**2.2. Solderable metal coatings on ceramic materials formed by physical and chemical deposition**

The methods for the formation of metallic layers and ceramic or glass substrates may be divided into several groups. The layers are obtained by chemical deposition from solutions of appropriate metal salts (e.g. Ni, Ag, etc.) by the aid of chemical agents, such as formaldehyde, hypophosphite, glucose and so on, with eventual heat treatment of the segregated coating. These layers are used in cases when small areas are metallized, eventually a motive composed of individual narrow conductive tracks, which are not mechanically loaded. Soldering of such layers requires the application of solders containing Sn, In, Cd and so on, with a low melting point (max. 200°C) and a minimum dwell time on soldering temperature [6].

Other known methods comprise the physical deposition by vacuum evaporation and sputtering of metallic layers as shown in **Figure 2**. These processes employ a high vacuum and temperatures within the range from 200 to 500°C. In case of these methods, the coating particles are deposited rather in physical manner, not by the aid of chemical reactions, as in the case of CVD (chemical vapour deposition) processes.

The thickness of evaporated or sputtered layers varies in the order of nm to μm. To prevent the de-alloying (de-wetting) of the metallic layer to the molten solder, the multiple deposition of system consisting of several metals, for example, Cr/Ni-Ag, Ni-Ag and Cr- Ni/V-Ag, is used.

To ensure a good spreadability of solder over the entire surface, the final layer is often formed by evaporation or sputtering of Au or Ag, which is dissolved in the used solder and will thus create suitable conditions for a good wettability of entire surface by the solder. Chemical reaction of the deposited layer with the material of ceramic substrate does not occur in both mentioned deposition methods. The bond between the layer and ceramics is mostly of adhesion character [2]. The solderable coatings for ensuring the wettability of poorly wettable surfaces are mostly employed in the electrotechnics and electronics. The advantage of well-solderable metal coatings consists in the fact that the soldering process can be accomplished in very short soldering times. The used coatings are either meltable or soluble (**Figure 3**). The soluble

**Figure 2.** Main physical deposition methods applied for metallizing of ceramics [7]: a, evaporation; b, diode sputtering; c, magnetron sputtering.

Recent Advances in Solderability of Ceramic and Metallic Materials with Application of Active... http://dx.doi.org/10.5772/intechopen.69552 67

**Figure 3.** Basic division of solderable coatings [7].

• The formed oxide then react with Al2

CVD (chemical vapour deposition) processes.

the product of reaction [5]:

66 Recent Progress in Soldering Materials

**deposition**

c, magnetron sputtering.

O3

point (max. 200°C) and a minimum dwell time on soldering temperature [6].

MnO + Al<sup>2</sup> O3 → MnAl2 O4 (2)

The methods for the formation of metallic layers and ceramic or glass substrates may be divided into several groups. The layers are obtained by chemical deposition from solutions of appropriate metal salts (e.g. Ni, Ag, etc.) by the aid of chemical agents, such as formaldehyde, hypophosphite, glucose and so on, with eventual heat treatment of the segregated coating. These layers are used in cases when small areas are metallized, eventually a motive composed of individual narrow conductive tracks, which are not mechanically loaded. Soldering of such layers requires the application of solders containing Sn, In, Cd and so on, with a low melting

Other known methods comprise the physical deposition by vacuum evaporation and sputtering of metallic layers as shown in **Figure 2**. These processes employ a high vacuum and temperatures within the range from 200 to 500°C. In case of these methods, the coating particles are deposited rather in physical manner, not by the aid of chemical reactions, as in the case of

The thickness of evaporated or sputtered layers varies in the order of nm to μm. To prevent the de-alloying (de-wetting) of the metallic layer to the molten solder, the multiple deposition of system consisting of several metals, for example, Cr/Ni-Ag, Ni-Ag and Cr- Ni/V-Ag, is used. To ensure a good spreadability of solder over the entire surface, the final layer is often formed by evaporation or sputtering of Au or Ag, which is dissolved in the used solder and will thus create suitable conditions for a good wettability of entire surface by the solder. Chemical reaction of the deposited layer with the material of ceramic substrate does not occur in both mentioned deposition methods. The bond between the layer and ceramics is mostly of adhesion character [2]. The solderable coatings for ensuring the wettability of poorly wettable surfaces are mostly employed in the electrotechnics and electronics. The advantage of well-solderable metal coatings consists in the fact that the soldering process can be accomplished in very short soldering times. The used coatings are either meltable or soluble (**Figure 3**). The soluble

**Figure 2.** Main physical deposition methods applied for metallizing of ceramics [7]: a, evaporation; b, diode sputtering;

**2.2. Solderable metal coatings on ceramic materials formed by physical and chemical** 

, forming thus the manganese-aluminium spinel as

coating may consist of one metal or of several metal layers deposited subsequently on the substrate. The meltable coating is formed by the deposition of the lead- and/or lead-free layer of soldering alloy on the substrate surface.

The system of dissolvable solderable coating in electronics mostly consists of an adhesion layer (Cr, Ni, Ti, Al), a diffusion barrier (Ni, Cr) and a solderable layer (Ag, Au, Cu, Ni), whereas its composition depends on the substrate material and parameters and conditions of soldering. For preserving a good solderability, for example tinning with pure tin, metallizing with Sn-Pb solders, gold and/or silver plating used to be applied until now. However, the application of leadfree soldering necessitates the new approaches also in the field of surface finishing, especially in printed circuits and also outlets of electronic components. The meltable coatings must be selected without lead content, regarding thus the environmental viewpoint. The coatings composed of SnBi and SnCu, eventually coating of pure tin, start to be more favourable nowadays [8].

A coating composed of a system of Cr/Ni-7%V/Ag layers, prepared by PVD sputtering, was approved experimentally. The chromium layer has the adhesion function, Ni-V is the diffusion barrier and Ag layer ensures a good wettability and spreadability in a short time as shown in **Figure 4** [8].

The SnIn52 solder with a melting point of 120°C was proposed for soldering. The soldering temperature was selected just by 10°C higher than its melting point with a minimum dwell time, needed for a proper spreading of the solder.

### **2.3. Soldering of ceramic materials with an active solder**

The wettability of ceramic material may be improved by reducing the interphase stress on the ceramics/substrate interface, by solder alloyed with an active element (mainly Ti, Zr or Hf is mostly mentioned in the literature) with a high affinity to oxygen, which reacts with ceramics during the soldering process, whereby bonds on the interatomic level are formed. The basic chemical reaction between Ti and oxidic ceramics has a general form [16]:

**Figure 4.** Distribution of metallic layers on a substrate [8].

$$\text{y/Ti/+MxOy} \rightarrow \text{yTiOi} + \text{x/M} \quad \text{(//-melt)}\tag{3}$$

Titanium can bond considerable amount of oxygen in ceramics of oxidic type. Therefore, several oxides can be formed, for example, between the AgCuTi solder and Al2 O3 ceramics: TiO, Ti2 O3 , Ti<sup>3</sup> O5 , Ti<sup>4</sup> O7 and TiO<sup>2</sup> . The reaction product formed on the contact area at individual types of oxidic and non-oxidic ceramics is several μm in thickness and depends on the soldering conditions and solder type. The reaction product alters the surface energy of ceramics and allows its wetting by the solder [17]. Higher concentration of active element may in some cases increase the joint brittleness; therefore, its amount must be limited. The foreign sources refer the maximum limit to 4 wt. % Ti in an eutectic solder-type Ag72Cu. Chemical changes occurring on the ceramics-active solder interface are very complex since the concentration gradients are formed [1].

The process of soldering ceramics with metal is significantly simplified by the application of active solders. Soldering ceramics with an active solder is also more economically efficient, since the multi-stage metallizing processes with demanding inter-operational annealing are unnecessary [9].

#### **2.4. Division of active solders**

The active solders, similar as the commercial solders, are classified by the melting temperature to solders, brazing alloys and high-temperature solders. The difference is seen just in the case of active solders, which may be further divided into high-temperature solders and the mechanically activated solders. Due to reaction capability of an active element, the soldering temperature for high-temperature activated active solders must be higher than 780°C (at the application of active Ti). Nevertheless, the active solders are molten at the temperature around 220°C. The division of active solders and their chemical base is given in **Table 1**.

#### **2.5. Active solders**

The brittle materials as vitreous glass (SiO2 ), silicon, graphite and so on can be soldered with active solders. Base metal of active solders is mostly tin, lead or indium and the alloys created


**Table 1.** Division of active solders by the melting temperature [10].

on their basis. However, the lead-containing solders are not suitable for soldering in vacuum, since considerable evaporation of lead and also furnace contamination occurs. When soldering with lead, a through-flow atmosphere with pure argon, eventually helium with overpressure attaining 0.1–0.2 MPa, should be used [11].

The solders used in joint assembly are capable to compensate the stresses resulting from different thermal expansivity, owing to their plastic straining by the shear or creep mechanism. In this way, the most significant reduction of residual stresses at preserved joint simplicity may be achieved. The presence of an active element ensures a good wettability of soldered parts.

The active solders can be used for soldering unusual combinations of metallic materials (e.g. CrNi steel, Mo, W, Ti, Cr, etc.) and non-metallic materials, mostly of brittle character (vitreous glass, sapphire, carbon, silicon and also almost all types of ceramics) [11]. Such joints are applied mainly in electronics and electrotechnics, where lower strength and thermal resistance of the joint are sufficient. For example, the soldering of glass in lasers and spectroscopes, creation of electric connections with graphite, connecting the heat exchangers to ceramic electronic substrate of Al<sup>2</sup> O3 , or AlN [12]. The solders also allow fabricating the vacuum-tight joints in vacuum and cryogenic technology, where the application of indium-based solder was well approved [11].

Great attention is paid to issues of soldering with active solders, especially for the electronic applications. However, there are still many issues, regarding the achievement of the desired utility properties of joints, to be solved.

As already mentioned, the active solders may be further divided according to the way of active element activation in the solder to the solders destined for:


y/Ti/+MxOy → yTiOi + x/M/ (//− melt) (3)

Titanium can bond considerable amount of oxygen in ceramics of oxidic type. Therefore, sev-

types of oxidic and non-oxidic ceramics is several μm in thickness and depends on the soldering conditions and solder type. The reaction product alters the surface energy of ceramics and allows its wetting by the solder [17]. Higher concentration of active element may in some cases increase the joint brittleness; therefore, its amount must be limited. The foreign sources refer the maximum limit to 4 wt. % Ti in an eutectic solder-type Ag72Cu. Chemical changes occurring on the ceramics-active solder interface are very complex since the concentration

The process of soldering ceramics with metal is significantly simplified by the application of active solders. Soldering ceramics with an active solder is also more economically efficient, since the multi-stage metallizing processes with demanding inter-operational annealing are

The active solders, similar as the commercial solders, are classified by the melting temperature to solders, brazing alloys and high-temperature solders. The difference is seen just in the case of active solders, which may be further divided into high-temperature solders and the mechanically activated solders. Due to reaction capability of an active element, the soldering temperature for high-temperature activated active solders must be higher than 780°C (at the application of active Ti). Nevertheless, the active solders are molten at the temperature around 220°C. The division of active solders and their chemical base is given in **Table 1**.

active solders. Base metal of active solders is mostly tin, lead or indium and the alloys created

**Active solders Application temperature Chemical composition**

up to about 150°C

Active brazing alloys up to 350°C Ag, Cu and Au based (Ag72CuTi1,5) High-temperature active solders up to 900°C Ni, Co, Pd and Pt based (e.g. Ni70Hf30)

. The reaction product formed on the contact area at individual

), silicon, graphite and so on can be soldered with

activation (e.g. Sn95Ag5Ti3)

(e.g. SnAg6Ti4Ce)

Sn, In and Pb based, for high-temperature

Sn and In based, for mechanical activation

O3

ceramics: TiO,

eral oxides can be formed, for example, between the AgCuTi solder and Al2

O7 and TiO<sup>2</sup>

gradients are formed [1].

68 Recent Progress in Soldering Materials

**2.4. Division of active solders**

unnecessary [9].

**2.5. Active solders**

The brittle materials as vitreous glass (SiO2

Active solders From cryogenic temperatures

**Table 1.** Division of active solders by the melting temperature [10].

Ti2 O3 , Ti<sup>3</sup> O5 , Ti<sup>4</sup>

> The active brazing alloys for high-temperature activation based on Ag and Ag-Cu, designated as CB 2, CB 4, CB 5 and CB 6, are supplied by Umicore-BrazeTec GmbH, Germany. The brazing alloy designated as CB 10, with composition AgCu25Ti10, and the alloy designated as CB 11, with composition AgTi10, are supplied in the form of brazing paste. The working temperature of these brazing alloys is 850–1050°C. The solders for ultrasonic activation, designated as CERASOLZER, are supplied by the Japanese company KURODA ELECTRIC.

> The solders for mechanical (ultrasonic) activation, designated as S-Bond, are supplied by the Euromat, GmbH, Germany. It concerns, for example, the solders type S-bond 140 (based on Bi-Sn-Ag-Ti), S-bond 220 (based on Sn-Ag-Ti), S-bond 400 (based on Zn-Al-Ag) and so on. The soldering temperature for the S-bond 140 solder is from 150 to 160°C, for the S-bond 220 solder it is 240–260°C and for the S-bond 400 solder the soldering temperature is 420–430°C. It was proved experimentally, that the S-bond 220 (SnAg6Ti4Ce) solder has wetted the Al2 O3 ceramics with the wetting angle of 62°, when applied at a temperature of 860°C in vacuum of 10-2 Pa. This experiment was performed in cooperation with the Institute of Materials and Machine Mechanics at the Slovak Academy of Sciences in Bratislava.
