# Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design
## 2 Experimental Section  
The fluorescent film used for packaging is made by TECORE SYNCHEM, Inc., Tianjin, China, our partner company, using their self-developed preparing technology. A red-emitting phosphor CASN: Eu\({}^{2+}\), with a wavelength of 618 nm, was used to make the fluorescent film with a mass fraction of 22.2 wt.%. The manufacturing process of CSP LEDs is shown in Figure 1. Firstly, under a certain pressure, rearranged chips were pressed into a softened film under vacuum conditions. Then, after being baked in an oven for three hours at a temperature of 300 \({}^{\circ}\)C, the pressed chips coated with film were cut into pieces, which we called CSP LEDs. After that, the red CSP LEDs were welded to a 135-mm long flexible substrate using a solid crystal machine. Each flexible filament substrate was welded with 51 CSP chips, which were connected in series. The uniform mixture of green-emitting phosphor LuAG:Ce\({}^{3+}\) and package resin was used as a second layer package for the CSP filament. Compared with the traditional dispensing packaging method, this packaging method is more complex but has a lower cost. Figure 1: Schematic stacked type chip-scale package (CSP) filament design and fabrication. As mentioned above, stacked packaging structure can improve the optical efficiency of remote encapsulation of an LED. However, due to the air gap between the interfaces of the two packages, there are many optical phenomena, such as refraction, reflection and scattering at the interfaces [24]. Therefore, LEDs with stacked packages will not reach the ideal light effect, and the light effect will even be reduced due to the excessive absorption and barrier of red phosphor to blue light when the red phosphor layer is under the green phosphor layer. Therefore, a dry-wet separation package method, on the basis of a stacked packaging structure, was adopted to eliminate the air gap by the wet layer wrapping the dry layer completely. As we know, red-emitting phosphors CASN:Eu\({}^{2+}\) have strong absorption and blocking effects on blue emissions [25]. The electro luminescence (EL) spectra obtained under the conditions of 30 mA and 140 V, and the corresponding Commission Internationale de L'Eclairage-1931 (CIE-1931) chromaticity coordinates tested by the Yuan Fang integral ball under a condition of 30 mA and 140 V, are shown in Figure 2a,b. The EL emission of the red CSP filament without the green package is composed of a blue emission at 450 nm, and a broad emission that occurs at wavelengths ranging from 550 to 700 nm; the former arises from the InGaN LED chip and the latter from the red CASN:Eu\({}^{2+}\) phosphors. However, the EL emission of the red CSP filament with a green package includes little blue emission at 450 nm, which means that the upper green phosphor almost completely absorbs the blue emissions emitted from the red CSP LEDs. With an increasing forward driving current, from 30 to 210 mA, the corresponding CIE-1931 chromaticity coordinates are presented in Figure 2b. A comparison of Commission Internationale de L'Eclairage (CIE) color coordination reveals that the emitting lights of these two samples are all in red. In order to fully stimulate the red-emitting phosphors and let parts of the blue light overflow stimulate the green emitting phosphors, a small improvement was applied to the filament structure. As shown in Figure 3, blue chips were arranged regularly between red CSP LEDs, and the proportions of red CSP and blue light chips were 2:1, 1:1, 1:2 and 1:3, respectively. This package can be seen as a combination of stacked-type and segmented-type packages. The mass fraction of red CASN:Eu\({}^{2+}\) in the film was determined, and the mass fraction of green LuAG:Ce\({}^{3+}\) and the proportion of CSP and blue chips is controllable. Thus, luminous performance can be designed by controlling these two parameters.