## 3 Results and Discussion
For the mechanically more robust polycrystalline Al\({}_{2}\)O\({}_{3}\), a much higher average shear strength of more than 50 MPa could be achieved (Figure 5), with failure within the soldering zone, while, for Cu, a much lower shear strength of about 20 MPa was obtained (again with failure within the soldering zone). The observed stark discrepancy in shear strength can be explained by the peculiar microstructure formed within the soldering zone for the specific time-temperature-profile resulting from the thermal properties of the materials: in case of Al\({}_{2}\)O\({}_{3}\) (and also Si), the reactive joining process leads to a joint microstructure featuring excellent mechanical properties upon shear testing: the heat generated by the reactive foil, in combination with the specific thermal conductivity of the material, is sufficient to completely liquefy the Sn solder layer and to achieve distribution of the alloying elements of the Ag-Cu-In layer throughout the solder layer. This leads to formation of intermetallics (especially Al\({}_{2}\)O\({}_{3}\)) evenly distributed within the solder layer. Notably, the shear strength of pure Sn solder is only about 18 MPa [29]. Thus, the additional in situ alloying of Sn by Ag, Cu and In from the protective Ag-Cu-In layer of the RF leads to pronounced strengthening of the solder with regard to pure Sn solder. Good bonding between the solder and the metallisation layer is realised by the Ni-Cu-Sn scallop formation along the solder-metallisation interface without excessive formation of intermetallics, which would otherwise lead to embrittlement of the joint. In case of Cu, the given joining system results in a soldering profile with relatively low temperatures over a relatively short time period. As a consequence, the alloying elements Ag, Cu and In are only distributed in a small region of the Sn solder layer, which prevents an overall strengthening of the solder layer. The shear strength obtained for Cu is therefore practically identical with that of pure Sn solder (about 20 MPa vs. 18 MPa). Notably, although Cu shows the largest thermal conductivity and hence also the fastest cooling rate after foil reaction, a positive effect of microstructural refinement due to rapid quenching of the liquid solder is not observed (cf. [1]). In order to increase the shear strength in case of the Cu substrates, instead of pure Sn a Sn-alloy solder, e.g., of the Sn-Ag-Cu-system, could be used to achieve a more homogeneous strengthening by intermetallics, albeit this may come at the price of an increase in joint brittleness due to the localised alloying effect by the Ag-Cu-In-layer of the reactive foil. The thermal conductivity of the employed substrates or components thus has a pronounced effect on the microstructure of the joining zone obtained upon reactive joining, and accordingly also on the joint quality: the time-temperature-profile developing within the joining zone as a function of the  
Figure 4: SEM micrographs (backscatter electron image) of the solder layer between substrate and RF layer for (**a**) borosilicate glass; (**b**) Al\({}_{2}\)O\({}_{3}\); (**c**) Si; and (**d**) Cu (compare Figure 1a). In case of Cu, the Ni-metallisation layer is virtually invisible due to the similar contrast of Cu and Ni. The micrographs show pronouncedly different microstructures, i.e., different types of intermetallics as well as a different spatial distribution of the intermetallics and of the porosity in the solder layer. thermal conductivity defines the formation, amount and spatial distribution of intermetallics within the joining zone, which again define the mechanical performance of the joint. Optimum reactive joining finally requires adjustment of two factors: first, balancing of heat production and heat dissipation (for given joining components conveniently done by adjustment of RF type and thickness), and second, careful adjustment of the joining system by deliberate selection of solder type and thickness, surface metallisations and RF coatings for in situ alloy formation.