**2.15. Analysis of Sn-Ag-Ti/Si-soldered joint**

The Sn3.5Ag4Ti (Ce, Ga) solder was used for soldering. Soldered joints were fabricated with the application of mechanical activation by power ultrasound. Heating was realized by a hot-plate method. The soldering temperature was 280°C. The dwell time at soldering temperature was 30 s and the time of ultrasound acting was 5 s. The test specimens were prepared

The microstructure of solder type Sn-Ag-Ti is documented in **Figure 26**. It consists of tin matrix. The tin matrix contains unevenly distributed constituents of intermetallic Ti-Sn

Sn phase was proved by X-ray diffractometer (XRD) analysis.

phases depends on manufacturing the temperature of the solder, the amount of titanium

**O3**

Sn5 phase was mostly represented. The formation of individual titanium

Sn, uniformly distributed along the tin grains. The

ceramics is documented in **Figure 27**. A pronounced

Sn5

O3

ceramics. An

and Ti<sup>2</sup>

Sn3

O3 ceramics, silicon as non-metallic material and Cu as metallic material.

XRD analysis revealed also the Ti-Sn phases. Actually identified were the Ti<sup>6</sup>

added to solder and also the way of Ti addition to solder during its manufacture.

O3

material, where a reaction layer is formed, which ensures the wettability of Al2

**Figure 26.** Microstructure of Sn-Ag-Ti solder (a) in polished condition, (b) after etching of tin matrix.

transition zone, reaction layer with an average thickness of 2.6 μm, is formed in the interface.

The energy-dispersive X-ray spectroscopy (EDX) analysis of chemical composition has revealed that the reaction layer (**Figure 28**) contains 5.35 wt.% Al; 37.33 wt.% Ti; 2.84 wt.% Ag and 54.48 wt.% Sn. The linear course of concentration of individual elements is documented

During soldering process, the titanium from solder is distributed to the interface with ceramic

oxidation-reduction reaction takes place between the active solder and ceramic material at the formation of reaction products, which allow the wetting of ceramics by an active solder (**Figure 28**).

of Al<sup>2</sup>

presence of Ag3

84 Recent Progress in Soldering Materials

phases, where Ti<sup>6</sup>

in **Figure 26**.

phases and fine needles of silver phase, Ag<sup>3</sup>

**2.14. Analysis of soldered joint of Sn-Ag-Ti/Al<sup>2</sup>**

The microstructure of Sn-Ag-Ti solder/Al2

The interface of Si/Sn-Ag-Ti solder joint can be seen in **Figure 29**. The Ag3 Sn phase is segregated along the grain boundaries of tin matrix of the solder. This phase increases the strength of soldering alloy type Sn-Ag-Ti. Also, the formation of a pronounced reaction layer, 1–2 μm in thickness, may be seen in **Figure 29**. The formation of this layer in the consequence of interactions is between the active element (Ti) and the surface of silicon substrate. Titanium is segregated to interface with silicon during the soldering process. The products of interaction ensure the wetting of silicon and bond formation. The diffusion mechanism takes place there. The ultrasound exerts two effects in this case, namely it speeds up the Ti diffusion and disrupts the substrate surface by the cavitation erosion.

**Figure 28.** Interface of the soldered joint of Al2 O3 /Sn-Ag-Ti solder.

The formation and composition of reaction layer may be well observed on the planar distribution of elements attained by EDX analysis. **Figure 30** shows the map of elements in the interface of soldered joint. In **Figure 30**, we may also see the segregation of Ti on Si/Sn-Ag-Ti interface; 28 wt. % Ti was observed in the reaction zone, while the balance consisted of silicon.

#### **2.16. Analysis of Sn-Ag-Ti/Cu-soldered joint**

In case of solder type Sn-Ag-Ti, the main role in bond formation with Cu substrate is played by tin. The Cu3Sn, Cu<sup>6</sup> Sn5 phases were identified in the solder/Cu substrate interface in all cases, which are growing in the direction from the phase interface to solder matrix. The Cu3Sn phase is closer to Cu substrate and the Cu6 Sn5 phase is in contact with Sn-Ag-Ti solder.

**Figure 29.** Reaction layer on Si/Sn-Ag-Ti solder interface.

**Figure 30.** Map of Si, Ag, Sn and Ti elements in the interface of Si/Sn-Ag-Ti joint.

Massive transition phases of Cu<sup>6</sup> Sn5 were formed, up to 12 μm in width. The Cu<sup>6</sup> Sn5 phase has an elongated acicular shape as shown in **Figure 31**.
