Modeling on phosphor sedimentation phenomenon during curing process of high power LED packaging
© Wang et al.; licensee Springer. 2014
Received: 16 October 2013
Accepted: 23 December 2013
Published: 23 April 2014
In the LED packaging, the phosphor particles are usually mixed with silicone matrix material, and then dispensed onto the LED chips directly. Since the density of the phosphor particle is bigger than that of the silicone, the phosphor particles may settle in the silicone matrix because of the gravity during dispensing and curing processes. In this paper, a model was developed to investigate the phosphor sedimentation phenomenon in silicone during isothermal curing according to the Stokes’ law, and the size distribution of phosphor particles and the viscosity-time function of silicone during curing were built for the model. We compared the modeling results with the experimental results and found that the model can well simulate the sedimentation phenomenon. The modeling results show that the sedimentation velocity is inversely proportional to the viscosity of silicone and proportional to the size of phosphor particle. Phosphor particles sedimentation lasts for about 5 minutes during isothermal curing at 85°C, and the big particles accumulate in the bottom of the sample.
High power light-emitting diode (LED) has grown explosively in recent years for its extraordinary characteristics, such as high luminous efficiency, low power consumption, high reliability, and long life. Thus, LEDs are considered as the most potential solid-state light source for the next generation lighting and have been widely used in our daily lives [1–3].
In the LED packaging shown in Figure 1, phosphor particles are usually mixed with silicone matrix material, and then dispensed onto LED chips directly . Since the arrangement of phosphor particles plays an important role in determining the optical performances of white LEDs, the concentration of phosphor particles should be as uniform as possible. However, the density of phosphor particles (about 4800 kg/m3) is bigger than that of silicone (about 1120 kg/m3). Phosphor particles would settle in silicone matrix because of the gravity during dispensing and curing processes as a consequence. With the settling of phosphor particles, the luminous efficiency would decrease and so did the homogeneity of the color. Thus, it is a serious problem in the LED packaging.
In recent years, few experiments and simulation studies have been performed to investigate phosphor sedimentation phenomenon. Sommer et al.  simulated phosphor sedimentation using the commercial software package ASAP (Advanced System Analysis Program). They found that LED optical performances, such as the color temperature and the flux output, are highly dependent on phosphor distribution. Lee et al.  investigated the influence of phosphor sedimentation on the white LEDs with different structure chips, and found that phosphor sedimentation cause 20% difference in the luminous efficiency of white LEDs. Hu et al.  established a multi-layer phosphor model by assuming the concentration gradient of phosphor, which was applied to study the effect of phosphor sedimentation on the light extraction efficiency. They found that phosphor sedimentation caused 13.04% drop of light extraction efficiency.
The above researches focused on studying the influence of phosphor sedimentation on the performance of white LEDs, while the investigations about phosphor sedimentation during curing process of white LED packaging have not been found yet. In this study, phosphor sedimentation during curing process was investigated by modeling according to the Stokes’ law. In order to validate the accuracy of the model, longitudinal cross-sectional microstructure photograph of a sample was acquired using the scanning electron microscope (SEM). The comparison between modeling results and experimental results proves that the model can well predict this sedimentation process.
where A = (4/3)πr3g(ρ p - ρ f ), B = 6πηr, and C = (4/3)πr3ρ p . Thus, the size distribution of the phosphor particles and the viscosity-time function of silicone during curing are the key to get the velocity of any phosphor particle at any time during the sedimentation process, according to A, B and C.
Size distribution of phosphor
Viscosity-time function of silicone
Substituting Eq. (6) and Eq. (7) into Eq. (5), the velocity of any phosphor particle at any time during the sedimentation is obtained, and the displacement of the particle can be calculated by integration as a result.
Results and disussions
Comparison with experiment
According to the Stokes’ law, a model was developed to investigate phosphor sedimentation in silicone during isothermal curing. Experiment was conducted and the comparison between the experiment and simulation results shows that the model can well simulate the sedimentation of the phosphor particles in silicone during isothermal curing.
The results indicate that the sedimentation velocity is inversely proportional to the viscosity of silicone and proportional to the size of phosphor particles. During isothermal curing, the resistance force (F Stokes = 6πηrv) acting on the phosphor particle increases with the increasing viscosity as time goes by. The velocity of the phosphor particle (d= 13 μm) is close to zero after about five minutes, and the displacement is 0.188 mm which accounts for 15.67% of the total thickness. Therefore, phosphor sedimentation occurs obviously during isothermal curing at 85°C. The way to suppress the occurrence of phosphor sedimentation is now under investigation. We suppose to find a curing condition in which the distribution can be more uniform using our sedimentation model in further study.
The authors would like to acknowledge the financial support in part by National Natural Science Foundation of China (51376070) and in part by 973 Project of The Ministry of Science and Technology of China (2011CB013105).
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