Research On Color Temperature Drift Of White Light Emitting LED Caused By Phosphor Sedimentation And Its Solution

The white light emitting LED(WLED) has the advantages of long service life, green environmental protection, high efficiency and energy saving, so it has a wide application in the field of lighting and display. The main form of realization of the WLED is the composition of blue LED chip and the yellow phosphor. In the packaging process of the WLED in dispensing and baking, the phosphor can produce obvious settlement, which will lead to serious color temperature drift and inconsistency problem of color performance. To study this phenomenon, a WLED spectrum real-time observation system has been designed, and the cross section of the actual type SMD of WLED encapsulation structure is observed to study the settlement of phosphor, which explains the reasons of settlement influence on the color temperature of WLED. On this basis, several methods have been tried to solve color temperature drift problems, and finally it is found that the incorporation of scattering particle with light density into in the phosphor rubber can effectively solve the problem of color temperature drift caused by the phosphor settlement.1) A WLED spectrum real-time observation system has been designed and made to study the relationship between the phosphor sedimentation phenomenon and the color temperature of WLED. The results show that the settlement will make the blue light decreased and yellow light intensity increased in the spectrum of WLED, which will lead to the dropping of color temperature. It is believed that the phosphor deposition would result in the changes of the distribution of phosphor in the packaging plastic. In the settlement process of aggregation, the transmittance of phosphor layer decreases and reflectance rises, which will lead to the rising of back scattering/reflection of light. The blue light is reflected between the bracket reflective layer and the phosphor layer and then more yellow light is excited, so the conversion rate increases and the color temperature decreases gradually.2) The research shows that the using of phosphor mixed with appropriate amount of SiO2 scattering particles or small diameter phosphor can reduce color temperature drift caused by the settlement, but would have a greater impact on the initial brightness of the WLED. A kind of hollow glass microspheres with light density and appropriate particle size is added into the fluorescent glue, and the microspheres will gradually rise to the surface in the packaging adhesive, leading to the decrease of back scattering/reflection gradually, and this has a contrary effect compared with the phosphor sedimentation. It proves that the incorporation of hollow glass microspheres can basically eliminate the drift of color temperature during the settlement process, and it has little influence on the initial brightness.3) A method of transparent silicone resin inlay is used to make a cross section of SMD type WLED, and the settlement phenomenon of phosphor in the actual SMD type WLED is studied by this method. It is found that as the blue LED chip has a certain height, part of phosphor can sink to the LED chip below the light output surface, leading to lateral blue light leak, thereby increasing the color temperature.4) A pre-smooth process is performed in the SMD type WLED, and meanwhile the selected hollow glass microspheres is added into the phosphor, which alleviates the color temperature drift caused by the sedimentation of the yellow phosphor. A practical SMD package test shows that the application performs well.