**3.2 Analysis of Cu-BiIn25Sn18 joint**

**Figure 17** shows the interface of BiIn25Sn18-Cu joint. In the interface of BiIn25Sn18 solder and copper, a noticeable increase in the proportion of the darker phase, formed mostly of Sn, may be seen. A continuous transition zone of reaction elements is formed in the solder interface. Based on the study of binary diagrams and performed analyses, formation of the following phases is supposed: Cu3Sn, Cu6Sn5 and Cu9In4. A thinner, non-wettable phase rich in Cu (Cu3Sn), which is followed with Cu6Sn5 phase, is formed in the joint interface. From the map of quantitative proportion of chemical elements, it is obvious that in the copper-solder interface mainly, the presence of In and Sn is exerted. However, Bi does not play any significant role in bond formation and does not create any phases with copper.
