## 4 Conclusions  
The objective of this research was to characterize the Zn-Mg type soldering alloy, specifically assessing its suitability for soldering SiC ceramics and a Cu substrate using ultrasonic soldering. The attained results are as follows:  
* The melting temperature of the solder was determined using TG/DTA analysis, which revealed two significant peaks in the test record. The first peak corresponds to the reaction resulting in the formation of the MgZn\({}_{2}\) phase, while the second peak corresponds to a eutectic reaction occurring at 365 \({}^{\circ}\)C. The liquidus temperature was determined to be 405 \({}^{\circ}\)C.
* The microstructure of type Zn1.5 Mg soldering alloy is formed of a zinc matrix. Segregation of binary eutectics in the form of (Zn) + Mg\({}_{2}\)Zn\({}_{11}\) lamellas occurred on the boundaries of Zn grains. The EDX and XRD analyses also proved the presence of magnesium phase type MgZn\({}_{2}\) in the solder matrix.
* The formation of a SiC/solder bond occurred as a result of magnesium distribution from the solder to the interface with SiC ceramics, where Zn is also present. It was found that magnesium reacts with silicon, which is infiltered in SiC ceramics. By mutual interaction, the Mg\({}_{2}\)Si phase is formed.
* In the boundary of a Cu substrate/solder joint, a massive interaction between the molten zinc solder and copper substrate took place at the formation of a broad transition zone, which was formed of two new phases-namely, a non-wettable \(\gamma\)Cu (Cu\({}_{5}\)Zn\({}_{8}\)) phase 30 \(\upmu\)m in width and a non-wettable \(\varepsilon\) (CuZn\({}_{4}\)) phase, 12 \(\upmu\)m in width. The bond formation is thus affected mainly by Zn from the solder.
* The shear strength measurement was performed on several combinations of ceramic materials with the Cu substrate. The average shear strength of a combined SiC/Cu joint fabricated using Zn1.5 Mg solder was 44 MPa. In oxidic ceramics, the average values of shear strength were considerably lower than in nitridide and carbidic ceramics. This is caused by the interaction of an active element (magnesium) with the surface of those ceramics.  
Conceptualization, R.K. and A.P.; methodology, R.K.; validation, R.K. and J.D.; formal analysis, J.D., P.G. and M.P.; investigation, R.K.; data curation, A.P.; writing--original draft preparation, R.K.; writing--review and editing, R.K.; visualization, M.S.; supervision, R.K.; project administration, A.P. All authors have read and agreed to the published version of the manuscript.  
Figure 20: The shear strength of soldered joints fabricated by use of Zn1.5 Mg solder.  
**Funding:** This work was supported by the Slovak Research and Development Agency under contract no. APVV-21-0054 and APVV-17-0025. The paper was also prepared with the support of the VEGA 1/0026/23: Modification of the structure and mechanical properties of solder alloys to increase the reliability of soldered joints in advanced electronic applications and VEGA 1/0237/24 Research of active soldering alloys filled with nanoparticles of ceramic materials.  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
**Acknowledgments:** This contribution was elaborated in cooperation with the Technical University Ostrava (VSB-TU Ostrava), FMT--Faculty of Materials Science and Technology, Department of Non-ferrous Metals, Refining and Recycling.  
**Conflicts of Interest:** The authors declare no conflicts of interest.