# Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design
## 3 Results and Discussion  
Figure 5 shows the practicality of two samples before and after the second-layer packaging. The first two samples were pure red CSP filaments, before and after the green LuAG:Ce\({}^{3+}\) packaging, respectively. The last sample was our designed filament, with a CCT of 2500 K, which emits the warm white light, and the upper one is the sample before the designed 2500 K filament packaging. It can be seen that the colors of each sample's emitting light were different. Although the last filament in Figure 5 is a segmented-type luminescence design, the luminescence appeared very homogeneous. This phenomenon was due to the second layer of green packaging providing ample space for light reflection, diffusion and deflection in the silicone matrix. The CSP filament of the laminated package is shown in Figure 6, and the dimensions of each part can be seen clearly. The thickness of the green packaging layer was nearly three times that of the red film thickness, which was a favorable space for photon multiple conversions, so the filament could give uniform illumination. The EL emission of the stacked package and traditional dispensing package filaments, shown in Figure 7, were composed of a blue emission at 450 nm, and a broad emission that occured at wavelengths ranging from 500 to 700 nm, the former from the InGaN LED chip and the latter from the green LuAG:Ce\({}^{3+}\) and red CASN:Eu\({}^{2+}\) phosphors. The green EL emission intensity of the stacked-type packaged filament was found to be higher than the traditional dispensing packaged filament. This may be ascribed to the decreased absorption of the LuAG:Ce\({}^{3+}\)'s green emission by the red-emitting CASN:Eu\({}^{2+}\) phosphor, thereby increasing green emission intensity. These phenomena clearly indicated that spectral overlapping was effectively reduced using a stacked-type package and segmented-type  
Figure 5: An actual image of the stacked type packaged filament. Figure 6: A microscopic image of stacked type packaged filament. luminescence design. The decrease in spectral overlapping observed in the current design of the CSP filament can be attributed to the successful organization of the proximity of the luminescent elements in the assembled CSP filaments. The interaction of these optical centers at an optimum distance promotes the spatial delocalization of energy, minimizes energy transfer between the luminescent centers and thereby reduces the spectral overlapping. With the same correlated color temperature of 2500 K, the CIE color coordinates of the aforementioned filaments are shown in Figure 8. The CIE color coordinates of both samples move to the left, but with two different trajectories, while the current increases from 30 to 210 mA. In the stacked-type filament, red fluorescent film can be considered to be in a state of excitation saturation; thus, with the increase in current, the emission of red light did not change obviously. Instead, the green phosphor stimulated more green light with an increasing intensity of blue light, so the color coordinates moved directly to the blue-green area. On the contrary, in the traditional dispensing package, phosphors were evenly dispersed in the silicone resins. As seen in Table 1, the luminous efficiency of the stacked-packaged filament was higher than that of the traditional dispensing packaged filament. With the increase in current, the luminous efficiency variations of the stacked-packaged filament were lower than that of the traditional dispensing packaged filament. This phenomenon can mainly be attributed to the reduction of photon reabsorption and the simplification of the photon transfer path. The general luminescence mechanism of Pc-White LEDs is the mixing of short wavelength light emitted by blue or UV LED chips and long-wavelength light emitted from phosphors pumped by the short wavelength light [26]. It is considered a complicated energy-conversion and optical tracing process when the input light interacts with a phosphor.