## 4 Conclusions  
* The influence of the thermal conductivity on the evolution of the joint microstructure upon reactive joining was systematically analysed for four substrate materials with thermal conductivities ranging from very low to very high (i.e., borosilicate glass, Al\({}_{2}\)O\({}_{3}\), Si and Cu). * Pronounced differences in formation of intermetallic phases, their amount and their spatial distribution within the joining zone, as well as in the spatial distribution of porosity were observed, reflecting the specific time-temperature-profiles developing for the different substrates upon reactive joining. * The differences in the joint microstructure could be directly related to the performance upon mechanical testing. Optimum mechanical performance was found for a homogeneous distribution of intermetallics within the soldering zone and formation of moderate amounts of intermetallics along the metallisation-solder interface, as obtained in case of Al\({}_{2}\)O\({}_{3}\). * Detailed understanding of the interplay between the time-temperature-profile on the one hand, defined by the properties of the RF and the thermal properties of the joining components, and the joining system of solder, surface metallisations and additional coatings for in situ alloying of the solder on the other hand, allows tailoring of an optimum microstructure of the joint. Conceptualization, B.R., I.S., A.S., S.K., L.P.H.J. and J.J.-R.; Investigation, B.R., I.S., A.S. and R.F. ; Writing--Original Draft Preparation, B.R. ; Writing--Review and Editing, B.R., I.S., A.S., R.F., S.K., L.P.H.J. and J.J.-R. Project Administration, S.K. and J.J.-R.; Funding Acquisition, S.K., L.P.H.J. and J.J.-R. This work was supported by the Swiss National Science Foundation, the European Union and the European Regional Development Fund within the Interreg \"Alpenrhein Bodensee Hochrhein\" program, under Grant ABH028 \"Schonendes reaktives Fugen von Mikrosystemen\". [PERSON] (Empa) is gratefully acknowledged for preparation of the cross-sections. The authors declare no conflict of interest. Figure 5: Shear strength measured for the different substrate materials represented as Tukey-type box plots. (*) For borosilicate glass (BS) and Si, the shear strength is defined by the failure of the substrate and not of the bond; red: median value, blue: lower (25%) and upper (75%) quartile; whiskers: maximum 1.5 inter-quartile range, red cross: outlier. * [10] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON] Reactive Joining of Thermally and Mechanically Sensitive Materials. _J. Electron. Packag._**2018**, _140_, 041006. [CrossRef]
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