# Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design
## 1 Introduction  
Attributed to their high efficiency, long life, low power consumption, environmental friendliness, tiny size, and directionality, the recently-developed phosphor-converted white light-emitting diode (WLED) have become the most promising substitute to conventional light sources [1; 2; 3; 4]. However, there are still many problems, including the improvement of phosphor conversion and luminous efficiency [5], and improvements in color purity for their substitution of cheaper incandescent and fluorescent lamps that currently occupy the global lighting market [6].  
Presently, light-emitting diode (LED) filaments are a fervent LED product. They have three-dimensional luminescence and high plasticity, and can control light distribution according to the application environment. In contrast, the shortcomings are also very prominent, such as low color purity, low light efficiency, and low heat dissipation, so it is difficult to meet the standards of indoor lighting. The dispensing method of traditional LED filament packaging technology has a large variation in optical performance and the drift of color temperature under different currents [7; 8]. Color purity is the key to improving luminous efficiency and light output. The blue InGaN LED chip pumped by blue/green phosphor is a mature technology; however, these lack red components [9].  
Therefore, the mixing of a red phosphor viz. Sr\({}_{2}\)Si\({}_{5}\)N\({}_{8}\): Eu\({}^{2+}\) and green/yellow phosphor, namely, yttrium aluminum garnet (YAG): Ce\({}^{3+}\) or SrSi\({}_{2}\)O\({}_{2}\)N\({}_{2}\): Eu\({}^{2+}\), is one method to overcome the problem of impure color. Nevertheless, low color purity can be caused by spectral overlapping, which happens when the partial reabsorption of emitted light by another phosphor, mixed with the design of the two phosphors, occurs. [10; 11; 12; 13; 14; 15]. To tackle this problem, stacked phosphor layers were developed by stacking green and red phosphor layers, which have been found to have higher efficiencies. This design showed that the reabsorption of green emissions by red components is reduced, when a red phosphor is used under the green phosphor layer [16].  
However, thus far, most of the research on stacked package structures is focused on remote type packages, such as phosphor in glass (PiG), fluorescent film, and other forms [17; 18]. For example, [PERSON] et al. [19] found that screen-printing a red phosphor layer on LuAG:Ce\({}^{3+}\) PiG obtained a thermally-stable warm WLED. [PERSON] et al. [20] designed a new phosphor design for reducing the spectral overlapping of PiGs, by cutting and reassembling commercial green Lu\({}_{3}\)Al\({}_{5}\)O\({}_{12}\): Ce\({}^{3+}\) (LuAG:Ce\({}^{3+}\)) and red CaAlSiN\({}_{3}\): Eu\({}^{2+}\) (CASN:Eu\({}^{+}\)) PiGs. Inspired by what has been explained above, our team planned to solve the performance problem of LED filaments by combining chip scale package (CSP) with a traditional dispensing method. In recent years, CSP has become a very popular research topic in the LED packaging field [21; 22]. The package size is only slightly larger than an LED chip, with an area ratio of 1.5:1 [23]. CSP has the advantages of having a small package size and high assembly density [21]. Thus, in this work, we report a new phosphor design for reducing the spectral overlapping of LED filaments by using a CSP layer and a layer of dispensing phosphor.