## 3 Results and Discussion  
Reactive joining was first performed for all four substrates using the standard joining configuration with a reactive foil of 60 \(\upmu\)m thickness and Sn solder foils of 10 \(\upmu\)m thickness. In this way, firm bonds were immediately obtained in case of Al\({}_{2}\)O\({}_{3}\), Si, and borosilicate glass. In the latter case, the substrates showed large cracks emerging from the joining interface, which can be attributed to an overexposure to heat (i.e., thermal shock, cf. [10]). In contrast to that, in case of Cu no bond was obtained with the standard joining configuration: although the Sn solder foils were completely melted and strongly fused to the reactive foil after the joining process, bonding of the Sn solder to the Ni-metallisation of Cu was not achieved. This difference in macroscopic joining behaviour clearly shows the effect of the different thermal conductivities of the substrate materials (see Table 1): in case of borosilicate glass, the very low thermal conductivity of the glass leads to a heat pile-up and very high temperatures within the joining zone adjacent to the reactive foil. As a consequence of the sudden and localised heat exposure, the glass substrate experiences a thermal shock close to its surface and crack formation is initiated. Residual stresses then lead to further crack growth (this problem and its prevention is discussed in detail in [10]). By contrast, the very high thermal conductivity in case of Cu leads to fast drainage of the heat of reaction into the substrates. As a consequence, only a temperature increase is achieved in the joining zone which is sufficient to melt the Sn shortly, but insufficient to activate the reaction of liquid Sn with the Ni surface metallisation. This problem could be solved e.g., by use of a RF with larger thickness providing more heat of reaction [11]. The solution applied here was, for sake of comparability, to pre-solder the (Ni-coated) Cu substrates by the galvanic deposition of Sn of same thickness as the Sn-foils. This way, a direct contact between the Sn solder and the Ni metallisation is created, and it is no longer necessary to liquefy the entire solder layer completely: local melting and re-solidification of the Sn solder surface in contact with the reactive foil is enough to achieve bonding. Pre-soldering of the substrates also has the advantage that two sets of interfaces in the joining setup can be eliminated, which reduces the amount of gas trapped during joining at atmospheric pressure, i.e., the amount of porosity in the solder layers (additionally, with regard to manufacturing efficiency, two process steps for positioning of the solder foils become obsolete). Although pre-soldering of Cu with Sn may eliminate the necessity of a surface metallisation, the use of a Ni metallisation is still advised in order to prevent formation of detrimental amounts of Cu-Sn intermetallics and Kirkendall voiding during joint operation at elevated temperatures. Another feasible approach to improve the solderability with Sn solder upon reactive joining would be the use of Au as a final surface metallisation layer: Since the temperature of the Sn-rich eutectic decreases from 231 \({}^{\circ}\)C for the system Ni-Sn [21] to 211 \({}^{\circ}\)C for the system Au-Sn [22], a lower temperature is sufficient to generate a liquid phase at the solder-substrate interface and thus to achieve bonding. Figure 2 shows the results of the non-destructive CT tests for the upper solder layer of the different substrate types. Due to the small thickness of the solder layer, the CT slices in Figure 2 also contain parts of the reactive foil. For all substrates, a considerable number of pores within the solder layer can be observed, as well as cracks within the reactive foil layer, which are partly filled by solder material. The cracks within the reactive foil layer are due to the pronounced volume reduction associated with the reaction of Ni and Al to NiAl, causing lateral shrinkage of the foil and formation of cracks.