**2.7. Mechanical activation**

The mechanically activated solders allow wetting the metallic and non-metallic materials at considerably lower temperature than in the case of high-temperature activation. Moreover, there is no need to solder in vacuum and/or in the shielding atmosphere. In dependence on soldering alloy type, this temperature may vary from 150 to 430°C, thus in the temperature range of soldering with solders, which are based on Bi-Sn, Sn-Ag or Zn-Al, as already mentioned. This unique capability is allowed by a slight addition of the following metals: lanthanum, cerium, yttrium and samarium (lanthanides), which occur in the soldering alloy matrix. These metals at the same time create a protective barrier for the active metal (Ti) as shown in **Figure 6** [13]. Mechanical activation can be performed by


**Figure 5.** Scheme of soldering with high-temperature activation in vacuum [14].

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**Figure 6.** Scheme of soldering with mechanical activation in air without flux—S-Bond process [14].

**2.6. High-temperature activation**

70 Recent Progress in Soldering Materials

element in the solder [7].

**2.7. Mechanical activation**

**Figure 6** [13]. Mechanical activation can be performed by

**Figure 5.** Scheme of soldering with high-temperature activation in vacuum [14].

occur.

• intrusions

• vibrations (50–60 Hz)

• ultrasound (20 60 kHz)

• friction (different processes)—**Figure 7**.

It is a process being performed at high temperature (850–950°C), mostly in vacuum furnace with the shielding atmosphere of argon as shown in **Figure 5**. It was found out experimentally that at activation with Ti-active element, the lowest temperature at which wetting of Al2

ceramics may be achieved is 780°C. However, wetting depends also on the content of Ti-active

With increasing soldering temperature (activation temperature), the wetting of ceramic material is enhanced by high-temperature activation but, in contrary, if a ceramic material is soldered in combination with metallic material, the degradation of base metal by erosion may

The mechanically activated solders allow wetting the metallic and non-metallic materials at considerably lower temperature than in the case of high-temperature activation. Moreover, there is no need to solder in vacuum and/or in the shielding atmosphere. In dependence on soldering alloy type, this temperature may vary from 150 to 430°C, thus in the temperature range of soldering with solders, which are based on Bi-Sn, Sn-Ag or Zn-Al, as already mentioned. This unique capability is allowed by a slight addition of the following metals: lanthanum, cerium, yttrium and samarium (lanthanides), which occur in the soldering alloy matrix. These metals at the same time create a protective barrier for the active metal (Ti) as shown in

O3

**Figure 7.** Scheme of different ways of mechanical activation of solder by friction [14].

#### **2.8. Wetting**

The wettability is qualitatively assessed by the wetting angle Θ and adhesion work Wad [15]. It is defined as the capability of molten solder to adhere to a clean surface of material joined at a certain temperature [16]. Wetting of surfaces is at the same time the primary precondition for joint formation. Wetting is realized either by Van der Waals bond and/or by chemical bonding. Thus, two basic types of wetting may be distinguished [15]:


The equations describing wetting of liquid droplet on a solid material surface were derived on the basis of physical and chemical principles. The droplet during wetting takes such a shape, at which the energy of solder-material-atmosphere (vacuum) system is minimum, and thus the interatomic forces may be exerted [16]. The basic Young's equation for a weak reaction system may then result from the facts mentioned [17]:

$$
\gamma \text{SV} - \gamma \text{SL} = \gamma \text{LV} \cos \Theta \tag{4}
$$

where *γ*SV is the surface energy between the material surface and atmosphere, *γ*SL is the surface energy between the material surface and solder, *γ*LV is the surface energy between the solder and atmosphere and Θ is the wetting angle.

The magnitude of wetting angle is the qualitative criterion of wetting. **Figure 8** shows two basic cases which may occur at material wetting with a liquid solder.

As obvious from **Figure 8**, the solder does not wet the material for which the value of wetting angle Θ > 90° (*γ*SL < *γ*SV). If the value Θ < 90°, the solder wets the material. By the equation of wetting (4), the driving force of process is the (*γ*SV < *γ*SL) difference. By the magnitude of wetting angle, we distinguish different degrees of wetting—**Table 2** [16].

Lowering the contact angle of wetting and increasing the value of adhesive work at a defined solid substrate may be affected by the selection of a suitable solder. However, for wetting

**Figure 8.** Scheme of wetting for a weak reaction system. (a) Solder does not wet the material; (b) solder wets the material.


**Table 2.** Assessment of wetting.

ceramic materials, it is necessary to use an active solder, that is, the solder containing the element chemically active towards ceramics (Ti, Zr or Hf). Such an element is chemically bond with some component of ceramics, so improving its wettability and thus also the joint strength [15]. Scheme of wetting for the system with reaction on the interface is shown in **Figure 9**. In such a case, Young's equation according to Ref. [18] attains the following form:

$$[\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny//\text{\tiny N}Cl\text{-}}}\text{\textsuperscript{\text{\tiny N}Cl\text{-}}}\text{\}}]}\text{)}\text{)}\text{)}}\text{)}}\text{)}}\text{)}}$$

where *γ*SV is the surface energy between the material surface and atmosphere, *γ*LV is the surface energy between the solder and atmosphere, *γ*RL is the surface energy between the solder and reaction layer, *γ*SR is the surface energy between the reaction layer and material, Θ is the wetting angle, Ω is the gram atomic volume, *t* is the thickness of reaction layer and Δ*G* is the change in free energy per one mole of reaction product.

#### **2.9. Compensation of different thermal expansivity of the ceramics/metal-soldered joints**

The residual stresses in soldered joint of two materials with different coefficients of thermal expansivity belong to one of the most serious issues for ensuring the reliable joints of ceramics/metal. Ceramic materials exert in general lower coefficients of thermal expansivity and are not capable of plastic strain. On the contrary, the metals exert much higher coefficients of thermal expansivity and are in certain extent capable of plastic strain. These are the main reasons for the formation of residual stressed during cooling down of ceramics/metal joints, which must be compensated.

**Figure 9.** Scheme of wetting for the system with reaction on the interface [18].

**Figure 8.** Scheme of wetting for a weak reaction system. (a) Solder does not wet the material; (b) solder wets the material.

The wettability is qualitatively assessed by the wetting angle Θ and adhesion work Wad [15]. It is defined as the capability of molten solder to adhere to a clean surface of material joined at a certain temperature [16]. Wetting of surfaces is at the same time the primary precondition for joint formation. Wetting is realized either by Van der Waals bond and/or by chemical

• Wetting, where chemical reaction in the solder-soldered material interface takes place,

The equations describing wetting of liquid droplet on a solid material surface were derived on the basis of physical and chemical principles. The droplet during wetting takes such a shape, at which the energy of solder-material-atmosphere (vacuum) system is minimum, and thus the interatomic forces may be exerted [16]. The basic Young's equation for a weak reaction

*γ*SV − *γ*SL = *γ*LV cos*Θ* (4)

where *γ*SV is the surface energy between the material surface and atmosphere, *γ*SL is the surface energy between the material surface and solder, *γ*LV is the surface energy between the

The magnitude of wetting angle is the qualitative criterion of wetting. **Figure 8** shows two

As obvious from **Figure 8**, the solder does not wet the material for which the value of wetting angle Θ > 90° (*γ*SL < *γ*SV). If the value Θ < 90°, the solder wets the material. By the equation of wetting (4), the driving force of process is the (*γ*SV < *γ*SL) difference. By the magnitude of wet-

Lowering the contact angle of wetting and increasing the value of adhesive work at a defined solid substrate may be affected by the selection of a suitable solder. However, for wetting

bonding. Thus, two basic types of wetting may be distinguished [15]:

system may then result from the facts mentioned [17]:

solder and atmosphere and Θ is the wetting angle.

basic cases which may occur at material wetting with a liquid solder.

ting angle, we distinguish different degrees of wetting—**Table 2** [16].

• Wetting without chemical reaction in the soldered material-solder interface.

whereby also the formation of reaction products (new phases) occurs.

**2.8. Wetting**

72 Recent Progress in Soldering Materials

Reducing the level of residual stresses may be more or less attained in the following ways [7]:


**Figure 10.** Complex interlayer [20].

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**Figure 11.** Reduction of residual stresses in contact zone by application of plastic metallic interlayer: a, joint without interlayer; b, joint with interlayer [21].

#### **2.10. Indium-based solders**

Reducing the level of residual stresses may be more or less attained in the following ways [7]: • *By the selection of suitable material couples*. Application of joined materials with similar coefficients of thermal expansivity is not a practical solution in most cases, since the joined materials are mostly selected on the basis of other desired properties and not on the basis

• *Soldering temperature*. By selection of a suitable solder (regarding thermal resistance of the

• *Shape of joint and thickness of the materials joined*. A simple type of joint should be selected, attaining thus the compressive loading in ceramic material. It should be taken into account that the ceramic materials do not exert constant value of strength limit. This is in practice exerted in such a way that during loading of ceramics a crack may be formed without any strain. • *Size of clearance between the soldered parts*. In soldering-combined materials, it is required that the joint clearance should be generally greater than in the case when soldering the same materials (**Figure 10**). It is recommended to select the clearance value within *s* = 0.2–0.5 mm [19].

• *Application of interlayer*. If allowed by the design solution of the joint, the reduction of residual stresses can be attained also by the application of an interlayer, where material is selected on the basis of elastic, plastic and thermal properties. At the application of interlayer with a small coefficient of thermal expansivity, close to that of ceramics (W, Mo), lower stresses are formed in the ceramics. In the case of application of interlayer with a low yield point (Ni, Cu, Al), the level of stresses is reduced due to their relaxation by the slip mechanism. At the application of a composite interlayer (e.g. cermet-sintered interlayer composed of ceramic and metallic powders) with gradual transition from metal expansivity to expansivity of ceramics, the creation of gradient of physical properties is considered as shown in **Figure 11**. In interlayer selection, it is also necessary to consider the interactions with the molten solder: segregation, formation of brittle structures on the interface

• *Application of solder*. At the application of metal solders in the joint assembly, these are capable to compensate the stresses resulting from different thermal expansivity by their plastic straining via the slip or creep mechanism. In this way, the most significant reduction

of residual stresses at preserved joint simplicity may be attained [2].

of their thermal expansivity.

74 Recent Progress in Soldering Materials

and dilution in the solder.

**Figure 10.** Complex interlayer [20].

joint), reduced soldering temperature may be achieved.

The indium-based solders, for example, type 100In or the solders type In-Sn with a higher indium content, are characteristic with their unique soldering properties. These solders allow to wet a wide range of metallic, non-metallic and ceramic materials, for example non-metallics such as glass, glazed ceramics, mica, mullite, quartz, fibre optic glass, lead glass, liquid crystal glass, metallized glass, optical glass, pyrex, quartz glass, sapphire, silica, silica glass, soda lime glass, and various metallic oxides [22]. During soldering process performed with high-indium solders in air, the indium suboxides are formed, which react with the surface oxides on the soldered substrate at the formation of a strong bond between the substrate and solder. An example of soldering the ITO ceramics (In2 O3 /SnO2 ) presents the study [23], which deals with the application of soldering alloy type Sn-In-Ag-Ti at ultrasonic soldering of ITO ceramics. It was found that the Sn-In-Ag-Ti solder reacts with the surface of ITO substrate, whereby the wettability of materials joined and bond formation was ensured.

The goal of the next research within the study [24] consisted of soldering metallic (Cu, Ni, Al, Ti, AISI 316 steel) and ceramic materials (Al2 O3 , SiC) by the aid of solders with a high content of indium at power ultrasound application. The solders with composition 100In (5N purity) and 70In30Sn (4N5 purity) were used in experiments. The UT equipment with a frequency of 40 kHz and an output power of 400 W with 2 μm amplitude was used for experiments. The scheme and description of this equipment is shown in **Figure 12**. Soldering temperature was 20°C above the liquidus of the solder tested. The dwell time at soldering temperature was 30 s and the time of ultrasound action on the soldered joint was 5 s. Heating of specimens was ensured by the hot-plate method with temperature control via a thermocouple type NiCr/NiSi. The shear strength of metallic and ceramic substrates was assessed. The test specimens of substrates were prepared in the form of disks with a diameter of Ø 15 mm and

**Figure 12.** Scheme of ultrasonic device used for soldering.

1.5–2 mm in thickness. The test specimen is shown in **Figure 13**. The procedure of specimen preparation is shown in **Figure 14**. The shear gap was selected to 0.1 mm, which corresponds to 2% from the sheared diameter of the roll formed of the solder. Uniform testing rate was 0.5 mm/min.

The 100In and 70In30Sn solders with a high indium content wetted all studied metallic (Cu, Al, Ni, Ti and AISI 316 steel) and ceramic materials (SiC and Al2 O3 ) at the application of power ultrasound with the frequency of 40 kHz. For comparison, the 100Sn solder wetted all metallic materials but it did not wet the ceramic materials in spite of ultrasound assistance. The 100Sn solder cannot be used for soldering ceramic and non-metallic materials.

The results of mechanical tests of soldered joints fabricated by the use of 100In solder are given in **Figure 15**. The highest shear strength of 12.5–54 MPa is achieved at the soldering of metals. The lowest shear strength was observed with aluminium and the highest with copper. Considerably lower shear strength values are achieved at the application of 100In solder on the ceramic materials, varying from 3.5 to 6 MPa. Higher strength was attained with Al2 O3 ceramics and lower with SiC ceramics.

**Figure 13.** Fabricated test specimen.

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**Figure 14.** Specimen layout at its preparation for the shear test.

**Figure 15.** Shear strength of joints fabricated with solder type 100In.

1.5–2 mm in thickness. The test specimen is shown in **Figure 13**. The procedure of specimen preparation is shown in **Figure 14**. The shear gap was selected to 0.1 mm, which corresponds to 2% from the sheared diameter of the roll formed of the solder. Uniform testing rate was

The 100In and 70In30Sn solders with a high indium content wetted all studied metallic (Cu,

ultrasound with the frequency of 40 kHz. For comparison, the 100Sn solder wetted all metallic materials but it did not wet the ceramic materials in spite of ultrasound assistance. The 100Sn

The results of mechanical tests of soldered joints fabricated by the use of 100In solder are given in **Figure 15**. The highest shear strength of 12.5–54 MPa is achieved at the soldering of metals. The lowest shear strength was observed with aluminium and the highest with copper. Considerably lower shear strength values are achieved at the application of 100In solder on the ceramic materials, varying from 3.5 to 6 MPa. Higher strength was attained with Al2

O3

) at the application of power

O3

Al, Ni, Ti and AISI 316 steel) and ceramic materials (SiC and Al2

**Figure 12.** Scheme of ultrasonic device used for soldering.

ceramics and lower with SiC ceramics.

**Figure 13.** Fabricated test specimen.

solder cannot be used for soldering ceramic and non-metallic materials.

0.5 mm/min.

76 Recent Progress in Soldering Materials

At comparison of the results of shear strength attained with 100In and 70In30Sn solders (**Figure 16**), it was found that with the solder containing tin higher strength values are achieved, both on the metallic and ceramic materials. This is caused by the fact that the matrix of 70In30Sn solder exerts the eutectic structure [25, 26]; therefore, it offers also higher mechanical resistance, when compared to pure indium solder. At the In content of 70 wt. %, the solder preserves also suitable wetting properties and sufficient interaction with the surface of ceramic material. The shear strength achieved on metallic materials ranged from 23 to 71.5 MPa. It was the highest on copper and lowest again on Al. On the ceramic materials, it was 6 and 7 MPa as shown in **Figure 16**.

The fractured surfaces on metallic materials remained always covered with a uniform layer of solder after shear test. In case of ceramic materials, the fractured surface remained covered with a solder layer in most cases. Partial separation of solder from the ceramic substrate was observed approximately with 40% of all specimens. The fracture mostly initiated in the solder and was of ductile character. The failure took place in shear mechanism (**Figure 17**). The motion of shearing tool is clearly visible on the fracture morphology.

#### **2.11. Active solder with lanthanum content**

The research [27] was aimed at direct soldering Al<sup>2</sup> O3 ceramics with a copper substrate by the application of Sn2La solder. It was studied whether the Sn-based solder alloyed with La

**Figure 16.** Shear strength of joints fabricated with solder type In70Sn30.

**Figure 17.** Fracture area of joint on the substrate of Al2 O3.

can wet the Al<sup>2</sup> O3 ceramics and create thus a strong bond. The possibility to substitute La with Ti in active tin solders was also studied. For this reason, the analyses for revealing the mechanism of bond formation were performed, and the shear strength of the joints was measured. Lanthanum, as a metal with high affinity to oxygen, was applied as an active element. Soldering was performed at a low temperature in air with the application of power ultrasound. The ultrasonic equipment type Hanuz UT2 with the parameters given in **Table 3** was employed for soldering. An ultrasonic transducer was used for solder activation, which uses an oscillating piezoelectric system and a titanium tool with an outlet diameter of Ø 3 mm. The scheme of soldering with ultrasound assistance is documented in **Figure 18**. Soldering takes place through the layer of molten solder. The titanium tool, sonotrode, is thus not in a direct contact with the ceramic substrate. The soldering temperature was selected to 290°C, which is 20°C above the liquidus temperature of the solder.

Soldering procedure runs in such a manner that a layer of solder is deposited on the substrate heated at soldering temperature. The liquid solder is then subjected to active ultrasound without the application of shielding atmosphere, thus in air during the time of 5 s. After ultrasonic activation, the redundant layer of molten solder and formed oxides are removed from the substrate surface. Both substrates are prepared in the same way. The substrates with the deposited solder layer are put on each other in such a manner to create contact with the molten phase. They are then centred and the desired joint is formed by a slight compression of this assembly. Graphical representation of this procedure is shown in **Figure 19**.


**Table 3.** Parameters and conditions of soldering.

**Figure 18.** Ultrasonic soldering.

**Figure 17.** Fracture area of joint on the substrate of Al2

**Figure 16.** Shear strength of joints fabricated with solder type In70Sn30.

78 Recent Progress in Soldering Materials

O3.

**Figure 19.** Procedure of joint fabrication by ultrasonic soldering.

A uniform distribution of La phases in tin matrix may be seen in the microstructure of Sn2La solder shown in **Figure 20**. No La was observed in the matrix of the studied solder. This fact was verified by the energy dispersive spectroscopy (EDS) analysis.

#### **2.12. Analysis of interface in Al2 O3 /Sn2La solder joint**

Comparison of microstructures of Al<sup>2</sup> O3 /SiC and Cu/Cu-soldered joints from the optical microscopy is shown in **Figure 21**.

As shown, much of lanthanum is oxidized in air during ultrasonic process. The lanthanum particles are distributed to the interface with ceramic material during ultrasonic activation as shown in **Figures 22** and **23**, and they are then combined with oxides on the surface of ceramic material. The concentration line of La in **Figure 22** proves increased La concentration on the

**Figure 20.** Microstructure of Sn2La solder.

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**Figure 21.** Comparing the microstructure of Sn2La solder after UT soldering at the same *parameters and conditions of soldering*.

interface with Al<sup>2</sup> O3 ceramics. A uniform, continuous layer of La oxides on the interface with ceramic material can be seen in **Figure 23**, which ensures the bond formation. The thickness of this layer is around 1.5 μm. In spite of this layer, the solder is more or less bonded to the ceramic substrate. The bond with ceramic material is of adhesion character. The formation of new intermetallic phases was not observed. This also causes lower shear strength of the bond with ceramic materials. The mechanism of bond formation is schematically outlined in **Figure 24**.

A uniform distribution of La phases in tin matrix may be seen in the microstructure of Sn2La solder shown in **Figure 20**. No La was observed in the matrix of the studied solder. This fact

**/Sn2La solder joint**

As shown, much of lanthanum is oxidized in air during ultrasonic process. The lanthanum particles are distributed to the interface with ceramic material during ultrasonic activation as shown in **Figures 22** and **23**, and they are then combined with oxides on the surface of ceramic material. The concentration line of La in **Figure 22** proves increased La concentration on the

/SiC and Cu/Cu-soldered joints from the optical

O3

was verified by the energy dispersive spectroscopy (EDS) analysis.

**O3**

**2.12. Analysis of interface in Al2**

80 Recent Progress in Soldering Materials

microscopy is shown in **Figure 21**.

**Figure 20.** Microstructure of Sn2La solder.

Comparison of microstructures of Al<sup>2</sup>

**Figure 19.** Procedure of joint fabrication by ultrasonic soldering.

**Figure 22.** Concentration profiles of Al, Sn, La and O elements on the interface of Al2 O3 ceramics/Sn2La solder.

**Figure 23.** Planar EDX analysis of soldered interface of Sn2La/Al2 O3 .

**Figure 24.** Mechanism of bond formation at UT activation of SnLa2 solder.

The research was primarily oriented to soldering ceramic substrate of Al<sup>2</sup> O3 and a copper substrate. The experiments carried out in the study of shear strength of soldered joints were extended to other metallic materials (Al, Ni, Ti and CrNi steel) and SiC ceramics in order to show broader applicability of Sn2La solder.

Measurement was performed on four specimens of each material. The results of average shear strength of joints are documented in **Figure 25**. The lowest shear strength was observed on Al<sup>2</sup> O3 ceramics (7.5 MPa). Little higher strength of 13.5 MPa was observed on SiC ceramics. The highest strength, when regarding the metallic materials, was achieved with Al and Ni. The strength on copper substrate was 26.0 MPa.

The Sn2La solder has shown relatively great differences in shear strength on the metallic and ceramic materials. It can be generally said that the shear strength of joints in metallic materials is almost three times higher than in the case of ceramic materials.

From the results of analysis of transition zone of soldered joints, it may be concluded that the bond with metallic material is of metallurgical-diffusion character. The bond with a ceramic material, namely Al2 O3 (at soldering with solder containing La), is of adhesion character.
