**3. Results and discussion**

#### **3.1 Phase analysis**

Results of the crystallization behaviour of glass-ceramic Ferro A6S substrate sintered at various sintering temperatures are presented in Fig. 3. It can be seen that the small intensity peaks which appeared for the 800 °C sintering belongs to crystalline phases of Calcium Silicates with various stoichiometric ratios Ca2SiO4, and Ca3SiO5. Increasing the temperature to 825 °C and 850 °C, the two phases of Calcium silicate phase are still maintained but accompanied by a new phase of Calcium Borate (CaB2O4). As the sintering temperature increases the formation of Danburite (CaB2Si2O8) start to appear in small intensities while crystalline phases of Ca2SiO4, Ca3SiO5 including CaSiO3 and CaB2O4 phase are maintain in this composite system. They are the stable phases when crystallization is complete. Due to the high peak intensity of CaSiO3 at 2θ ≈ 29.9572, it can be noted that the main phase in this system is Calcium Silicate CaSiO3. The above results were consistent with the work done by Chang and Jean (1999). However, from their studies, they concluded that no crystalline phase was detected at temperatures below 850 °C. The crystallite size as a function of sintering temperature was calculated from the most intense peak (3 1 1) by using the Scherrer equation (equation 1) and tabulated in Table 1.

Fig. 3. XRD pattern of the LTCC substrate sintered at various sintering temperature.

Generally sintering temperature plays an important role in determining the crystallite size of the silver paste. However the range of sintering temperature 800 °C and 825 °C in this study did not play an important role in the variation of the crystallite size evolution due to the stable growth of the grains. However, increasing sintering temperature slightly variation of crystallite size was obtained. Thus a further investigation of sintering temperature whether increase or decrease is necessary in order to see the significance different of the crystallite size.


Table 1. Parameter of XRD.

### **3.2 Physical measurements**

#### **3.2.1 Density**

66 Sintering of Ceramics – New Emerging Techniques

Results of the crystallization behaviour of glass-ceramic Ferro A6S substrate sintered at various sintering temperatures are presented in Fig. 3. It can be seen that the small intensity peaks which appeared for the 800 °C sintering belongs to crystalline phases of Calcium Silicates with various stoichiometric ratios Ca2SiO4, and Ca3SiO5. Increasing the temperature to 825 °C and 850 °C, the two phases of Calcium silicate phase are still maintained but accompanied by a new phase of Calcium Borate (CaB2O4). As the sintering temperature increases the formation of Danburite (CaB2Si2O8) start to appear in small intensities while crystalline phases of Ca2SiO4, Ca3SiO5 including CaSiO3 and CaB2O4 phase are maintain in this composite system. They are the stable phases when crystallization is complete. Due to the high peak intensity of CaSiO3 at 2θ ≈ 29.9572, it can be noted that the main phase in this system is Calcium Silicate CaSiO3. The above results were consistent with the work done by Chang and Jean (1999). However, from their studies, they concluded that no crystalline phase was detected at temperatures below 850 °C. The crystallite size as a function of sintering temperature was calculated from the most intense peak (3 1 1) by using the

Fig. 3. XRD pattern of the LTCC substrate sintered at various sintering temperature.

Generally sintering temperature plays an important role in determining the crystallite size of the silver paste. However the range of sintering temperature 800 °C and 825 °C in this study did not play an important role in the variation of the crystallite size evolution due to the stable growth of the grains. However, increasing sintering temperature slightly variation of crystallite size was obtained. Thus a further investigation of sintering temperature whether increase or decrease is necessary in order to see the significance different of the

**3. Results and discussion** 

Scherrer equation (equation 1) and tabulated in Table 1.

**3.1 Phase analysis** 

crystallite size.

The physical data of the LTCC substrate fired at various temperatures are tabulated in Table 2. The relationship of density with sintering temperatures is shown by Fig. 4. The results reveal decreasing trend of the density with sintering temperature with the highest density being 2.992 g/cm3 at 800 °C and the lowest density is 2.806 g/cm3 at 900 °C. The densities of all the samples were between 92 to 99 % of theoretical density (3.018 g/cm3). This trend seems to contradict the normal expected phenomenon that increasing sintering temperature should increase the density. As is well known the sintering process of a ceramic based material is the sintering of the powder compact into the final material. During this step, the porosity decreases and the microstructure of the material develop; this determines its final performance. During the sintering of a homogeneous material the porosity induced during the preparation of the green compact gradually decreases, depending on the powder morphology, agglomeration, the presence of liquid phase sintering and the sintering condition itself. However, the sintering of heterogeneous materials in the LTCC substrate, reactive sintering occurs in the concurrent process of reaction and densification during sintering. A variety of reactions are possible: oxidation-reduction, phase transition or solid solution formation. In this way reaction caused by impurities, additives or other product formed during heating which are often included in the normal sintering process may imply some sort of reactive sintering which usually generates additional porosity. This sintering process is complicated because the phase changes are involved. Thus the understanding of material behaviors such as binder burnout, densification mechanisms of LTCC, pore evolution and deformation of suspended LTCC is important in optimizing the fabrication process for multilayer LTCC substrates as well as for tailoring new LTCC systems (Kemethmuller et al., 2007).


Table 2. Physical properties of LTCC tape samples fired at various sintering temperature.

The Effects of Sintering Temperature Variations on Microstructure Changes of LTCC Substrate 69

In general, dimensional control is a fundamental problem in ceramic processing. In this cofired package the dimensional control, reproducibility and consistency of the shrinkage value to exact tolerance are strongly required and critical. Better reproducibility increases the uniformity of finished product characteristics and thereby increases the process yield. Each process step must complement the preceding and subsequent steps to achieve reproducibility. The main factor governing the reproducibility is the control of shrinkage. This rigid dimensional control in terms of shrinkage is the key to manufacturing high quality, high yield LTCC devices especially when the dimensions of lines and vias continuously decrease. Furthermore, repeatable shrinkage results are also needed for accurate positioning of circuit features such as vias, landing pads, cavities, surface mount component placement and post-fired printing and testing alignment (Sawhill, 1988). Dimensional uncertainties can result in disregistries of the laminated substrate due to the difficult control of the shrinkage amount (Raj and Cannon, 1999). So, to achieve desired properties and desired dimension, the process engineer must establish control of critical

It is clear that the linear shrinkage as tabulated in Table 2 increases up to 875 °C and drops slightly at 900 °C. It was due to the reactivity of the cofired material components such as ceramic oxide, glass, metal and organic solvent. This reaction also depends on the firing condition such as temperature, time and ambient atmosphere (Rabe et al., 2005). Typically, the shrinkage of the LTCC substrate across its width (XY) will be nearly identical while for the Z direction it has big variations. In the commercial practice, the standard values for XY and Z shrinkage are about 14-17 % and 20-25 % respectively. In this work the shrinkage was about 13-15 % for XY and 21-24 % for Z direction (Table 2). Chiang et al. (2011) has noted that in his work the shrinkage in the thickness direction is always larger than that in-plan shrinkage due to more organic material accumulated on the top surface of the green tape caused by unidirectional drying. However for the XY direction the better particle packing is

Microstructural characterization of ceramic packaging materials plays an important role in the understanding and improvement of most material properties such as thermal, electrical and mechanical properties (Pinckney and Beall, 2008). The properties of LTCC material are determined not only by the chemical composition and crystal structure but are also governed by the microstructural features such as density, grain size, porosity, intra- and inter-granular pore distribution, phases, crystalline morphology, crystallography and the chemistry of the interfaces. Such a microstructural arrangement can produce property inhomogeneities. The inhomogeneities usually occur during sample preparation. Brook (1988) has start earlier investigation on the role of inhomogeneities in the sintering process of alumina. Since early work on the subject, abnormal grain growth in alumina has often been attributed to inhomogeneities. He also noted that there are two principal kinds of inhomogeneities that may exist in a green or sintered body; extrinsic and intrinsic. Extrinsic inhomogeneities are associated with imperfect processing such as chemical segregation during drying, agglomeration or defect in powder consolidation. Intrinsic inhomegeneities are associated with anisotropy in the material. In a later investigation by Cho et al., (2000)

**3.2.2 Linear shrinkage** 

process variables.

**3.3 Microstructure** 

believed to reduce shrinkage during sintering.

Fig. 4. The plot of the density with the variation of sintering temperature for substrate with metal conductor.

According to the Ferro guideline the tape A6S used in this work is a crystallizing Ca-B-Si-O system and CN33-391 used as a thick-film material (Ferro; Muralidhar et al., 1992). Both of them typically consist of glass and/or oxide dispersed in organic medium. Owing to the coexistence of ceramic filler and glass in LTCCs, liquid phase sintering can be considered; due to the large amount of glass, viscous sintering can be assumed, so the liquefaction of glass has a dominant role. The presence of these mechanisms increased the volume and size of pores inside the material. The density drop due to porosity is based on

the same weight for all the samples i.e.; for *<sup>M</sup> V* ρ= ; M does not change but V increases to

accommodate higher porosity (more pores) within the samples. However in the present case, the porosity present inside the substrate cannot support the decreasing trend of density with increasing sintering temperature. So, it is believed that the main reason of the decreasing trend of the density with increasing sintering temperature could be due to the chemical reaction e.g. between some components such as silver in the system with the oxygen. The chemical reaction can result in an expanded volume for the system. Referring back to the XRD analysis in Fig. 3, increasing sintering temperature the phases of Calcium Silicate (CaSiO3) become more stable with the highest intensity for each samples. This phase has the lowest theoretical density (2.92 g/cm3) among other phases. This finding was consistent with the results found by Erol and his co-workers in 2009 from their studies on the influence of the binder on the properties of sintered glass-ceramics produced from industrial wastes (Erol et al., 2009). They concluded that the densities of the sintered glass-ceramic samples changed depending on the amount of crystalline phase. The higher the content of crystalline phases with high density, the higher the measured density; the higher the amount of crystalline phase with the lower density, the lower the measured density will be obtained. So this might be a reason why the decreasing density was obtained in this present work.

#### **3.2.2 Linear shrinkage**

68 Sintering of Ceramics – New Emerging Techniques

780 800 820 840 860 880 900 920 **Sintering temperature (**<sup>ο</sup>**C)**

Fig. 4. The plot of the density with the variation of sintering temperature for substrate with

According to the Ferro guideline the tape A6S used in this work is a crystallizing Ca-B-Si-O system and CN33-391 used as a thick-film material (Ferro; Muralidhar et al., 1992). Both of them typically consist of glass and/or oxide dispersed in organic medium. Owing to the coexistence of ceramic filler and glass in LTCCs, liquid phase sintering can be considered; due to the large amount of glass, viscous sintering can be assumed, so the liquefaction of glass has a dominant role. The presence of these mechanisms increased the volume and size of pores inside the material. The density drop due to porosity is based on

> *V* ρ

accommodate higher porosity (more pores) within the samples. However in the present case, the porosity present inside the substrate cannot support the decreasing trend of density with increasing sintering temperature. So, it is believed that the main reason of the decreasing trend of the density with increasing sintering temperature could be due to the chemical reaction e.g. between some components such as silver in the system with the oxygen. The chemical reaction can result in an expanded volume for the system. Referring back to the XRD analysis in Fig. 3, increasing sintering temperature the phases of Calcium Silicate (CaSiO3) become more stable with the highest intensity for each samples. This phase has the lowest theoretical density (2.92 g/cm3) among other phases. This finding was consistent with the results found by Erol and his co-workers in 2009 from their studies on the influence of the binder on the properties of sintered glass-ceramics produced from industrial wastes (Erol et al., 2009). They concluded that the densities of the sintered glass-ceramic samples changed depending on the amount of crystalline phase. The higher the content of crystalline phases with high density, the higher the measured density; the higher the amount of crystalline phase with the lower density, the lower the measured density will be obtained. So this might be a reason why the

= ; M does not change but V increases to

2.75

the same weight for all the samples i.e.; for *<sup>M</sup>*

decreasing density was obtained in this present work.

2.8

2.85

2.9

**Density (g/cm**

metal conductor.

 **3)**

2.95

3.05

3

In general, dimensional control is a fundamental problem in ceramic processing. In this cofired package the dimensional control, reproducibility and consistency of the shrinkage value to exact tolerance are strongly required and critical. Better reproducibility increases the uniformity of finished product characteristics and thereby increases the process yield. Each process step must complement the preceding and subsequent steps to achieve reproducibility. The main factor governing the reproducibility is the control of shrinkage. This rigid dimensional control in terms of shrinkage is the key to manufacturing high quality, high yield LTCC devices especially when the dimensions of lines and vias continuously decrease. Furthermore, repeatable shrinkage results are also needed for accurate positioning of circuit features such as vias, landing pads, cavities, surface mount component placement and post-fired printing and testing alignment (Sawhill, 1988). Dimensional uncertainties can result in disregistries of the laminated substrate due to the difficult control of the shrinkage amount (Raj and Cannon, 1999). So, to achieve desired properties and desired dimension, the process engineer must establish control of critical process variables.

It is clear that the linear shrinkage as tabulated in Table 2 increases up to 875 °C and drops slightly at 900 °C. It was due to the reactivity of the cofired material components such as ceramic oxide, glass, metal and organic solvent. This reaction also depends on the firing condition such as temperature, time and ambient atmosphere (Rabe et al., 2005). Typically, the shrinkage of the LTCC substrate across its width (XY) will be nearly identical while for the Z direction it has big variations. In the commercial practice, the standard values for XY and Z shrinkage are about 14-17 % and 20-25 % respectively. In this work the shrinkage was about 13-15 % for XY and 21-24 % for Z direction (Table 2). Chiang et al. (2011) has noted that in his work the shrinkage in the thickness direction is always larger than that in-plan shrinkage due to more organic material accumulated on the top surface of the green tape caused by unidirectional drying. However for the XY direction the better particle packing is believed to reduce shrinkage during sintering.

#### **3.3 Microstructure**

Microstructural characterization of ceramic packaging materials plays an important role in the understanding and improvement of most material properties such as thermal, electrical and mechanical properties (Pinckney and Beall, 2008). The properties of LTCC material are determined not only by the chemical composition and crystal structure but are also governed by the microstructural features such as density, grain size, porosity, intra- and inter-granular pore distribution, phases, crystalline morphology, crystallography and the chemistry of the interfaces. Such a microstructural arrangement can produce property inhomogeneities. The inhomogeneities usually occur during sample preparation. Brook (1988) has start earlier investigation on the role of inhomogeneities in the sintering process of alumina. Since early work on the subject, abnormal grain growth in alumina has often been attributed to inhomogeneities. He also noted that there are two principal kinds of inhomogeneities that may exist in a green or sintered body; extrinsic and intrinsic. Extrinsic inhomogeneities are associated with imperfect processing such as chemical segregation during drying, agglomeration or defect in powder consolidation. Intrinsic inhomegeneities are associated with anisotropy in the material. In a later investigation by Cho et al., (2000)

The Effects of Sintering Temperature Variations on Microstructure Changes of LTCC Substrate 71

of its curvature (See Fig. 5). Owing to this movement the grain growth takes place i.e. an increase in the size of the grains with many neighbors at the expense of grains with few

> A B

Across the curved grain boundaries, there is the difference in the chemical potential resulting from the stresses which should be directed from the concave side of the curved interface to its convex side. Thus atoms on the convex side are in a state of compression and those on the concave side are in tension. This brings about a change in the chemical potential of the atoms (Rahaman, 1995). The gradient of the chemical potential constitute the driving force for the process of transferring atoms from the convex to the concave side across the curved grain boundary; this causes the movement of the boundary towards the centre of its curvature i.e. in a manner increasing the radius of the curvature. Owing to this movement, grain growth takes place, i.e. an increase in the size of the grains with many neighbors at the expense of grains with few neighbors. The rate of growth is proportional to the driving force and the driving force is proportional to the total amount of grain boundary energy. The driving force for each crystal to grow or shrink is given by the free energy difference between the atoms on the convex side and its difference across the interface

> 

*VG* (7)

2 1

 +=Δ

Where ∆G is the change in the free energy on going across the curved interface, γ is the boundary energy *V* is the molar volume and r1 and r2 are the principal radii of the

Glass-ceramics formed by the controlled nucleation and crystallization of a glass precursor. As mention in the previous section, the Ferro A6S tape system consists of ceramic filler in glass matrix. The formation of a liquid phase is helpful to improve the development of microstructure behavior of the substrate. For glass-ceramics based on internal nucleation and grain growth, a general evolutionary pattern is observed in the crystallization cycle: amorphous phase separation and/or precipitation of primary crystalline nuclei, nucleation

γ

1 1 *rr*

neighbors.

Fig. 5. Classical picture of a grain boundary.

amounts which given by the equation 7,

**3.3.2 Glass-ceramic tape microstructure** 

curvature.

and Sone et al., (2001) has shown that the impurity inhomogeneities in alumina would also produce abnormal grain growth. So, the desirable properties at particular frequencies can be improved by carefully controlling of the microstructure where the change of microstructure is the main issue in engineering materials.

### **3.3.1 Grain growth**

One of the microstructural aspects normally studied is the grain growth. To date there exists a fairly detailed understanding of how grains of a sintered material can grow. According to Kingery (1976) the normal grain growth is the process by which the average grain size of the material increases continuously without a change the graphical shape of the grain size distribution during the heat treatment. Grain growth in polycrystalline materials is conventionally considered to be the results of the migration of grain boundary in response to the driving force provided by the system energy associated with the reduction in the total interface area by interface migration (Dillon and Rohrer, 2009). Since mass is conserved, some grains get smaller and disappear while others grow larger. In grain growth, the fundamental process is transferring an atom across the boundary from one grain to another as shown in Fig. 5. In this respect, it is well known that sintering is an interfaced related process in terms of driving force (Jo et al., 2006). This curvature–drive motion of grain boundary can lead to a normal behavior.

The occurrence of grain growth involves the elimination of free surface, i.e. the elimination of pores during sintering and is one of the ways in which there occurs a spontaneous decrease in the free energy during heating of a system with large interfacial areas (Pampuch, 1976). Interface energy is associated with the boundary between individual grains. A decrease in the free energy of such a system is also possible when the area of interfaces between the grains (grain boundary) decreases because of the excess energy possessed by these boundaries. The decrease in the area of the grain boundaries means an increase in the size of the grains. This is possible when some grains grow at the expense of others as the consequence of the movement of the grain boundaries toward their centers of their curvature. The boundary velocity has been observed to be linearly dependent on the curvature. This phenomenon can be explained in terms of the capillary force. The driving force for the grain growth process is the difference between the fine grain material and the larger grain size product resulting from the decrease in grain boundary area and the total boundary energy (Kingery, 1976). Grain growth must be controlled by even significant defects of the texture of the poly-crystal. It may be stopped by the presence of foreign inclusions (Shackelford, 1992). Besides the solid phase inclusions, the most important influence is exerted by pores (inclusion of gaseous phase). This explains the fact that, in some cases, grain growth does not take place at all, or wherever it is observed, it takes place only after the elimination of most of the pores, or after a considerable decrease in the volume fraction of pores on sintering.

Atoms can move across the grain boundary as well as within crystals when heated at high temperature. Equal numbers of atoms cross a plane boundary in opposite directions. However, atoms on a concave-surfaced boundary are likely to have more neighbors, and hence less energy than atoms on a convex-surfaced boundary. This brings about the chemical potential of the atoms. This causes a movement of the boundary towards the center of its curvature (See Fig. 5). Owing to this movement the grain growth takes place i.e. an increase in the size of the grains with many neighbors at the expense of grains with few neighbors.

Fig. 5. Classical picture of a grain boundary.

70 Sintering of Ceramics – New Emerging Techniques

and Sone et al., (2001) has shown that the impurity inhomogeneities in alumina would also produce abnormal grain growth. So, the desirable properties at particular frequencies can be improved by carefully controlling of the microstructure where the change of microstructure

One of the microstructural aspects normally studied is the grain growth. To date there exists a fairly detailed understanding of how grains of a sintered material can grow. According to Kingery (1976) the normal grain growth is the process by which the average grain size of the material increases continuously without a change the graphical shape of the grain size distribution during the heat treatment. Grain growth in polycrystalline materials is conventionally considered to be the results of the migration of grain boundary in response to the driving force provided by the system energy associated with the reduction in the total interface area by interface migration (Dillon and Rohrer, 2009). Since mass is conserved, some grains get smaller and disappear while others grow larger. In grain growth, the fundamental process is transferring an atom across the boundary from one grain to another as shown in Fig. 5. In this respect, it is well known that sintering is an interfaced related process in terms of driving force (Jo et al., 2006). This curvature–drive motion of grain

The occurrence of grain growth involves the elimination of free surface, i.e. the elimination of pores during sintering and is one of the ways in which there occurs a spontaneous decrease in the free energy during heating of a system with large interfacial areas (Pampuch, 1976). Interface energy is associated with the boundary between individual grains. A decrease in the free energy of such a system is also possible when the area of interfaces between the grains (grain boundary) decreases because of the excess energy possessed by these boundaries. The decrease in the area of the grain boundaries means an increase in the size of the grains. This is possible when some grains grow at the expense of others as the consequence of the movement of the grain boundaries toward their centers of their curvature. The boundary velocity has been observed to be linearly dependent on the curvature. This phenomenon can be explained in terms of the capillary force. The driving force for the grain growth process is the difference between the fine grain material and the larger grain size product resulting from the decrease in grain boundary area and the total boundary energy (Kingery, 1976). Grain growth must be controlled by even significant defects of the texture of the poly-crystal. It may be stopped by the presence of foreign inclusions (Shackelford, 1992). Besides the solid phase inclusions, the most important influence is exerted by pores (inclusion of gaseous phase). This explains the fact that, in some cases, grain growth does not take place at all, or wherever it is observed, it takes place only after the elimination of most of the pores, or after a considerable decrease in the

Atoms can move across the grain boundary as well as within crystals when heated at high temperature. Equal numbers of atoms cross a plane boundary in opposite directions. However, atoms on a concave-surfaced boundary are likely to have more neighbors, and hence less energy than atoms on a convex-surfaced boundary. This brings about the chemical potential of the atoms. This causes a movement of the boundary towards the center

is the main issue in engineering materials.

boundary can lead to a normal behavior.

volume fraction of pores on sintering.

**3.3.1 Grain growth** 

Across the curved grain boundaries, there is the difference in the chemical potential resulting from the stresses which should be directed from the concave side of the curved interface to its convex side. Thus atoms on the convex side are in a state of compression and those on the concave side are in tension. This brings about a change in the chemical potential of the atoms (Rahaman, 1995). The gradient of the chemical potential constitute the driving force for the process of transferring atoms from the convex to the concave side across the curved grain boundary; this causes the movement of the boundary towards the centre of its curvature i.e. in a manner increasing the radius of the curvature. Owing to this movement, grain growth takes place, i.e. an increase in the size of the grains with many neighbors at the expense of grains with few neighbors. The rate of growth is proportional to the driving force and the driving force is proportional to the total amount of grain boundary energy. The driving force for each crystal to grow or shrink is given by the free energy difference between the atoms on the convex side and its difference across the interface amounts which given by the equation 7,

$$
\Delta G = \gamma \overline{V} \left( \frac{1}{r\_1} + \frac{1}{r\_2} \right) \tag{7}
$$

Where ∆G is the change in the free energy on going across the curved interface, γ is the boundary energy *V* is the molar volume and r1 and r2 are the principal radii of the curvature.

#### **3.3.2 Glass-ceramic tape microstructure**

Glass-ceramics formed by the controlled nucleation and crystallization of a glass precursor. As mention in the previous section, the Ferro A6S tape system consists of ceramic filler in glass matrix. The formation of a liquid phase is helpful to improve the development of microstructure behavior of the substrate. For glass-ceramics based on internal nucleation and grain growth, a general evolutionary pattern is observed in the crystallization cycle: amorphous phase separation and/or precipitation of primary crystalline nuclei, nucleation

The Effects of Sintering Temperature Variations on Microstructure Changes of LTCC Substrate 73

a b c

**1** μ**m 1** μ**m 1** μ**m** 

e

**1** μ**m** 

Fig. 6. SEM micrograph of laminated sample sintered at a) 800 °C, b) 825 °C, c) 850 °C, d) 875

d

**1** μ**m** 

The whole densification rate at the final stage, is a summation of the developed local internal defect such as the presence of pores that are no longer large enough to prevent grain growth and this is the major process going on in addition to the final densification. With the increasing grain size, the densification rate decreases as the distance of the defects to the grain boundaries increases. Grain growth also gives pore coalescence where smaller pores are merged together into larger ones: this also reduces the densification rate and explains the density results obtained. In the case of the sintering process of glass ceramic material, if crystallization occurs before densification, the viscosity of samples will be increased. It is due to the contribution of glass composition into a crystalline phase structure, resulting in the reduction of viscous flow of the system. As a result, densification through viscous flow sintering will not occur properly and a porous body will be formed

In the LTCC technology, the microstructure changes involves the combination of a substrate and conductor material where LTCC tape materials contain a glass binder and organic solvent for tape casting purpose, thick-film conductor also contain glass frit as adhesion element with the substrate via formation of a vitreous bond (Kuromitsu et al., 1994). However, knowledge about the interaction of ceramic filler and amorphous matrix is very limited. Moreover the understandings of how an amorphous matrix influence the crystal grain growth in the metal film is not yet clear (Liu and Shen, 2004). According to Yajima and Yamaguchi (1984) the densification process of the substrate and printed film are depends upon sintering where the conductor and the substrate consisting of CaO-B2O3-SiO2 sinter simultaneously. So the liquid phase formed is believed to help densification of metal

°C and e) 900 °C.

(Banijamali et al., 2009).

**3.3.3 Silver grain microstructure** 

powder to a denser packing of grains (see Fig. 7).

and growth of metastable crystalline phases and approach to a stable crystalline structure (Pinckney and Beall, 2008). Amorphous phase separation is generally the first stage in glassceramic formation. This phase is highly unstable as a glass and will precipitate primary crystalline nuclei on heating at temperature near the annealing point of the host glass. The next nucleation stage usually involves the heterogeneous nucleation and growth of metastable crystalline phases on the primary crystalline nuclei, resulting in a fine-grained metastable solid solution assemblage. Finally with increasing temperature, this metastable phase assemblage breaks down into stable crystalline phases by means of crystal phase transformation, reaction between metastable phases or a combination of several of these mechanisms.

The effects of sintering temperature on microstructure of glass-ceramic tape are shown in Fig. 6. It is clearly seen that there is inhomogeneous microstructure for the entire sample with some sample showing a big size pore meaning that probably only a few crystallite sizes present in the bulk samples. This feature of the microstructure could be due to the agglomeration within the sample. In the LTCC process, some particles are densely packed and some particles are loosely packed due to the agglomeration of particle at some places. The presence of agglomeration is a common problem in ceramics processing and influenced the microstructure behavior of the whole substrate. As mentioned by Lange (1984) and Hirata et al., (2009), when a laminated substrate or powder compact is heated, the inhomegeneity of the packing provides the different densification rate and the produce a microstructure which is usually not uniform when agglomeration is severe. An inhomogeneous distribution of particles leads to an inhomogeneous liquid distribution such that there is no driving force for redistribution of the liquid, so the densification rate is not homogeneous and the microstructure development also becomes inhomogeneous.

Based on their research, Deng et al., (2007) studied the microstructure of porous ZrO2 ceramics and found that the agglomeration resulted in localized non-uniform shrinkage and the large pores are believed to originate from the large interagglomerated particles and greater microstructure non-uniformity. The agglomerated tape however leads to lower densities with large shrinkage deviations in particular direction giving a poor quality (Raj and Cannon, 1999). Agglomeration promotes uneven sintering which sometimes results in a mechanically weak and porous product. Thus, to achieve a high density material and good microstructure development, the agglomeration needs to be controlled (Forrester et al., 2008).

The densification process of the glass-ceramic composite can be described by the conventional three-stage liquid phase sintering; particle rearrangement, dissolution and precipitation and solid state sintering. The presence of glass in this composite system, acts as a sintering aid which produces liquid phase formation at a temperature lower than the sintering temperature and may considerably increase the rate of sintering. The viscous liquid which occurs during the sintering process may promote an additional diffusion mechanism of dissolution/precipitation, particle rearrangement and capillary forces and finally affect the densification at high temperature. Compared to the solid state sintering, liquid phase sintering enhances densification by two mechanisms; the densification is caused by mass transport via lattice diffusion from grain boundaries and grain boundary diffusion that should be encouraged to obtain high density when the sintering temperature is increased.

Fig. 6. SEM micrograph of laminated sample sintered at a) 800 °C, b) 825 °C, c) 850 °C, d) 875 °C and e) 900 °C.

The whole densification rate at the final stage, is a summation of the developed local internal defect such as the presence of pores that are no longer large enough to prevent grain growth and this is the major process going on in addition to the final densification. With the increasing grain size, the densification rate decreases as the distance of the defects to the grain boundaries increases. Grain growth also gives pore coalescence where smaller pores are merged together into larger ones: this also reduces the densification rate and explains the density results obtained. In the case of the sintering process of glass ceramic material, if crystallization occurs before densification, the viscosity of samples will be increased. It is due to the contribution of glass composition into a crystalline phase structure, resulting in the reduction of viscous flow of the system. As a result, densification through viscous flow sintering will not occur properly and a porous body will be formed (Banijamali et al., 2009).
