# Effect of Different Bonding Materials on Flip-Chip LED Filament Properties
## 2 Experimental Section  
A flip chip (Zhongke0620, Shenzhen, China) with an Au pad was welded to a flexible aluminum substrate. The flip chip size was 6 \(\mathrm{mil}\times 20\)\(\mathrm{mil}\), the working voltage was 2.51-2.60 V, and the mainly blue wavelength range was 452.5 \(\mathrm{nm}\) to 454.9 \(\mathrm{nm}\). In this research, JUST BT-929 (JUST, Xiamen, China) LED automatic bonding equipment was used for attaching the flip chip to the substrate; Sn90 Sb10 (Proudbull, Shenzhen, China) and SAC0307 (Proudbull, Shenzhen, China) solders were used as bonding materials. A flexible aluminum substrate was placed on a horizontal movable platform, and the flip chip was fixed on the wafer substrate; then, the wafer substrate was removed. All samples were welded using the reflow process. The physical morphology of the flip chip is presented in Figure 1; Figure 2 shows the welding schematic diagram, composed of, from top to bottom, the flip chip, the lead-free solder, an Au layer, a Cu layer, and the flexible aluminum substrate.  
Figure 3a shows both the SAC0307- and the Sn90 Sb10-welded filaments, with SAC0307 filaments on the left, and Sn90 Sb10 filaments on the right. Figure 3b,c show the two types of filament with input current. The substrate for these types of filament was 175 \(\mathrm{mm}\) thick, the working voltage was about 265 \(\mathrm{V}\), and each filament could fix 100 chips in total. A thermocouple was attached to the substrate for thermal performance analysis in a reflow welding test. The peak welding temperature was measured by the LED-T300B thermal analysis system (LEETS, Shanghai, China). It was found in the reflow welding test that the performance of the filaments welded at the peak temperature range was better. More precisely, the peak welding temperature of SAC0307 was about 553.5 \(\mathrm{K}\), and its melting point was  
Figure 1: Physical morphology of the flip-chip.  
Figure 2: Flip chip welding schematic diagram. FC-LED: flip-chip light-emitting diode.  
about 500.5 K. The melting point of Sn90 Sb10 was 513.5-518.5 K, and the peak welding temperature of the solder was about 573.5 K [17; 18].  
The physical morphology of this flip chip was observed by two-dimension analysis. The microstructure surface of the solder joints after the shear test were photographed by SEM (Hitachi, Tokyo, Japan), with a field-emission scanning electron microscope at a working voltage of 15.0 kV and a working distance of 6 mm during the test. The shearing force of the two types of solder joint was examined using a shearing strength tester (XYZETC, Shenzhen, China). In the shearing test, the instrument working at room temperature provided a thrust of 200 Kgf and detached the flip chip from the bonded solder joint. A LED-T300B thermal analysis system (LEETS, Shanghai, China) was used for measuring the junction temperature of the FC-LED filament enclosed in the bulb at room temperature, at a test voltage of 285 V with input current of 15 mA. A GDJS-100 (Aoke, Hangzhou, China) high-temperature and high-humidity aging equipment was used for the FC-LED filament aging test at 85 \({}^{\circ}\)C/85% relative humidity (RH). The steady-state voltage of these FC-LED filaments was measured by a constant current source of 285 V/15 mA, lighted for 30 min at room temperature. The blue light luminous flux of these filaments was calculated by the LED300 integrating sphere (EVERFINE, Hangzhou, China).