## 3 Results and Discussion
Cross-sectional analysis of the Sn solder foils (respectively, the Sn solder plating in case of Cu) by SEM showed a negligible amount of initial porosity in the solder before joining, i.e., the porosity  
predominantly originates from air enclosed within the joining setup during assembly. Although porosity is present for all substrate materials, CT shows a clear difference in the distribution of the porosity within the solder layer, resulting from the different time-temperature-profiles experienced by the joint: in case of borosilicate glass, large agglomerates of pores can be observed, while for Al\({}_{2}\)O\({}_{3}\), Si and Cu, a much more homogeneous distribution of pores is present. Since the thermal conductivity defines not only the maximum temperature which is reached within the joining zone during the reaction, but also the period during which the solder is in the liquid state [11], the solder remains liquid for a comparatively long period in case of borosilicate glass. This allows for pronounced pore condensation, i.e., coarsening of the porosity (Figure 2a). Due to a comparatively much larger thermal conductivity in case of Al\({}_{2}\)O\({}_{3}\) and Si, the solder is for a shorter time in the liquid state. As a consequence, the pore distribution is much finer (Figure 2b,c). For Cu, the high thermal conductivity leads to rapid re-solidification of the Sn solder, and almost no coarsening of the pores can take place, leading to the very fine distribution of porosity found in the centre of the bonding area with CT (Figure 2d). The cross-sections of the joints prepared for the four different substrate types (Figure 3) show the typical features of solder joints prepared with reactive Ni-Al-foils: the reactive foil layer is embedded within the two solder layers and occasionally shows some cracks partly filled with solder material. Notably, the solder layers for borosilicate glass, Al\({}_{2}\)O\({}_{3}\) and Si are much thinner than expected from the initial thickness of the Sn foil of 10 \(\upmu\)m (also see Figure 4). This shrinkage can be partly attributed to the filling of the cracks in the reactive foil layer (For some applications, the necessity to fill the cracks sets a lower limit for the thickness of the solder layer: if thinner solder layers are used, the cracks can either not be filled, which may lead to problems if e.g., a hermetic joint is required, or the remaining solder layer becomes too thin, which may lead to problems in case of components with high surface roughness and waviness.). However, the here observed shrinkage can mainly be attributed to the pronounced lateral outflow of liquid Sn out of the joining zone during the bonding step, driven by the application of the uniaxial pressure (also see [23]). Note that this practical issue prevents modelling of the actual time-temperature-profiles as e.g., performed in [11]: the observed lateral outflow of liquid solder leads to an undefined heat loss which makes correct modelling of the heat flow and of the corresponding time-temperature-profile impossible. For Cu, almost no shrinkage of the solder layers was observed (especially for the lower solder layer, which is in direct contact with the larger substrate acting as a large heat sink). This indicates that, during reactive joining, the Sn layers were likely not liquefied completely throughout the whole bonding area, thus preventing lateral outflow of liquid Sn under pressure. With a speed of the reaction front of approximately 8 m s\({}^{-1}\) for the commercial Ni(V)-Al RFs used here (determined with a high-speed camera), the total reaction time from one side of the 4 mm \(\times\) 4 mm bonding area to the other is only about 0.5 ms (also see [23]). Hence, in case of Cu, the solder layer was in the liquid state for less than 0.5 ms, which corresponds reasonably well to model predictions for a similar scenario [11].