**Figure 7.**

*Schematic of the wetting process of low melting point filler metal with mechanical activation [47].*

Another promising option for solving the problems of oxidation and wetting of the filler metal on substrate is ultrasonic-assisted soldering technology. An ultrasonic vibration soldering system is illustrated in **Figure 8** [32]. The wettability study

**Figure 8.** *Schematic of the ultrasonic vibration soldering system [32].*

reported by Hillen et al. [4] presented an ultrasonic vibration method using an ultrasonic soldering iron, as shown in **Figure 9**. Ultrasonic vibration can effectively improve the wettability of active solder. Yu et al. [48] have reported that applying ultrasonic vibration during the soldering process causes the active solders Sn–Ag–Ti and Sn–Ag–Ti–Al to spread on the graphite surface at 450°C in air. Koleňák et al. [49] successfully used ultrasonic-assisted soldering to join SiC and copper with In10Ag4Ti solder at 230°C.
