## 2 Materials and Methods
Grinding stages involved the utilization of silicon carbide (SiC) emery papers with granularities of 240, 320, and 1200 grains per square centimeter. Subsequent polishing steps employed diamond suspensions with grain sizes of 9 \(\upmu\)m, 6 \(\upmu\)m, and 3 \(\upmu\)m. A final polish was executed using OP-S Struers (Struers, Paris, France) polishing emulsion with a granularity of 0.2 \(\upmu\)m.  
To examine the microstructure of the solder, scanning electron microscopy (SEM) was conducted on TESCAN VEGA 3 (Tescan Group Ltd., Brno, Czech Republic) and JEOL 7600 F microscopes (JEOL Ltd., Tokyo, Japan), equipped with an X-ray micro-analyzer (Microspec WDX-3 PC, Microspec Corp., Fremont, CA, USA) for both qualitative and semi-quantitative chemical analyses. The solder's phase composition was determined using X-ray diffraction analysis, carried out on specimens measuring 10 \(\times\) 10 mm using a PANalytical X'Pert PRO XRD diffractometer (Malvern Panalytical Ltd., Malvern, UK). Thermal analysis (TG/DTA) of Zn1.5 Mg solder was conducted using the DTA SE-TARAM Setsys 18 TM equipment (KEP Technologies Inc., Austin, TX, USA). The measurement system consisted of a cylindrical oven featuring a graphite heating element, a control thermocouple, a measuring bar, and a cooling medium. Both measured and reference specimens were positioned in Al\({}_{2}\)O\({}_{3}\) crucibles, freely placed on the measuring bar, and in contact with two thermocouples, monitoring the temperature difference between the tested and reference samples. Prior to each analysis, the inner space underwent a 15-min wash with argon (6N purity), followed by vacuuming and replenishing with argon. Throughout the analysis, a constant dynamic atmosphere in the oven was maintained, with an argon flow rate of 2 L/h. Temperatures were recorded as the specimen was heated at a rate of 4 \({}^{\circ}\)C/min from room temperature to complete fusion. These temperatures were documented and analyzed using the SETSOFT program. Phase transformation temperatures within the liquidus-solidus range and in the solid state were determined from the evaluated DTA curves. Enthalpies of phase transformation were also derived from the areas under significant peaks. \begin{table}
\begin{tabular}{l c c} \hline Ultrasound power & 400 & [W] \\ Working frequency & 40 & [kHz] \\ Amplitude & 2 & [\(\upmu\)m] \\ Soldering temperature & 380 & [\({}^{\circ}\)C] \\ Time of ultrasound activation & 5 & [s] \\ \hline \end{tabular}
\end{table} 
Table 2: Soldering specifications. Figure 4: Illustration outlining the soldering process with the aid of ultrasound. For the assessment of mechanical properties in soldered joints, a shear strength test was conducted. A schematic representation of the test specimen is depicted in Figure 3b. The shear strength was measured using versatile equipment type 5.250 SP1-VM (Labortech Ltd., Opava, Czech Republic). A specially designed jig with the defined shape of the test piece was employed to alter the direction of the axial loading force on the test specimen (Figure 5). The shearing jig ensured uniform specimen loading for shear in the plane of the substrate/solder boundary.