# Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties
## 1 Introduction  
The light-emitting diode (LED), which is the most promising cold light source in the 21 st century due to its energy-saving, environmental protection, high reliability, and flexible design qualities and other advantages, has been widely developed and applied in the field of lighting [1]. Furthermore, the flip-chip LED (FC-LED) filament, which is a new generation of LED lighting sources because of its flat coating technology, 360\({}^{\circ}\) luminescence, no blue light, long life, and slow decay, is a popular choice among those who recall older forms of lighting. Additionally, in terms of appearance, FC-LED filament bulbs are closer to incandescent lamps. With the state ban on incandescent lamps, FC-LED filament bulbs will gradually replace incandescent lamps [2; 3; 4]. In contrast with the ordinary filament, the FC-LED filament, which has a flexible aluminum substrate, can be bent at any angle to satisfy therequirements of different shapes. During transport, the FC-LED filament will not break easily and will not stop functioning when used, which greatly reduces transportation costs and improves the service life of the filament [5,6].  
Although the welding line is omitted in the process description on FC-LED filament packaging, solid crystal welding is still an extremely important part of the process. Presently, the main welding method on the market is direct welding by heating platform. Direct welding means that the heating platform is preheated to the welding temperature, and then the substrate is put in the fixture on the heating platform, so that the aluminum substrate is closely linked to the heating platform. Due to the good thermal conductivity of aluminum substrate, the direct welding method can make solder paste reach the melting point quickly and shorten the time needed for welding. However, using this method, the solder joint is exposed to air, and the solder paste is easily oxidized during the welding process. Vacuum furnace welding improves upon that method. During vacuum furnace welding, the filament is placed in the vacuum furnace, the vacuum is activated and then, according to the heating curve, the furnace is slowly set to the temperature [7, 8, 9, 10, 11, 12, 13, 14]. Relatively speaking, vacuum furnace welding avoids the oxidation of the solder. Although much research has been conducted about different welding methods used on LED, there is limited literature reporting relevant studies for the comparison of direct welding and vacuum furnace welding. In order to select the most suitable welding method according to actual production, the effect of two welding methods on the properties of FC-LED filaments was studied in this paper [15, 16, 17].  
In this study, shearing force testing was conducted using the strength tester. The microstructure of the fracture surface was observed by SEM. The steady-state voltage and the luminous flux of filament were measured by integration sphere, respectively. Then, the two filaments were lit and aged with a constant current source of 285 V/15 mA. The steady-state voltage, the luminous flux and the main wavelength were measured after aging for 72, 216, 336, and 504 h, respectively. By using the above testing methods, the shearing force, the microstructure of the solder joint after the shear failure test, the steady-state voltage, the luminous flux, and the main wavelength curve were obtained, respectively. By summarizing and analyzing the above data, the effects of two different welding methods on the performance of the FC-LED filament were obtained. Subsequently, the suitable welding methods can be selected according to the different production requirements.