**1.2 Active solders**

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


#### **Table 1.**

*Classification of active solders by the melting point.*

vacuum-tight joints used in the vacuum and cryogenic technology, where indiumbased solder has proved as suitable. The solders containing lead are not suitable for soldering in vacuum, since considerable evaporation of lead and also oven contamination may occur. In the case of soldering with lead, it is necessary to employ the through-flow atmosphere of pure argon, eventually helium [15]. The active solders may be activated mechanically (by scrapping or ultrasound) in dependence on material which they wet (metal/ceramics) within the temperature interval from 200 to 400°C. An essential group of these solders applicable at lower soldering temperatures comprises the tin-based, lead-free solders which are enriched by a small amount of an active element as titanium. An example of such active solders destined for soldering a wide scope of materials is the solders type S-Bond, where also SnAg3.5Ti4(Ce,Ga) solder [16, 17] may belong.

Titanium is moved from the solder matrix to the interface; combines with carbon, nitrogen or oxygen from the ceramic material; and thus creates intermetallic compounds which allow the wetting of ceramic substrate and creation of a metallurgical bond. Fluxless soldering process in the air is concerned, where no corrosion owing to flux remnants occurs [18, 19].

The active solders for high-temperature activation necessitate considerably higher soldering temperature and may be used only in a vacuum and/or shielding atmosphere. Soldering temperature of the tin- and lead-based solders varies within 850–950°C. The solders for mechanical activation (e.g. SnAg3.5Ti4(Ce,Ga)) may be used also for high-temperature activation. However, greater wetting angles than in the case of mechanical activation are attained [14]. As an example of using S-Bond 220-1 (SnAg3.5Ti4(Ce,Ga)) solder in electrotechnics, joining of Al2O3 with copper in **Figure 2** may be mentioned. This solder allows to join the materials with different

**29**

**Figure 3.**

*Scheme of thermal cycle of vacuum soldering [21].*

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

coefficients of thermal conductivity and expansivity (metals, light metals,

The solder type S-Bond contains Ti as an active element and elements from the group of lanthanides. These active elements migrate to the interface of the soldered joint and act upon the material surface by the active disruption and removal of oxides. Disruption of oxide layer, which prevents the formation of contact between the solder and substrate surface, is called as 'activation'. As soon as the oxide layer is disrupted, the solder volume reacts with the substrate surface, and a strong bond

• *Metallurgic bonding*—for example, with Cu and/or Al surfaces. Besides the active element, also other elements of solder, as Sn-Ag-Ti, react with the substrate elements. These contribute to bond formation by creation of phases as Al-Ti, Cu-Sn, Ag-Sn, etc. system. This process may be used for a wide scope of metallic materials.

• *Gravity bond*—in the case of metals with very thin dielectric surface oxides. These are, for example, Ti or stainless steel with TiO2 eventually Cr3O2 oxides. Bonding is ensured by the gravity of surfaces with opposite electric charge. The active elements of solder and the elements of joined substrate are attracted across

In the case of active solders containing Ti (or other reactive elements), the hightemperature activation in shielding atmosphere of vacuum is employed. The active element reacts with the surface oxides of substrates at the soldering temperatures of 850–950°C. Similarly also nitrides, carbides or silicides of the active element are formed in the solder in the case of non-oxide ceramics. One of the greatest disad-

The work cycle in **Figure 3** consists of a rapid heating to soldering temperature, dwell time for about 9 minutes and a long free cooling down (for about 320 minutes) in the oven. The short dwell time at the soldering temperature prevents formation of brittle phases and grain coarsening. The slow cooling down is needed in order to prevent high residual stresses, which may cause the joint cracking. The diffusion processes take place during slow cooling down, which will be exerted in the growth of diffusion zone in the parent metal and the solubility zone in the solder. Soldering with an active solder may be performed in a vacuum furnace at

ceramics, composites with metal matrix, carbides, glass, etc.) [20].

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

with the joined surface is thus formed.

The bond is formed by two mechanisms:

the interface by the van der Waals forces.

*1.2.1 High-temperature activation of an active solder*

vantages of this process consists in the necessity of a vacuum.

**Figure 2.** *Detailed view of Al2O3-Cu joint fabricated with S-Bond solder [20].*

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

*Lead Free Solders*

High-temperature

solders

**Table 1.**

**28**

**Figure 2.**

*Detailed view of Al2O3-Cu joint fabricated with S-Bond solder [20].*

also SnAg3.5Ti4(Ce,Ga) solder [16, 17] may belong.

Solders From cryogenic temperatures

*Classification of active solders by the melting point.*

owing to flux remnants occurs [18, 19].

vacuum-tight joints used in the vacuum and cryogenic technology, where indiumbased solder has proved as suitable. The solders containing lead are not suitable for soldering in vacuum, since considerable evaporation of lead and also oven contamination may occur. In the case of soldering with lead, it is necessary to employ the through-flow atmosphere of pure argon, eventually helium [15]. The active solders may be activated mechanically (by scrapping or ultrasound) in dependence on material which they wet (metal/ceramics) within the temperature interval from 200 to 400°C. An essential group of these solders applicable at lower soldering temperatures comprises the tin-based, lead-free solders which are enriched by a small amount of an active element as titanium. An example of such active solders destined for soldering a wide scope of materials is the solders type S-Bond, where

Brazing alloys Up to 400°C Based on Ag, Cu and Au (e.g. Ag72CuTi1.5)

Up to 900°C Based on Ni, Co, Pd and Pt (e.g. Ni70Hf30)

**Active solders Application temperature Chemical composition**

High-temperature activation, based on Sn, In and Pb (e.g. Sn90Ag10Ti3) Mechanical activation based on Sn (e.g. SnAg3.5Ti4(Ce,Ga))

up to 200°C

Titanium is moved from the solder matrix to the interface; combines with carbon, nitrogen or oxygen from the ceramic material; and thus creates intermetallic compounds which allow the wetting of ceramic substrate and creation of a metallurgical bond. Fluxless soldering process in the air is concerned, where no corrosion

The active solders for high-temperature activation necessitate considerably higher soldering temperature and may be used only in a vacuum and/or shielding atmosphere. Soldering temperature of the tin- and lead-based solders varies within 850–950°C. The solders for mechanical activation (e.g. SnAg3.5Ti4(Ce,Ga)) may be used also for high-temperature activation. However, greater wetting angles than in the case of mechanical activation are attained [14]. As an example of using S-Bond 220-1 (SnAg3.5Ti4(Ce,Ga)) solder in electrotechnics, joining of Al2O3 with copper in **Figure 2** may be mentioned. This solder allows to join the materials with different

coefficients of thermal conductivity and expansivity (metals, light metals, ceramics, composites with metal matrix, carbides, glass, etc.) [20].

The solder type S-Bond contains Ti as an active element and elements from the group of lanthanides. These active elements migrate to the interface of the soldered joint and act upon the material surface by the active disruption and removal of oxides. Disruption of oxide layer, which prevents the formation of contact between the solder and substrate surface, is called as 'activation'. As soon as the oxide layer is disrupted, the solder volume reacts with the substrate surface, and a strong bond with the joined surface is thus formed.

The bond is formed by two mechanisms:


### *1.2.1 High-temperature activation of an active solder*

In the case of active solders containing Ti (or other reactive elements), the hightemperature activation in shielding atmosphere of vacuum is employed. The active element reacts with the surface oxides of substrates at the soldering temperatures of 850–950°C. Similarly also nitrides, carbides or silicides of the active element are formed in the solder in the case of non-oxide ceramics. One of the greatest disadvantages of this process consists in the necessity of a vacuum.

The work cycle in **Figure 3** consists of a rapid heating to soldering temperature, dwell time for about 9 minutes and a long free cooling down (for about 320 minutes) in the oven. The short dwell time at the soldering temperature prevents formation of brittle phases and grain coarsening. The slow cooling down is needed in order to prevent high residual stresses, which may cause the joint cracking.

The diffusion processes take place during slow cooling down, which will be exerted in the growth of diffusion zone in the parent metal and the solubility zone in the solder. Soldering with an active solder may be performed in a vacuum furnace at

**Figure 3.** *Scheme of thermal cycle of vacuum soldering [21].*

the pressure of 10<sup>−</sup><sup>1</sup> –10<sup>−</sup><sup>3</sup> Pa. In some special cases, soldering may be performed also in the overpressure of argon or helium. Nitrogen cannot be used as a shielding gas since it deteriorates the wetting of ceramics. This is caused by the fact that nitrogen has a high affinity to Ti, and thus it depletes the solder by Ti. The high-temperature activation cannot be used for soldering of metallic materials with the coatings of Au, Ag, Cu, Al and Mg, which exert a high dilution rate in Sn solder. The working parameters exert a significant effect upon the properties of soldered joints [21].
