# Effect of Different Welding Methods on Flip-Chip LED (FC-LED) Filament Properties
## 3 Results and Discussion
### Steady-State Voltage and Blue Light Luminous Flux  
Figure 4 shows the box plot of the steady-state voltage of 10 FC-LED filaments after lighting 15 min. It can be seen from the diagram that most of the first group have higher voltages than the entire latter group. The average voltage of the DW group was 261.44 V, and the average voltage of the VFW group was 260.72 V. \(D\)-value was less than 1%. However, according to the formula of variance:  
\[\sigma^{2}=\frac{\sum(\mathrm{X}-\mu^{2})}{N}, \tag{1}\]  
where \(\sigma^{2}\) is the variance, which can reflect the degree of dispersion of the data; X is the value of each sample; \(\mu\) is the total average value; \(N\) is the sample number. The authors calculated the variance of the previous set of voltages \(\sigma^{2}_{1}\) = 0.884, the latter \(\sigma^{2}_{2}\) = 0.0225. \(\sigma^{2}_{1}>>\sigma^{2}_{2}\) illustrates that the steady-state voltage of the direct welding filament is more discrete, whereas the vacuum furnace welding steady-state voltage is more concentrated. It can be concluded that there is a big difference in voltage between directly welded filaments. During the practical use of the filament, if it is used in parallel or in series, it may accelerate the aging of the filament because of its partial pressure or shunt unevenness, and shorten the service life of the filament.  
Figure 3: Microstructure of the solder joint after the shear failure test using (**a**) direct welding and (**b**) vacuum furnace welding.  
Figure 2: Average shear strength of 10 FC-LED filaments in two sample package.  
Figure 5 is the steady-state luminous flux of two groups of filaments. It can be seen from the figure that the luminous flux of the VFW group is larger than that of the DW group. The average luminous flux in the DW group was 94.8 lm, and the average luminous flux in the VFW group was 101.0 lm, higher by 6.2 lm than the previous group. In the packaging process of white light LED, the conversion ratio of blue light luminous flux to white light is approximately 1:7. Therefore, assuming that suitable phosphors are chosen, the integral luminous flux of filaments using vacuum furnace welding is much higher than that using direct welding.  
This occurs because the soldering temperature rises too quickly and the insufficient volatilization of the fluxing agent in the solder causes voids in the solder layer; the solder joint is exposed to the air and oxidizes to form an aperture gap, so that the resistance of the chip increases and the voltage increases. In addition, the size and number of the voids in each filament vary from one to the other, which causes the resistance of each filament to be different, further reflected in that fact that the voltage dispersion degree becomes larger. By comparison, the solder fluxing agent in the vacuum furnace is volatilized relatively completely, the solder joint is not oxidized, the voids formed in the solder layer are less porous, the resistance of the filament is relatively consistent, and the voltage fluctuation is also  
Figure 4: Steady-state voltage measured for 10 FC-LED filaments after 15-min lighting.  
Figure 5: Steady-state luminous flux measured for 10 FC-LED filaments after 15-min lighting.  
small. At the same time, because the defects of the DW group are large and the internal resistance increases, the current density through the luminous region decreases when the filament is lit under the same conditions, leading to the low luminous flux [23; 24].