## 4 Results  
In using the method described in Section 3, all the LEDs described in Section 2 were tested. The results of the performed investigations are shown in Figures 5-12. In these figures, individual samples are marked with a number from 1 to 9. The soldering alloy is given in brackets according to the following code: REF--SACX0307 alloy; TiO\({}_{2}\)--SACX0307 alloy with an addition of TiO\({}_{2}\) (200 nm); and nanoTiO\({}_{2}\)--SACX0307 (20 nm) alloy with an addition of TiO\({}_{2}\) nanopowder. During the measurements, some of the tested LEDs were damaged. A photo presenting all samples after the accelerated aging test is shown in Figure 5.  
Figure 5: Views of lens and chips of tested LEDs after aging test. Figure 8: Luminous flux changes in LEDs during the aging process. Figure 6: I-V characteristics of tested devices obtained at t = 0 (solid lines) and at t = 1000 h (dashed lines). Figure 7: Thermal resistance of LEDs during the aging process. Figure 11: Optical power of LEDs during the aging process. Figure 10: Luminous efficacy LEDs during the aging process. Figure 9: Thermal resistance changes in LEDs during the aging process. It can be seen in Figure 5 that after the accelerated aging test [45; 46], the lenses in samples S05 (SACX0307-nanoTiO\({}_{2}\)) and S09 (SACX0307-TiO\({}_{2}\)) were damaged. This damage caused a decrease in the obtained optical parameters of the tested power LEDs. It should also be noted that samples S02, S03, S04, S06, and S08 were damaged due to fatigue of the soldered joint, which caused a lack of current flow through the tested power LED. It can be emphasized that after 1000 h, only four out of nine samples were electrically efficient (S01, S05, S07, S09). Figure 6 shows the I-V characteristics of the tested LEDs. The solid lines indicate the characteristics measured for the time t = 0 (before starting the aging process), and the dashed lines indicate the characteristics measured for the time t = 1000 h (at the end of the aging test). As can be seen, some LEDs were damaged before the end of the tests, and characteristics for these LEDs after the 1000 h aging test were unavailable. For many of the tested diodes, there are no visible differences between the characteristics measured before the aging test and after it. Diode S09 recorded the most significant changes in its characteristics. At forward current I\({}_{\rm F}\) = 3 A, the forward voltage increased beyond 0.4 V after the aging test. In contrast, for diode S07, such changes do not exceed 10 mV. The observed differences between the characteristics of diodes S07 and S09 assembled with the use of the same soldering alloy suggest that individual properties of the tested devices can influence the speed of the degradation of their electrical properties. Figure 7 illustrates the effect of the aging test duration on the thermal resistance of the LEDs. Significant changes in R\({}_{\rm th}\), up to 25%, were observed for diode S01. After completing the tests, the lowest R\({}_{\rm th}\) value was obtained for the diode soldered with an alloy with the addition of TiO\({}_{2}\) and the highest for the diode soldered with the reference alloy. A similar shape of the considered dependence is observed for diodes S06 and S08, but the changes in the value of R\({}_{\rm th}\) do not exceed 6%. Figure 8 shows the impact of aging on the luminous flux (\(\Phi_{\rm V}\)) emitted by individual diodes. For most LEDs, the during of the aging process did not significantly affect the luminous flux. Only for diode S05 is a significant drop (even by 75%) of \(\Phi_{\rm V}\) visible. It results from damage to the diode lens during heating. Figure 9 illustrates the effect of the aging test duration on the thermal resistance of the LEDs. Significant changes in R\({}_{\rm th}\), up to 25%, were observed for diode S01. After completing the tests, it is also visible that the lowest R\({}_{\rm th}\) value was obtained for the diode soldered  
Figure 12: Luminous flux of LEDs during the aging process.