**4. Active soldering**

Active soldering is a flux-free soldering process. The active solders can be activated by exposure to high temperature or mechanical agitation [41, 42]. Joining with low melting point active solders such as Sn10Ag4Ti and Pb4In4Ti is always conducted at elevated temperatures above 700°C, owing to the decent thermodynamic activation. Chai et al. [22] investigated the wettability of Sn10Ag4Ti on SiC and Al2O3 substrates. They indicated that the contact angles decreased with increases in temperature and heating time. The contact angle of the Sn10Ag4Ti filler metal on SiC decreased almost to 0° when the temperature was raised above 680°C. Ti aggregated strongly in the Sn10Ag4Ti/SiC and Sn10Ag4Ti/Al2O3 interfaces after brazing at 700°C. Koleňák et al. [43] indicated that the wettability of Sn3.5Ag4Ti(Ce, Ga) solder depended on temperature and wetting time. Wettability of the Sn3.5Ag4Ti(Ce, Ga) solder on Al2O3 was achieved with heating at 850°C for 43 min [42]. The schematic in **Figure 5** illustrates the wetting process of low melting point filler metal with high-temperature activation [43].

This soldering process, normally implemented under low temperature, requires mechanical activation to destruct the oxide layer forming on the liquid molten filler, after which the active elements Ti and rare earth elements can allow metallurgical

reaction with the substrate. Smith [41] has reported that mechanical agitation such as edge abrasion, brushing, vibration, and ultrasonic pressure can disrupt the molten active solder's surface oxide, thus permitting metallurgical interaction between the active elements, Ti and rare earth elements, and substrate. **Figure 6** illustrates mechanical agitation to disrupt the oxide layer to activate the molten active solder [41].

**57**

**Figure 8.**

**Figure 7.**

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

*Active Solders and Active Soldering*

*DOI: http://dx.doi.org/10.5772/intechopen.82382*

Chang et al. [26, 29, 30] have investigated ITO/Cu, ZnS–SiO2/Cu, and Al2O3/ Cu joints using Sn3.5Ag4Ti(Ce, Ga) and mechanical agitation at 250°C. They have indicated that the affinity of rare earth elements to oxygen gives rise to the reaction of Ti with ITO, ZnS-SiO2, and Al2O3 at a low temperature of 250°C. Their results have also shown a strong tendency of Ti to segregate at the ITO/solder, ZnS-SiO2/ solder, and Al2O3/solder interfaces. Cheng et al. [45, 46] investigated the influences of the active element Ti on interfacial reaction and soldering strength between Sn3.5Ag4Ti(Ce, Ga) alloy filler and Si substrate as well as SiO2/SiO2 joints. They also found that Ti played a critical role in obtaining reliable bonds for active soldering. The chemical adsorption of Ti on the substrate and the interfacial reaction between Ti and substrate were the active mechanisms. Similar to the cases in previous studies [24, 27], the joining process of ceramics can be performed using Sn56Bi4Ti(Ce, Ga) filler at temperatures lower than 180°C. The schematic in **Figure 7** illustrates 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

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

**Figure 6.** *Schematic of mechanical activation and soldering process [41].*
