Research On Improving The Accuracy Of Color Coordinates Of The White LED

With the advantages of long lifetime, high luminous efficiency, solid-state, energy saving and environmental protection, LED lamps have been substituting the low luminous efficiency incandescent lamp and the fluorescent lamp which contain mercury heavy metal. It has been considered as one of the most promising forth generation of lighting source. In this paper, the development history and the prospect, the structure and working principle of the LED, the main implementation of white LED, the phosphor for white LED, as well as the common packaging process of white LED products in factory has been brief introduced and three kinds of technology methods to improved color coordinates accuracy of the white LED have been discussed in detail.The first method to improve the color coordinates accuracy of the white LED is doping the resistance precipitation powder into the phosphor power during the preparation process. The size of the resistance precipitation powder is very small, generally less than0.5um. It is hydrophobic, high transparency and has large specific surface area. It can be used to regulate the viscosity and flow freely, keep the phosphor powder flow and anti-caking. Stirring in the phosphor glue, it can form a three-dimensional network structure, which can increase the viscosity of the solution and avoid the phosphor sedimentation. With the resistance precipitation powder, the phosphor glue is not easy to precipitate, as a result, the concentration and the accuracy of the color coordinate of the LED can be improved. Under the white LAMP LED qualified rate higher than92.5%condition, the vary interval of the color coordinates Ay can be reduced to0.5by comparing that of0.6which does not dope resistance precipitation powder. For the3528SMD package white LED98%qualified rate requirement, Ay can be reduced from0.4to0.32.The second method is to precipitate the phosphor glue completely before sintering. The phosphor can be distributed uniformly within the glue, thus, the concentration and the accuracy of color coordinate of the white LED products can be improved and the qualified rate of the products can be increased. The precipitation process is to carry out as the following:the LED chips which coat with phosphor glue were fixed at the dispenser machine. Power on the dispenser machine, the phosphor can be precipitated by the centrifugal force. By using this precipitation process, in the case of Ay equal to0.3, the accuracy of color coordinate can be increased3%.The third method is the rapid solidification of the phosphor glue. With this process, a heating station was installed in the vicinity of the, dispenser machine. After dispensing phosphor glue, the white LED chips were constant temperature heated immediately by the heating station at120"C for1minute. So the phosphor glues can be solidified quickly and the precipitation probability of the phosphor can be greatly reduced. By using the rapid solidification of the phosphor glue method, the qualified rate of the COB package white product can be increased from80%to94%, and the dispensing efficiency can be increased6times by comparing with that of without rapid solidification process.