Densification and characterization of SiO2-B2O3-CaO-MgO glass/Al2O3 composites for LTCC application
Introduction
In the past decades, low temperature cofired ceramics (LTCC) substrate and related packaging technology have been extensively studied due to their practical and viable merits such as the utilisation of the low melting point of the highly conductive internal electrode metals (e.g., 961 °C for silver and 1083 °C for copper) and the increment of integration density [1], [2], [3]. LTCC substrates commonly combine many layers of ceramic and conductors resulting in multilayer modules and have been developed to achieve the criteria of high signal propagation speed, good reliability, and low cost [1], [4], [5]. Furthermore, in order to meet the requirement for desired LTCC substrates, the permittivity of the LTCC materials should be lower than that of Al2O3 (∼9.8) to diminish signal propagation delay.
Several low-permittivity dielectric compositions have been reported, including production of glass ceramic and glass/ceramic composites [6]. The starting material used in the glass ceramic approach is a pure glass such as cordierite glass, which densifies first, followed by crystallisation. The physical properties of the resulting composition are controlled by the degree of crystallisation, which can be enhanced by addition of a small amount of crystalline phase which acts as a nucleating agent. For the glass/ceramic approach, the low melting glass acts as a densification flux to enhance densification, and the ceramic fillers act to adjust the physical properties of the resulting composites [1], [7], [8]. Comparing the two approaches, it is a more general method to add glass frits into the ceramic to attain good electrical properties together with acceptable densification at temperature range of less than 900 °C [1]. Hence, the glass/ceramic approach is also taken in the present study.
Most of the known glasses in the LTCC materials are ZnO-B2O3 [9], BaO-ZnO-B2O3 [10], [11], Li2O-B2O3-SiO2-CaO-Al2O3 [12], MgO-B2O3-SiO2 [13], BaO-B2O3-SiO2 [13], CaO-B2O3-SiO2 [14], PbO-B2O3-SiO2 [10], SiO2-B2O3-ZnO [10], B2O3-La2O3-MgO-TiO2 [15], etc. Normally, the compositions of the low melting glass have typically three or more oxides present. SiO2 and B2O3 commonly form the network structures of glass [16]. PbO, MgO, CaO, Na2O, K2O, and Li2O are modifier oxides and can be used to tailor the physical properties of glass such as lowering the softening point, increasing the thermal expansion or enhancing the chemical durability [1], [16].
In this work, attention was focused on the LTCC system based on lead-free SiO2-B2O3-CaO-MgO-Na2O-K2O glass (SBCM glass) and Al2O3 filler. The effects of the glass on sintering, shrinkage, thermal expansion, thermal conductivity, flexural strength, and dielectric properties were investigated in this work.
Section snippets
Preparation of the glass and composites
The batch of raw materials corresponding to the low melting glass with the composition 25 wt% SiO2, 52 wt% B2O3, 10 wt% CaO, 10 wt% MgO, 1 wt% Na2O, and 2 wt% K2O was prepared by melting powders containing appropriate amounts of reagent grade SiO2 (99.9%; Bodi Chemical Co. Ltd., China), H3BO3 (99.0%; Damao Chemical Reagent Co. Ltd., China), MgO (99.9%; Bodi Chemical Co. Ltd., China), CaCO3 (99.9%; Bodi Chemical Co. Ltd., China), Na2CO3 (99.9%; Damao Chemical Reagent Co. Ltd., China) and K2CO3 (99.9%;
Phase composition
The X-ray diffraction patterns of the samples containing 50 wt% Al2O3 recorded between 825 and 925 °C are shown in Fig. 1(a). The pre-existing Al2O3 was the main crystalline phase presented in the composites, whereas the additional crystalline phases, anorthite (CaAl2Si2O8) and cordierite (Mg2Al4Si5O18), were found at all sintering temperatures. The formation of anorthite and cordierite might involve the reaction of Al2O3 filler with glass. Moreover, the anorthite phase increased with the
Conclusions
A glass/ceramic composite based on lead-free low melting glass (SiO2-B2O3-MgO-CaO glass) with Al2O3 filler was fabricated at a sintering temperature of ∼875 °C. In the sintering process, the chemical reactions between glass and Al2O3 resulted in the appearance of anorthite and cordierite phases. The sintering behaviour, dielectric and thermal properties were sensitive to the amount of Al2O3 filler and sintering temperature. At least 50 wt% of SBCM glass content was required for sufficient
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