**1.3 Activation mechanism of an active element by ultrasound**

The surface of soldered materials is covered with an oxide layer which must be gradually disrupted during soldering. This is performed by the mechanism called 'solder activation'. For the soldering of metals, it is mostly sufficient to activate mechanically by scratching; however, in the case of ceramic materials, it is necessary to employ the ultrasonic activation with the frequency over 20 kHz.

Mechanical activation is realised by:


*Lead Free Solders*

the pressure of 10<sup>−</sup><sup>1</sup>

–10<sup>−</sup><sup>3</sup>

*1.2.2 Mechanical activation of solder*

solder deposition is eliminated [21].

• spreading with a metal brush

exclusively by the application of ultrasound.

• scratching

• vibrations

• ultrasound

solder is shown in **Figure 4**.

Mechanical activation may be realised by:

Pa. In some special cases, soldering may be performed also

in the overpressure of argon or helium. Nitrogen cannot be used as a shielding gas since it deteriorates the wetting of ceramics. This is caused by the fact that nitrogen has a high affinity to Ti, and thus it depletes the solder by Ti. The high-temperature activation cannot be used for soldering of metallic materials with the coatings of Au, Ag, Cu, Al and Mg, which exert a high dilution rate in Sn solder. The working parameters exert a significant effect upon the properties of soldered joints [21].

Mechanical activation seems to be a new trend in the field of active solders at present. Soldering is realised at temperatures of 250–280°C with the dwell time from 30 seconds up to 3 minutes. The time- and power-demanding high-temperature activation is in this process replaced with the mechanical activation of an active element. In this way, the necessity of a vacuum, shielding atmosphere or multistep

The primary reason for activation consists in the fact that the surface of metallic materials is covered by an oxide layer, which must be gradually disrupted during the soldering process. Activation of surface layers on ceramic materials is possible

In order to allow soldering in the air without the necessity of flux, the active solder is alloyed with the elements from the group of lanthanides. These are the rare earth metals, for example, Ce and Ga, which protect Ti against oxidation during heating and soldering [22]. The soldered joint fabricated with Sn3.5Ag4Ti(Ce,Ga)

The work cycle of soldering with ultrasound activation is considerably shorter than in the case of high-temperature activation. The soldering temperatures are also significantly lower than at high-temperature activation. The structure of soldered

materials is less affected, and therefore lower residual stresses are formed.

**30**

**Figure 4.**

*Interface of ZnSiO2, Sn3.5Ag4Ti(Ce,Ga), Cu joint [23].*

• ultrasound with the frequency over 20 kHz (suitable for soldering ceramic and non-metallic materials)

The most used technologies for fabrication of combined soldered joints type ceramics-metal are derived from ultrasonic soldering. This results from the finding that only ultrasonic activation is sufficiently efficient for disrupting the surface layers on ceramic materials [15, 21]. The physical principle of ultrasonic soldering consists in the fact that cavitation of sufficient intensity occurs in liquids and molten metals affected by ultrasonic field. The erosive activity of cavitation attacks, disrupts and removes the oxides from the surface of the soldered part. If a solder with a sufficient content of active elements is used, the reliable, diffusion and metallurgical bonding with the parent material is attained. Ultrasonic cavitation reduces the surface tension and enhances the spreadability and capillarity of solders. It also significantly affects the distribution of an active element in the solder matrix and supports the diffusion processes in the phase interface. The time of solder activation by ultrasound partially depends on the resistance of surface oxide layers against the cavitation erosion. However, the times of working cycles are incomparably shorter than the times of activation at high temperatures in vacuum. Application of ultrasonic method is sometimes limited by the soldering material used. In the case of brittle substrates, the damage of specimen by cracking the surface layers may occur [24].

### *1.3.1 Principle of solder activation by power ultrasound*

Majority of power ultrasound applications, where also ultrasonic soldering belongs, necessitate semi-wave transducers with the resonance frequencies of 20–60 kHz. Ultrasonic transducer transfers the electric power to mechanical—the so-called ultrasonic—oscillations. Ultrasonic head consists of an oscillating system fastened in a case made of plastic. The protective case serves for an ergonomic grasping of the tool, eventually its clamping on a stand. The oscillating system is formed by a piezoelectric transducer, a concentration adapter and an exchangeable working tool. The exchangeable tools—sonotrodes—which are screwed on the adapter may be of different shapes. In most cases it is a conical point made of titanium alloy [25]. The principal scheme of an equipment for ultrasonic soldering is shown in **Figure 5**.

The sonotrode point is oscillating with the frequency of alternating current supplied by the generator through the connecting cable. The amplitude of oscillation is variable, and it is altered with the frequency of the supplied current. It generally varies at the level of 10<sup>−</sup><sup>4</sup> mm. The rate and intensity of applied UT oscillation vary within a certain range, and it is selected with regard to the process conditions and the character of materials. In this way, it is possible to affect both the strength characteristics of the

**Figure 5.** *Principal scheme of an equipment for ultrasonic soldering.*

#### **Figure 6.**

*Work cycle of soldering with application of mechanical activation.*

joint and its technological parameters (e.g. the joint width) as well [26]. The work cycle of soldering with application of mechanical activation by ultrasound is roughly 10 times shorter than in the case of high-temperature activation (**Figure 6**).

Acting upon the active solder by ultrasound may be performed in several ways. The technology depends on the size and geometry of the joint, soldering temperature, type of parent metal, number of produced pieces and other parameters. The most used technique of laboratory activation of melts by ultrasound comprises the method, where the solder is molten on the surface of a joined unit by an external heat source, and the ultrasonic power is supplied through the solder from a sonotrode point on the part surface. This method makes it possible to solder selectively the localised surfaces on large parts or to cover the entire surfaces of parts. This principle is shown in **Figure 7** [24].

**33**

**Table 2.** *Solder composition.*

**Figure 8.**

*Microstructure of the binary Ti-Sn system (SEM).*

*Soldering by the Active Lead-Free Tin and Bismuth-Based Solders*

**Figure 8** shows the microstructure of SnTi2 solder. This solder was manufactured by free casting in a vacuum. The solder exerts heterogeneous composition. The matrix consists of 100% Sn. The round, dark and oblong grey phases are the intermetallic compounds of the Ti-Sn system. The zones of solder containing 35.2% Ti and 64.8% Sn consist of Ti6Sn5 phase. Composition of SnTi2 solder in the selected spots is shown in **Table 2**. Diffraction analysis performed on a sample of SnTi2 (**Figure 9**) solder has revealed the presence of the following phases: Ti6Sn5, Ti3Sn, Sn3Ti5 and Sn5Ti6. Solder microstructure consists of a tin matrix (100% Sn) with non-uniformly distributed phases of the Ti-Sn system. The coarse dark-grey phases in **Figure 8** represent the Ti6Sn5 phase; the smaller bright-grey phases with Sn designation contain 3.6% Ti and

The SnAg3.5Ti4(Ce,Ga) solder shown in **Figures 10** and **11** consists of a tin matrix with non-uniformly distributed constituents of intermetallic phases of binary Ti-Sn, Ag-Sn and Ag-Ti systems. The dark-grey phases contain an average of 31.5% Ti and 68.5% Sn, while the constituents of the Ti6Sn5 phase are concerned. The dark, clearly limited zones are formed by almost pure Ti. The composition of

**Ti [wt. %] Sn [wt. %]**

A1 35.2 64.8 A2 4.36 95.64 A3 13.8 86.2 A4 0 100

SnAg3.5Ti4(Ce,Ga) solder in the selected spots is shown in **Table 3**.

*DOI: http://dx.doi.org/10.5772/intechopen.81169*

**2.1 SnTi2 solder**

96.4% Sn.

**2.2 SnAg3.5Ti4(Ce,Ga) solder**

**2. Active solders for ultrasonic activation**

**Figure 7.** *Manual soldering with ultrasound assistance.*
