**3.1 Analysis of Al2O3-BiIn25Sn18 joint**

**Figure 16** shows a combined soldered joint of BiIn25Sn18-Al2O3 ceramics. A longitudinal crack was observed in the interface of BiIn25Sn18-Al2O3 joint, which has occurred along the entire specimen length. This crack was formed during the metallographic preparation of specimen due to poor plastic-elastic properties of the solder used.

**Figure 16.** *Interface of Al2O3-BiIn25Sn18 joint (SEM) + concentration profiles of elements.*

**Figure 17.** *Interface of Cu-BiIn25Sn18 joint (SEM) + concentration profiles of elements.*

Based on the performed SEM and EDX analyses, it can be supposed that In primarily contributes to bond formation by creating indium oxide—In2O3. Besides In, Bi also partially contributes to bond formation. The presence of Sn is indifferent, and it does not exert any effect on the bond formation. Its presence was revealed in the grey phase. The interface contains exclusively a pale matrix composed of Bi and In. The formed reaction products are brittle with poor toughness that caused the crack formation in the bond plane.
