Experimental And Numerical Study On Phosphor Sedimentation Phenomenon Of High-power LED Packaging

So far, the most common approach for white LEDs is based on mixing the blue lightemitted from LED chip with the yellow light reemitted from the phosphor material. Thephosphor particles are usually mixed with silicone matrix material, and then dispensed ontothe LED chips directly. Since the density of the phosphor particle is bigger than that of thesilicone, the phosphor particles may settle in the silicone matrix because of the gravityduring dispensing and curing process. With the sedimentation of the phosphor, the opticalperformance of LED would decrease. Thus, it is a serious problem in the LED packaging.In this paper, based on the viscosity performance of silicone measured, a model wasdeveloped to investigate the phosphor sedimentation phenomenon in silicone according tothe Stokes’ law, and some experiments were performed to investigate the effect ofphosphor sedimentation on LED optical performance.The relationship between the viscosity of silicone and temperature, shear rate, timewas found through experiment. The results show that viscosity of silicone is proportional tothe time and inversely proportional to the temperature, and decreases slightly with theincreasing shear rate. Based on the experiment data, a model was developed for theviscosity of silicone in low cure extent case using N. Kiuna’s model.According to the Stokes’ law, a model was developed to investigate the phosphorsedimentation phenomenon, combining with the size distribution of phosphor particles andthe viscosity-time function of silicone during isothermal curing. We compared themodeling results with the experimental results and found that the model can well simulatethe sedimentation. The results show that the sedimentation velocity is inverselyproportional to the viscosity of silicone and proportional to the size of phosphor particle.Phosphor particles sedimentation lasts for about5minutes during isothermal curing at85℃,and the big ones remain in the bottom of the sample.Experiments were performed to investigate the effect of phosphor sedimentation onLED optical performance. The results show that phosphor sedimentation nearly has noeffect on the efficiency and average correlated-color temperature of LED, while has greateffect on angular color uniformity of LED. With the process of sedimentation, the color temperature fluctuation changes from913K to2354K and then to970K.