## 3 Results and Discussion
### Joining with Activating Materials
#### 3.2.2 Joining with Sn as Activating Material
Figure 18: SEM image of the interface between the Q&P steel and the joining seam (activating material: Sn; joining temperature: 237 \({}^{\circ}\)C, holding time: 5 min). Therefore, in further tests, Sn PVD coatings of the Q&P substrates were examined, because via PVD significantly thinner Sn layers can be deposited. Sn layer thicknesses of 3 \(\upmu\)m, 10 \(\upmu\)m, and 20 \(\upmu\)m were tested with a constant nanopaste application of 20 \(\upmu\)m (Table 5). During the experiments, a further advantage was recognized when using the PVD layers. The nanopaste can be applied more easily to the rigid coatings than to the flexible foils. Indeed, several interesting phenomena, which cannot be fully explained yet, were observed when using the PVD coatings. Whisker formation in the joining seams was often observed (Figures 19 and 20). Such whisker growth is known from the use of lead-free Sn solders with copper substrates [28]. It is assumed that in particular external or internal stresses lead to the formation of whiskers. Often, intermetallic phases (IMC), especially Cu\({}_{6}\)Sn\({}_{5}\), were identified as starting point for the formation of whiskers [28], but also oxide layers on the surface can initiate whisker growth [29]. This whisker formation and growth is not completely understood and discussed controversially in the literature [28]. Also, in the present work, it could not be clarified why whisker formation suddenly starts when using the PVD Sn layers, because the whisker formation was not observed when using Sn foils with the same thickness of 20 \(\upmu\)m. Intermetallic phases do not seem to be the starting point for whisker growth, since neither Ag\({}_{3}\)Sn nor FeSn\({}_{2}\) are formed when using the PVD Sn layers (Figure 19). In contrast, IMC were observed with the same holding time (5 min) and the same Sn thickness (20 \(\upmu\)m) when using Sn foils (Figure 17). Moreover, when using PVD Sn coatings, the joining seam exhibits a completely different microstructure of the matrix compared to the microstructure when using foils: With the same parameters (T = 237 \({}^{\circ}\)C, t = 5 min, Sn thickness = 20 \(\upmu\)m), no eutectic structure formed when using the PVD coating (Figure 17 vs. Figure 19). In addition, the Ag was not used for the formation of Ag\({}_{3}\)Sn, as it was observed when using the foils. Instead, sintered Ag particles were distributed within the joining seam (Figure 21). A gap remained in the middle of the joining seam, where the Ag nanopaste was applied (Figure 19). The Sn layers did not melt together sufficiently, which was likely to have a negative effect on the mechanical properties of these joints. A solution to this issue would be the application of light pressure during the joining process, or a slightly increased joining temperature. The differences in the formation of the microstructure--no eutectic, no IMC--when using PVD coatings compared to the use of foils with otherwise identical process parameters cannot be explained yet and requires further in-depth studies. In principle, the changed formation of the microstructure can be considered as positive, since Ag is not used up for IMC formation, and therefore, Ag is available in the joining seam. The variations of the Sn layer thickness show that a 10-\(\upmu\)m layer is still too thick, since the proportion of Ag in the joining seam is still lower compared to Sn (Figure 21). In contrast, with a coating thickness of 3 \(\upmu\)m, the joining seam exhibits a high porosity, Figure 22. The proportion of Sn does not seem to be sufficient to fill the pores that are inevitably formed when using the nanopaste. Therefore, an optimum thickness of the Sn layer should be determined in further experiments. This means, on the one hand, that the layer should be thin enough so that Ag dominates the properties of the joining seam, and on the other hand, a sufficient amount of Sn should be available to fill the pores in the joining seam. In addition, the interface between joining seam and substrate must be optimized in further tests.