**2. Active solders**

Both brazing and soldering are joining processes that use the principle of capillary action to distribute a molten filler metal between the surfaces of base materials [5]. In all cases, filler metal melting temperatures are below the melting temperatures of the base materials. Brazing filler metals melt completely at temperatures above 450°C, while soldering filler metals melt below that temperature [5]. A variety of alloys are used as filler metals for brazing, depending on the workpieces and the intended use or application method. In general, braze alloys are made up of Cu, Ag, Ni, or precious metals [6]. Low melting point metals such as Pb, Sn, Zn, Sb, and In are usually used for soldering filler [7]. Filler metals can be divided into three types according to their melting points, namely, high melting point, medium melting point, and low melting point fillers, as shown in **Figure 1** [8].

Chemical bonding at the interfaces of the filler metal and base materials is evaluated by wettability, defined as the ability of the molten filler metal to spread uniformly onto the surface of a base material. The molten filler metals must be able to be wetted on the surface of the base materials during the joining process, whether it is brazing or soldering. Ceramics and some materials, which easily form an oxide passivation layer by natural oxidation, such as aluminum alloys, magnesium alloys, titanium alloys and stainless steels, have surfaces of extreme physical and chemical stability that prevent filler metals from wetting the surface.

One critical parameter of wettability is the contact angle between the drop of molten filler and the wetting surface, as shown in **Figure 2a** and **2b** [2]. The contact angle can be calculated by Young's equation [9]:

$$\cos \Theta = \frac{\mathbf{\gamma}\_{\rm SV} - \mathbf{\gamma}\_{\rm SL}}{\mathbf{\gamma}\_{\rm LV}} \tag{1}$$

**49**

**Figure 1.**

*Active Solders and Active Soldering*

*DOI: http://dx.doi.org/10.5772/intechopen.82382*

To obtain better bonding integrity, difficult-to-wet materials, especially ceramics, are usually metallized prior to brazing or soldering. To overcome the non-wetting of common filler metals on the surfaces of difficult-to-wet materials, pre-metallization by molybdenum-manganese method, electroless plating, physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spraying, or ion implantation can be used to increase the wettability [10]. The indirect joining process includes two steps and is costly; thus, it is difficult to implement. Recently, direct active brazing has been developed to join difficult-to-wet materials such as ceramic and graphite. The direct brazing process, without the need for pre-metallizing, is simpler than the indirect brazing. Active brazing fillers that include an active element, such as Ti, Zr, Ta, Nb, or Hf, are used to promote wetting [11–14]. It is believed that the addition of an active element to filler metals can effectively improve the wettability of difficultto-wet materials by reducing the solid-liquid interfacial free energy and allowing chemical reactions in the interfaces between filler metals and substrates [15]. Due to its high chemical activity, titanium is often chosen as the active element in filler metals to improve the wetting on ceramic surfaces. For example, Ag–Cu–Ti active filler has been used widely to join ceramics because of its wettability and good bond strength [16–20]. However, the active filler metals for ceramic brazing have a high

*The melting ranges of some typical solder and braze materials [8].*

where θ is the contact angle, γSV is the solid-vapor interfacial energy, γLV is the liquid-vapor interfacial energy, and γSL is the solid-liquid interfacial energy.

If the solid-vapor interfacial energy (γSV) is higher than the solid-liquid interfacial energy (γSL), the right side of Young's equation will be positive, so cosθ must be positive and the contact angle will be less than 90° [9]. Small contact angles correspond to high wettability, as shown in **Figure 2a**. A contact angle of less than 90° indicates that wetting of the surface is favorable, and most strong chemical bonds can be formed at the interface. To improve the wettability of the filler, it is necessary to reduce the surface tension between the bonding material and the filler, usually by forming a chemical reaction between the filler and the bonded material surface, as shown in **Figure 3** [9].

*Active Solders and Active Soldering DOI: http://dx.doi.org/10.5772/intechopen.82382*

*Fillers - Synthesis, Characterization and Industrial Application*

during the joining process [4].

**2. Active solders**

metal [2]. In lightweight construction, the bonding of difficult-to-wet materials such as aluminum alloys, magnesium alloys, or titanium alloys, which easily forms an oxide passivation layer by natural oxidation, entails numerous difficulties. Brazing and soldering with fillers as interlayers are considered to be more feasible ways to bond dissimilar materials such as ceramics and difficult-to-wet metals. However, because ceramics provide mostly covalent or ionic bonding, have a very stable electron configuration, and are chemically inert, most brazing or soldering fillers cannot be wetted on their surfaces [3]. Thus, the filler metal is the most important key factor in determining wettability. Direct bonding has been developed to simplify wetting of difficult-to-wet materials by using active fillers containing active elements that improve the wettability of the filler on the difficult-to-wet material surface and eliminate the need for pre-metallization

Both brazing and soldering are joining processes that use the principle of capillary action to distribute a molten filler metal between the surfaces of base materials [5]. In all cases, filler metal melting temperatures are below the melting temperatures of the base materials. Brazing filler metals melt completely at temperatures above 450°C, while soldering filler metals melt below that temperature [5]. A variety of alloys are used as filler metals for brazing, depending on the workpieces and the intended use or application method. In general, braze alloys are made up of Cu, Ag, Ni, or precious metals [6]. Low melting point metals such as Pb, Sn, Zn, Sb, and In are usually used for soldering filler [7]. Filler metals can be divided into three types according to their melting points, namely, high melting point, medium

melting point, and low melting point fillers, as shown in **Figure 1** [8].

stability that prevent filler metals from wetting the surface.

angle can be calculated by Young's equation [9]:

cosθ = \_\_\_\_\_\_\_\_\_\_\_\_

Chemical bonding at the interfaces of the filler metal and base materials is evaluated by wettability, defined as the ability of the molten filler metal to spread uniformly onto the surface of a base material. The molten filler metals must be able to be wetted on the surface of the base materials during the joining process, whether it is brazing or soldering. Ceramics and some materials, which easily form an oxide passivation layer by natural oxidation, such as aluminum alloys, magnesium alloys, titanium alloys and stainless steels, have surfaces of extreme physical and chemical

One critical parameter of wettability is the contact angle between the drop of molten filler and the wetting surface, as shown in **Figure 2a** and **2b** [2]. The contact

where θ is the contact angle, γSV is the solid-vapor interfacial energy, γLV is the

If the solid-vapor interfacial energy (γSV) is higher than the solid-liquid interfacial energy (γSL), the right side of Young's equation will be positive, so cosθ must be positive and the contact angle will be less than 90° [9]. Small contact angles correspond to high wettability, as shown in **Figure 2a**. A contact angle of less than 90° indicates that wetting of the surface is favorable, and most strong chemical bonds can be formed at the interface. To improve the wettability of the filler, it is necessary to reduce the surface tension between the bonding material and the filler, usually by forming a chemical reaction between the filler and the bonded material surface, as

liquid-vapor interfacial energy, and γSL is the solid-liquid interfacial energy.

γSV − γSL

<sup>γ</sup>LV (1)

**48**

shown in **Figure 3** [9].

**Figure 1.**

To obtain better bonding integrity, difficult-to-wet materials, especially ceramics, are usually metallized prior to brazing or soldering. To overcome the non-wetting of common filler metals on the surfaces of difficult-to-wet materials, pre-metallization by molybdenum-manganese method, electroless plating, physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spraying, or ion implantation can be used to increase the wettability [10]. The indirect joining process includes two steps and is costly; thus, it is difficult to implement. Recently, direct active brazing has been developed to join difficult-to-wet materials such as ceramic and graphite. The direct brazing process, without the need for pre-metallizing, is simpler than the indirect brazing. Active brazing fillers that include an active element, such as Ti, Zr, Ta, Nb, or Hf, are used to promote wetting [11–14]. It is believed that the addition of an active element to filler metals can effectively improve the wettability of difficultto-wet materials by reducing the solid-liquid interfacial free energy and allowing chemical reactions in the interfaces between filler metals and substrates [15]. Due to its high chemical activity, titanium is often chosen as the active element in filler metals to improve the wetting on ceramic surfaces. For example, Ag–Cu–Ti active filler has been used widely to join ceramics because of its wettability and good bond strength [16–20]. However, the active filler metals for ceramic brazing have a high

*The melting ranges of some typical solder and braze materials [8].*

## **Figure 2.**

*Contact angle of sessile drop configurations (a) wetting and (b) nonwetting [2], where θ is the contact angle, γSV is the solid-vapor interfacial energy, γLV is the liquid-vapor interfacial energy, and γSL is the solid-liquid interfacial energy.*

melting point of about 750–850°C. Brazing is conducted at temperatures generally higher than 800°C. For some applications, the brazing temperature is so high that it causes hot cracking or functional degradation of the ceramics. Moreover, due to the difference in the thermal expansion coefficients of the metal and ceramic materials, high residual stress will develop upon cooling from the elevated temperature. Hence, the bonding temperature should be as low as possible to minimize the residual stresses. To solve this problem, low melting point filler metals containing titanium, such as Sn10Ag4Ti and Pb4In4Ti, have been developed, and they exhibit excellent wettability on ceramic substrates at 850°C. Unlike the widely used Ag–Cu-based active filler metals, the low melting point filler metals possess a melting range below 300°C. However, brazing ceramics with low melting point active filler metals is always conducted above 850°C, such as the Ag–Cu–Ti brazing temperature, owing to the decent thermodynamic activation [21–23]. Although the act of brazing with low melting point filler metals must be conducted at elevated temperatures far above

**51**

**Solder** Sn Ag 4 4Ti

Sn Ag 3 Cu 3 4Ti

Sn5In4Ti

Zn Al 6 Ag 6

Zn Ag 4

Zn Ag 4

**Table 1.**

*Active solders developed by Hillen et al. [4].*

4TiNi0.3

—

Bal.

4

—

—

4

—

—

0.3

<0.5

180–220

4TiCr0.7

—

Bal.

4

—

—

4

—

0.7

—

<0.5

220–232

Bal.

Bal.

Bal.

—

Bal.

6

6

—

—

—

—

—

<0.5

140–220

—

—

—

—

4

5

—

—

<0.5

220–232

—

3

—

1

4

—

—

—

<0.5

220–232

—

4

—

—

4

—

—

—

<0.5

220–232

**Sn**

**Zn**

**Ag**

**Al**

**Cu**

**Ti**

**In**

**Cr**

**Ni**

**Ce, Ga**

**Melting range, °C**

*Active Solders and Active Soldering*

*DOI: http://dx.doi.org/10.5772/intechopen.82382*

**Figure 3.** *Schematic of the spreading process of solder with the formation of reaction layer [9].*
