**1. Introduction**

#### **1.1 Soldering with active solders**

The term 'active solders' is occurring in the technologies oriented to fabrication of combined joints already for several decades. These solders contain an active element which reacts with the surface of the parent material during soldering process. This reaction takes place owing to the fact that an active element exerts higher affinity to the elements in the chemical composition of the substrate. The role of an active element is to ensure a good wetting of substrate with solder by reactive decomposition of the surface layer of the parent metal and by reducing the interfacial stress in the ceramics-solder interface. The active solder then may be used for fabrication of joints with different, either ceramic or metal, substrates. The most used active metals are titanium, zirconium or hafnium [1]. The chemical bonds in the interface of an active solder (with titanium content) and a solid substrate (ceramics/metal) are shown in **Figure 1**. Concentration of an active element should be sufficiently high in order to cause the wetting on a ceramic substrate, while it must not cause the formation of brittle intermetallic

#### **Figure 1.**

*Chemical bonds in the interface of (a) solder-ceramics, (b) solder-metal.*

phases. Though a good wetting is achieved at higher content of active elements in a soldering alloy, nevertheless the joint then exerts poorer mechanical properties [2]. The commercially used active solders usually contain relatively small amounts of active elements (not more than 4 wt. %). Addition of indium to solder supports the wetting of substrate and lowers the soldering temperature and thus also the thermal expansivity of the fabricated joint [3]. The additional elements with higher affinity to oxygen (cerium and lanthanum) may protect the active solders against excessive oxidation of an active element during soldering in the air. At the same time also, the elements supporting wetting, for example, gallium [4, 5], are added to active solders.

The active element is moved from the entire solder volume to both soldered materials during the soldering process. Thus, a reaction layer in the thickness of several μm, containing the reaction products of an active element and substrate, is formed in the interface of soldered joint. The thickness of this reaction layer depends on the solder type and soldering conditions. The active element with a high affinity to oxygen, which reacts with the ceramics during soldering, creates the bonds in an interatomic level. The active element (e.g. Ti) contained in the solder bonds the oxygen from the surface layers of oxide ceramics. The following oxide types are formed on the ceramics by the reaction of an active element: TiO, Ti2O3, Ti3O5, Ti4O5 and TiO2. The reaction product alters the surface energy of ceramics and enables wetting of the solder [6]. The following type of reaction takes place between the active element and Al2O3 ceramics [7].

$$\text{M/AO}\_x \rightarrow \text{ M/mMA}\_a / \text{nM}\_b \text{A}\_c \text{O}\_d / \text{AO}\_x$$

where

M metal element of the solder/interface.


This type of chemical reaction is valid for the active soldering with an active element as Ti, Zr and/or Hf. The layer formed after such a reaction will depend on the soldering parameters (temperature and time) and environment atmosphere (vacuum oven and inert gas). For the active element Ti on Al2O3 substrate, it is valid [7]:

$$\text{Ti/Al}\_2\text{O}\_3 \rightarrow \text{Ti/Ti}\_3\text{Al/TiAlO}\_x\text{/Al}\_2\text{O}\_3$$

In the selection of an active element for soldering ceramic materials, it is necessary to take into account the fact that higher concentration of an active metal may result in joint embrittlement. This is caused by the formation of brittleness phases in the ceramic-solder zone [8].

**27**

*Soldering by the Active Lead-Free Tin and Bismuth-Based Solders*

The use of Ti as an active element in a solder has been investigated by many authors. Titanium in the SnAgTi4 solder provided a reduction in the wetting angle to the sapphire substrate. The absorption of Ti together with the release of Al from the sapphire substrate provides an interaction between the solder and the substrate [9]. Another investigated aspect of the Sn-Ag-Ti-based solder was its spreadability on porous graphite when applying ultrasonic vibrations. The authors' results [10, 11] have shown that the application of ultrasonic waves during soldering allows the active solder to spread on the surface of the graphite at a relatively low temperature under atmospheric conditions. The oxide layer on the molten solder forms a compact layer of a certain thickness which prevents the liquid solder from wetting the base material. The reduction of Ti oxide formation is possible by the addition of Y, as documented by the authors [12]. They found that the oxide layer of the solder consists mainly of titanium dioxide. A small amount of Y improves the resistance to oxidation because it suppresses the oxidation of Ti in the molten solder and reduces the amount of oxygen atoms entering the molten solder. The composition of the surface oxide of SnAg4Ti2Y0.5 solder was mainly Y2O3 and a low amount of titanium dioxide. After ultrasonic application, the oxide layer was disrupted, and the solder was able to wet the materials. The authors found that the spreadability and wettability of Sn-Ag-Ti-based solders on the graphite substrate could be improved by increasing the time of ultrasound activation. Similar observations were published in [13], where the authors deal with the soldering of aluminium-graphite composite material using a SnAg3.5Ti4Cu0.5 solder at soldering temperature of 250°C. Aluminium from the composite was dissolved in the active solder and formed a solid solution of Al-Ag-Sn at the interface. The average shear strength

The main factors affecting the solder selection comprise different melting points of soldered materials, different surface stresses of the substrates and the residual stresses formed during solidification process. Therefore such a solder is proposed where the matrix exerts a sufficient plasticity reserve, capable to compensate the residual stresses formed in the joint [14]. To overcome the mentioned problems, the solder meeting a complex set of desired criteria for the quality of resultant joint is selected. The active solders, similarly as the commercial solders, are classified by the melting temperature to solders, brazing alloys and high-temperature solders. The active solders are further classified by the mechanism of bond formation to high-temperature and mechanically activated ones. For reaction capacity of an active element, the soldering temperature in the case of high-temperature-activated solders must be higher than 780°C. The classification of active solders and their

The primary materials of an active solder are usually tin, lead, bismuth, zinc or indium and the alloys based on these metals. Active solders allow to join also unusual combinations of metallic materials (e.g. CrNi steel, Mo, W, Ti, Cr, etc.) and non-metallic materials as siliceous glass, sapphire, carbon, silicon and several types of ceramics. In soldering with active solders, the solder is capable to compensate the stresses formed due to different thermal expansivity of joined materials by its plastic strain, shear mechanism or yield. In such a manner, the highest reduction of residual stresses may be achieved at a preserved joint stability. In the case of soldering untraditional material combinations with extremely different coefficients of thermal expansivity (e.g. glass with aluminium/copper), heavier solder thickness should be selected, in order to prevent the cracks in the joint interface. Such joints are used mainly in electrotechnics, where lower strength and thermal resistance of joint are sufficient. These solders also allow to fabricate the

*DOI: http://dx.doi.org/10.5772/intechopen.81169*

of the Al-Gr/Al-Gr joints was 8.15 MPa.

chemical composition is shown in **Table 1**.

**1.2 Active solders**

#### *Soldering by the Active Lead-Free Tin and Bismuth-Based Solders DOI: http://dx.doi.org/10.5772/intechopen.81169*

*Lead Free Solders*

**Figure 1.**

phases. Though a good wetting is achieved at higher content of active elements in a soldering alloy, nevertheless the joint then exerts poorer mechanical properties [2]. The commercially used active solders usually contain relatively small amounts of active elements (not more than 4 wt. %). Addition of indium to solder supports the wetting of substrate and lowers the soldering temperature and thus also the thermal expansivity of the fabricated joint [3]. The additional elements with higher affinity to oxygen (cerium and lanthanum) may protect the active solders against excessive oxidation of an active element during soldering in the air. At the same time also, the elements supporting wetting, for example,

The active element is moved from the entire solder volume to both soldered materials during the soldering process. Thus, a reaction layer in the thickness of several μm, containing the reaction products of an active element and substrate, is formed in the interface of soldered joint. The thickness of this reaction layer depends on the solder type and soldering conditions. The active element with a high affinity to oxygen, which reacts with the ceramics during soldering, creates the bonds in an interatomic level. The active element (e.g. Ti) contained in the solder bonds the oxygen from the surface layers of oxide ceramics. The following oxide types are formed on the ceramics by the reaction of an active element: TiO, Ti2O3, Ti3O5, Ti4O5 and TiO2. The reaction product alters the surface energy of ceramics and enables wetting of the solder [6]. The following type of reaction takes place

This type of chemical reaction is valid for the active soldering with an active element as Ti, Zr and/or Hf. The layer formed after such a reaction will depend on the soldering parameters (temperature and time) and environment atmosphere (vacuum oven and inert gas). For the active element Ti on Al2O3 substrate, it is

In the selection of an active element for soldering ceramic materials, it is necessary to take into account the fact that higher concentration of an active metal may result in joint embrittlement. This is caused by the formation of brittleness phases

gallium [4, 5], are added to active solders.

*Chemical bonds in the interface of (a) solder-ceramics, (b) solder-metal.*

between the active element and Al2O3 ceramics [7].

M metal element of the solder/interface.

M different intermetallic compounds

M/AOx → M/m.MAa/n.MbAcOd/AOx

Ti/Al2O3 → Ti/Ti3Al/TiAlOx/Al2O3

**26**

where

valid [7]:

AOx ceramic element O may be an oxide

in the ceramic-solder zone [8].

n different combined oxides

The use of Ti as an active element in a solder has been investigated by many authors. Titanium in the SnAgTi4 solder provided a reduction in the wetting angle to the sapphire substrate. The absorption of Ti together with the release of Al from the sapphire substrate provides an interaction between the solder and the substrate [9]. Another investigated aspect of the Sn-Ag-Ti-based solder was its spreadability on porous graphite when applying ultrasonic vibrations. The authors' results [10, 11] have shown that the application of ultrasonic waves during soldering allows the active solder to spread on the surface of the graphite at a relatively low temperature under atmospheric conditions. The oxide layer on the molten solder forms a compact layer of a certain thickness which prevents the liquid solder from wetting the base material. The reduction of Ti oxide formation is possible by the addition of Y, as documented by the authors [12]. They found that the oxide layer of the solder consists mainly of titanium dioxide. A small amount of Y improves the resistance to oxidation because it suppresses the oxidation of Ti in the molten solder and reduces the amount of oxygen atoms entering the molten solder. The composition of the surface oxide of SnAg4Ti2Y0.5 solder was mainly Y2O3 and a low amount of titanium dioxide. After ultrasonic application, the oxide layer was disrupted, and the solder was able to wet the materials. The authors found that the spreadability and wettability of Sn-Ag-Ti-based solders on the graphite substrate could be improved by increasing the time of ultrasound activation. Similar observations were published in [13], where the authors deal with the soldering of aluminium-graphite composite material using a SnAg3.5Ti4Cu0.5 solder at soldering temperature of 250°C. Aluminium from the composite was dissolved in the active solder and formed a solid solution of Al-Ag-Sn at the interface. The average shear strength of the Al-Gr/Al-Gr joints was 8.15 MPa.

The main factors affecting the solder selection comprise different melting points of soldered materials, different surface stresses of the substrates and the residual stresses formed during solidification process. Therefore such a solder is proposed where the matrix exerts a sufficient plasticity reserve, capable to compensate the residual stresses formed in the joint [14]. To overcome the mentioned problems, the solder meeting a complex set of desired criteria for the quality of resultant joint is selected. The active solders, similarly as the commercial solders, are classified by the melting temperature to solders, brazing alloys and high-temperature solders. The active solders are further classified by the mechanism of bond formation to high-temperature and mechanically activated ones. For reaction capacity of an active element, the soldering temperature in the case of high-temperature-activated solders must be higher than 780°C. The classification of active solders and their chemical composition is shown in **Table 1**.
