounted on a silicon substrate with the flip chip configuration. A silicon mold plate is fabricated with etched cavities that match with the dimensions and pattern of LED array. The silicon mold plate is then placed over the substrate that carries the LED array and serves as a printing mask. The yellow phosphor powder is pushed into the apertures by a squeegee blade and bonded to the LED with UV curable epoxy. The silicon mold plate is released after curing. Compared with other coating approaches, this yellow phosphor printing method is relatively simple and can make good quality white LEDs