*Active Solders and Active Soldering DOI: http://dx.doi.org/10.5772/intechopen.82382*

*Fillers - Synthesis, Characterization and Industrial Application*

**3. Effects of active elements**

their melting points, their lower solidification temperature can alleviate the thermal stresses in the ceramic-metal joint. Active soldering techniques, which provide for bonding of various ceramics and difficult-to-wet materials at low temperature, have also been developed [4]. The solders used for active soldering consist of a low melting point metal such as tin or zinc with active elements such as titanium and rare earth elements. There have been reports of active solders developed by adding rare earth elements (Ce, La) and a wetting promoter (Ga) into Sn–Ag–Ti, Zn–Ag–Ti, Sn–Bi–Ti, and In–Sn–Ti alloys [4, 24]. Hillen et al. initially developed active solders for soldering difficult-to-wet materials, as listed in **Table 1**. With these active solders, the joining process of ceramics can be performed at temperatures lower than 450°C without flux and without the need for premetallization or a protective atmosphere.

A number of active soldering filler metals for direct soldering of difficultto-wet materials have been reported. S-Bond technologies LLC has developed a series of active solder metals and an active soldering process [25]. In a prior study, the active filler metallic alloy Sn–Ag–Ti(Ce, Ga) was successfully used to join indium tin oxide (ITO) targets with Cu backing plates at 250°C in air [26]. Moreover, a lower-melting-point-active-filler metal Sn56Bi4Ti(Ce, Ga) with a low bonding temperature of 180°C was used to join ZnS–SiO2 targets with Cu backing plates [27]. Due to the high chemical activity of Ti, it can easily form the required subsequent chemical reaction between filler metal and substrate. Fu et al. [28] studied the effect of Ti content on the wetting behavior of the Sn0.3Ag0.7Cu/AlN system. They demonstrated that the addition of Ti to Sn–Ag–Cu filler resulted in a significant enhancement of wettability for Ti content of 4–10%. Chang et al. [24, 26, 29, 30] have shown that the affinity of rare earth elements to oxygen gives

rise to the reaction of Ti with some difficult-to-wet materials at a low temperature. Moreover, rare earth elements have a very strong affinity for oxygen, nitrogen, carbon, or almost all metals and hence create chemical reactions at the interface [31]. Qu et al. [32] investigated the effect of Ti content and Y additions on the oxidation behavior of Sn–Ag–Ti solder. Their results indicated that the addition of Y significantly improved the oxidation resistance of Sn–Ag–Ti solder. Due to the higher affinity of Y for oxygen than Ti, the Y added to the solder efficiently inhibited the oxidation of Ti during the soldering process. Many researchers have demonstrated that rare earth element dopants can significantly enhance solder bonding with difficult-to-wet materials [33–36]. The addition of Ti and rare earth elements to Sn or In alloys improves the solderability by increasing the wettability on difficult-to-wet materials. Furthermore, the oxidation resistances of these soldering filler metals are somewhat limited. For example, the addition of the rare earth element Ce to Sn3.5Ag4Ti solder protects Ti from oxidation and enhances the activity of Ti [26, 28, 32]. Magnesium is also very chemically active and has good electric and thermal conductivity. It is also a suitable additive active element for Sn or In alloys used to increase the wetting in some applications of electronic packaging [37, 38]. Chang et al. [39, 40] have also developed a series of magnesium-containing active solders, as listed in **Tables 2–4**. Sn3.5Ag0.5Cu1Mg filler metal was used for joining alumina with alumina at 250°C in air. The

microstructure of the bonding interface of Al2O3/Al2O3 joint is shown in **Figure 4**, which demonstrates a good wettability of the filler metal on the alumina. Hence, a satisfactory joint can be obtained using the magnesium-containing active filler. A good bonding strength of 6.54 MPa can be achieved using the Sn3.5Ag0.5Cu1Mg

**52**

filler metal.

