*1.2.2 Mechanical activation of solder*

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 solder deposition is eliminated [21].

Mechanical activation may be realised by:


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 exclusively by the application of ultrasound.

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) solder is shown in **Figure 4**.

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.

**31**

at the level of 10<sup>−</sup><sup>4</sup>

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

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

sary to employ the ultrasonic activation with the frequency over 20 kHz.

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 neces-

• scratching and spreading with a metal brush (suitable for soldering metals as

• ultrasound with the frequency over 20 kHz (suitable for soldering ceramic and

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].

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

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

*1.3.1 Principle of solder activation by power ultrasound*

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

Mechanical activation is realised by:

Cu, Al, Ni, CrNi steel, etc.)

• vibrations (50–60 Hz)

non-metallic materials)

**Figure 4.** *Interface of ZnSiO2, Sn3.5Ag4Ti(Ce,Ga), Cu joint [23].*
