Polymer Morphology Forming Technology Based On Electrohydrodynamic And Its Applications In LED Packaging

With the development of high-power light-emitting diode (LED) lighting, high quality LEDs with controllable light pattern and high angular color uniformity (ACU) have obtained very high recognition. However, the directly related packaging processes of lens fabrication and phosphor coating of high-power LEDs show many limitations for further promotion of LED in high quality lighting areas. In this thesis, polymer morphology forming method based on electrohydrodynamic (EHD) was studied by modeling and experimental investigation. In this new method, the electric field was introduced and utilized for morphology adjustment of the polymer. Then, its application in lens fabrication and phosphor coating process were proposed and studied. The contents in details and their innovations were described as follows:The mechanism of polymer morphology variations based on EHD were systematically researched by experiments, and the effect of electric field and physical property parameter of polymer on flow mechanism of polymer were studied in detail. Conclusions of these studies include:voltage values, electrode gap, initial height of polymer and temperature load were the main factors that influence the flow behavior of the polymer. In addition, for different polymer materials, it was the coefficient determined by relative permittivity and surface tension of polymer that affected its morphology variation.The conservative level set method coupling with EHD equations was employed to simulate the electrically induced polymer flow process, and the effect of different process parameters on the morphology adjustment of polymer were analyzed. The results show that larger voltage values, initial height and relative permittivity of polymer, and lower electrode gap and surface tension of polymer would result in larger flow deformation of polymer, while the effect of viscosity of polymer on its stable morphology was not remarkable. Moreover, simulation results show that four kinds of desired polymer morphologies, namely cone, ellipse, asymmetric and crater could be achieved by plate-plate, ring-plate, nonaxisymmetrical cylinder-plate and cone-plate electrode pairs, respectively.Phosphor coating process based on EHD was proposed and cone phosphor layer was obtained to enhance the ACU of white LEDs. Optical simulations were performed by the Monte Carlo ray-tracing method. Results show that compared to the conventional phosphor layer of spherical cap with the same correlated color temperature (CCT), the ACU of cone phosphor layer was increased by more than 26.0% and the phosphor volume was reduced by more than 7.66% to save material cost. The phosphor coating experiments were conducted and cone phosphor layers were obtained. Compared with the traditional spherical cap phosphor layer, the angular CCT deviations were reduced from the value larger than 1000K to 180K.Fabrication of adjustable-morphology lens based on EHD were proposed and studied. Three kinds of typical lens morphologies, namely cone, ellipse and asymmetry were fabricated by varying the electric field type. Moreover, the detailed characteristics of each of morphologies were significantly and accurately regulated by electric field. Results show that the cone lens realized convergent light and its strongest light intensity increased up by 35%; while divergent light was obtained by flat lens and its strongest light intensity increased up by 31%and its occurring view angle varied from 0° to ±46°. Meanwhile, asymmetric LID was acquired by asymmetric lens and its strongest light intensity increased up by 39% and its occurring view angle changed from 0° to -48°.Crater lens fabrication method was proposed through combination of droplet alignment based on EHD with droplet evaporation for uniform illumination of LED. In such a method, an immiscible and vaporizable droplet was deposited on the top surface of a liquid silicone lens. By application of electric field, the alignment of the vaporizable droplet was realized. Then, lenses with craters were fabricated by droplet evaporation. The effect of the volume of droplet on lens morphology and LID of packaging LEDs were studied. The optical test results show that the light pattern emitting from crater lens was modulated into the Batwing type which benefited the illumination uniformity; the view angles of the maximum light intensity of the crater lens changed from 0° to ±5° while its light intensity ratio increased from 1 to 2.55. Meanwhile, the maximum light intensity increased by 17%.