## 3 Results and Discussion
### Joining with Activating Materials
#### 3.2.2 Joining with Sn as Activating Material  
Due to the formation of brittle phases when using the solders, the use of pure Sn as activating material was tested in further experiments, especially to prevent formation of Ag\({}_{3}\)Sn. In addition, the thickness of nanopate application and the holding time were varied in these tests. When varying the thickness of paste application, the aim was to provide as much Ag as possible in the joining seam compared to Sn, so that Ag dominates the properties of the joining seam. A paste application of 40 \(\upmu\)m and 80 \(\upmu\)m in combination with a Sn foil thickness of 20 \(\upmu\)m was tested. The purpose of varying the holding time was to find an optimum time to minimize the formation of the brittle phases FeSn\({}_{2}\). Simultaneously, the aim was to achieve a good bonding as well as diffusion between joining seam and substrate even without flux. Holding times of 3 min and 5 min were investigated. Due to the use of pure Sn, the joining temperature was increased to 237 \({}^{\circ}\)C (compared to 230 \({}^{\circ}\)C when using the solders), Table 4.  
Figure 17 shows an SEM image of the joining seam with a holding time of 5 min and a paste application of 40 \(\upmu\)m. In comparison to a holding time of 30 min (Figure 8), in which the FeSn\({}_{2}\) phases are distributed over the entire joining seam, these phases are only formed at the edges of the joining seam when using a holding time of 5 min. In addition, a significantly smaller number of FeSn  
Figure 16: SEM image of the Q&P steel substrate after joining with SnAg4 at 230 \({}^{\circ}\)C with a holding time of 10 min. Figure 15: SEM image of the interface between the Q&P steel and the joining seam (activating material: SnAg5, with flux 2.2.2; joining temperature: 230 \({}^{\circ}\)C, holding time: 10 min). precipitates are formed compared to a holding time of 10 min (Figures 9 and 10). However, as can be seen in Figure 17, despite the use of a pure Sn foil, Ag\({}_{3}\)Sn is formed in the joining seam. It must be assumed that these phases are formed extremely quickly due to the presence of Ag from the nanopaste. Furthermore, the matrix consists of a Sn-Ag eutectic (Sn:Ag 94:6 at%), Figure 17. Thus, Ag from the nanopaste is \"used up\" for the formation of Ag\({}_{3}\)Sn and the eutectic microstructure. Consequently, no high Ag content was available in the joining seam, which was actually expected as an important factor to obtain good mechanical properties for the joint. Figure 18 shows the interface between joining seam and substrate with a holding time of 5 min. Compared to the microstructure after a holding time of 10 min (Figures 14 and 15), the diffusion zone with a depth of up to 0.5 \(\upmu\)m was significantly thinner. Further work on the effect of the thinner diffusion zone on the mechanical properties is currently under way. With a paste application of 80 \(\upmu\)m, an increased porosity was observed in the joining seam, which was a result of the thick paste application. A thick application of nanopaste inevitably leads to an increased evaporation of organic substances, and therefore to a higher porosity, which was already observed previously [24]. In this work, an optimum paste application of 20 \(\upmu\)m was determined, in which the porosity was \"acceptable\", because it was low and finely distributed. The application of 20-\(\upmu\)m paste was already tested with the SnAg solders but was not suitable due to the lack of options for rolling foils with a thickness of \(<20\)\(\upmu\)m, which is a requirement for the properties of Ag to dominate the joining seam. Accordingly, the goal of further work was to apply thinner Sn layers to the substrates, which would also allow a thinner application of the Ag nanopaste. Figure 17: SEM image of a joint with Sn as activating material and a thickness of Ag nanopaste application of 40 \(\upmu\)m (joining temperature: 237 \({}^{\circ}\)C, holding time: 5 min).