## 1 Introduction  
Since the ban on using lead for soldering alloys, owing to its toxicity [1; 2; 3; 4; 5], immense research and development of new lead-free soldering alloys is in progress. This R&D is primarily oriented to the study of properties such as excellent wettability, thermal and electrical conductivity, corrosion resistance, and, last but not least, a favorable price. In recent years, mainly high-temperature, lead-free soldering alloys, based, for example, on Au-Sn, Zn-Al, or Zn-Sn [6; 7; 8; 9], have been developed. The Sn-based lead-free alloys with very low melting temperatures are typical. At present, these solders are usually employed in the electronic industry as a connecting material in soldering printed circuit boards [10; 11]. However, due to their melting point, which is lower than 250 \({}^{\circ}\)C, they cannot meet the high demands for the applications in electronic power devices employed at elevated temperatures. Therefore, this fact requires further research on soldering alloys, such as Zn-based alloys that are capable of meeting the mentioned requirements [12; 13]. The authors of the study [14] were successful in joining SiC ceramics by application of ultrasonic soldering technology. Two types of Zn-based solders were applied--namely, the non-eutectic alloy type Zn5 Al3 Cu and the eutectic alloy type Zn5 Al. The specimens were heated at the temperature of 420 \({}^{\circ}\)C, followed by ultrasound assistance with a frequency of 20 kHz, 300 W power, and with acting times from 5 to 20 s. The authors studied the effect of ultrasound on the mechanical properties of soldered joints. The shear strength of Zn5 Al soldered joints was 102 MPa with ultrasound acting for 5 s. At a prolonged time of ultrasound assistance, up to 20 s, the shear strength of 138 MPa was achieved. In the case of joints fabricated using Zn5 Al3 Cu solder and ultrasound acting for 5 s, the shear strength attained was 139 MPa, which approaches the maximum shear strength of a joint fabricated using Zn5 Al solder. Thus, Cu addition resulted in improved shear strength. Additionally, the shear strength remained relatively constant with ultrasound assistance for 20 s.  
The authors of the study [15] studied the microstructural and mechanical properties of soldered joints formed of a SiC substrate and Zn-Al-Mg type solder fabricated by ultrasonic soldering technology at 420 \({}^{\circ}\)C. From the measurements of DTA analysis, it follows that TS and TL temperatures occur within the range of 371.5 \({}^{\circ}\)C to 394.5 \({}^{\circ}\)C. The highest measured shear strength of a joint reached 148 MPa in the case of ultrasound assistance for 8 s. The formation of a 2 to 6 nm thick amorphous phase was observed on the substrate/solder boundary. In the study [16], a Zn-based soldering alloy was studied. It was developed for ultra-high temperature applications, with alloying elements such as Al (4-6) wt. % and Cu (1-5) wt. %. That solder was designed in such a manner that it would exert the liquidus temperature within 655-675 K. The presence of Al in the solder improved the solder's spreadability and its electric resistance. The content of selected alloying elements increased the hardness and tensile strength of the solder. The study [17] dealt with Zn-Sb-based binary alloys destined for higher application temperatures. The alloy with a eutectic composition of Zn with 1.7 wt. % Sb exerted better mechanical properties. By increasing the Sb content to 2.2 wt. %, the alloy provided the highest tensile strength. The EDX and XRD analyses proved the presence of Zn and Zn\({}_{4}\)Sb\({}_{3}\) phases in Zn-xSb alloys. Increasing the Sb content in the solder also increases electric resistance. The experimental results suggest that this solder could be a prospective alternative to Pb-containing solders. Research on soldering ceramic or metallic materials using Zn-based solders is actively underway. [PERSON] et al. [18,19] carried out experiments employing Zn-3 Al solder with ultrasound assistance to bond Cu with Cu or Cu with Al.