## 3 Results and Discussion
Figure 2: CT scans (inverted grayscale) of joints prepared by reactive joining for (**a**) borosilicate glass; (**b**) Al\({}_{2}\)O\({}_{3}\); (**c**) Si; and (**d**) Cu, showing the region of the solder layer between the RF and the upper substrate plate. The red box defines the position of the upper substrate plate (4 mm \(\times\) 4 mm). Pores (darker regions) are present in the solder layer, as well as cracks (dark lines) in the reactive foil layer, which are partially filled with solder (bright lines). As expected, the microstructure of the joining zone resulting from the reactive bonding step is strongly affected by the different time-temperature profile evolving for the different component materials (Figure 4): as discussed above, in case of borosilicate glass, the low thermal conductivity leads to high temperatures within the joining zone over a prolonged time. The resulting microstructure (Figure 4a, cf. the initial setup depicted in Figure 1) clearly shows the consequences of this extreme time-temperature profile: the Ni-metallisation of the borosilicate glass surface is completely dissolved and only the (thermodynamically very stable) Ti-W adhesion layer remains (also see [10]). In addition, the elements of the Ag-Cu-In protection layer of the reactive foil are completely redistributed throughout the solder layer, leading to in situ alloying: the presence of Ag leads to formation of Ag-Sn intermetallic precipitates within the solder zone, most likely Ag\({}_{3}\)Sn (cf. e.g., [24]; note that the interaction volume of SEM-EDS is considerably larger than the size of the precipitates found in this study, which prevents the exact determination of the composition by this method). In direct vicinity of the Ag\({}_{3}\)Sn precipitates, small particles associated with an enrichment in Ni and Cu are found, indicating the separate formation of ternary Ni-Cu-Sn intermetallic particles (most likely of the Sn-rich (Ni,Cu)\({}_{3}\)Sn\({}_{4}\)-phase): while a number of mixed Ni-Cu-Sn intermetallics exists in the system Ni-Cu-Sn, the mutual solubility of Ag and Ni/Cu in the respective Sn-intermetallic phases is negligible [21; 25; 26]. For the minor alloying element In, no clear enrichments could be detected. The reactive foil layer consists of NiAl phase with some few at.% of V. Notably, at the interface between the solder layer and the reactive foil layer, pronounced scallop formation of the NiAl-phase can be observed, which is typical for the growth (and/or coarsening) of solid-phase particles in contact with a liquid phase. In case of Al\({}_{2}\)O\({}_{3}\) (Figure 4b), the Ni-metallisation layer on top of the Ti-W-adhesion layer is still intact and pronounced formation of Ni-Cu-Sn-phase scallops, probably (Ni,Cu)\({}_{3}\)Sn\({}_{4}\), into the Sn-solder layer can be observed (also here, due to the smallness of the intermetallic particles with respect to the beam interaction volume, exact determination of their composition is not possible). It can thus be estimated that the temperature at the solder-metallisation interface was above the eutectic temperature of Sn-rich Ni-Cu-Sn of about 228 \({}^{\circ}\)C, but below the melting temperature of (Ni,Cu)\({}_{3}\)Sn\({}_{4}\) of about 798 \({}^{\circ}\)C [27]. Within the solder layer, again comparatively large intermetallic Ag-Sn particles can be detected, while, at the boundary between solder layer and reactive foil, formation of some Ni-Cu-Sn intermetallics can be observed. This difference in spatial  
Figure 3: Cross-sections (optical microscopy) of joints prepared by reactive joining for (**a**) borosilicate glass; (**b**) Al\({}_{2}\)O\({}_{3}\); (**c**) Si; and (**d**) Cu. The micrographs were recorded in pore-free regions of the solder layers (cf. the CT results shown in Figure 2).