Wolfgang A. Schiller

LTCC-Modules with Integrated Ferrite Layers-Strategies for Material Development and Co-Sintering

The integration of passive components (resistors, capacitors, inductors) into LTCC modules is a challenging task in multilayer ceramics technology. We report on multilayer assemblies consisting of ...

Development of Advanced Low Temperature Co-Fired Ceramics (LTCC)

An important advantage of LTCC is the huge variability concerning an attainable property spectrum. Both crystalline and glassy raw materials can be combined and different material concepts (e.g. glass ceramic composites, glass bonded ceramics) are available. The combination of tapes with tailored properties (e.g. permittivity, sintering behaviour....) inside of one laminate further expands the variability of LTCC. Special miniaturized microwave modules were produced by combination of LTCC tapes with low and medium permittivities. Low shrinkage in lateral direction and low tolerances of shrinkage are an indispensable precondition for highdensity component configuration in LTCC modules. A proper combination of tapes with separate sintering ranges can be used to prevent lateral shrinkage. First results of this alternative “zero shrinkage” concept (without sacrificial tapes) are presented. Introduction LTCC are preferably used where integration of passive components (resistors, capacitors, inductors...), miniaturization (high savings in volume and mass) and high reliability is demanded. LTCC possesses great potentials on several promising application sectors like wireless communication, sensor technology, electronic control units and micro-systems [1]. But of course it exists a sharp competition with alternative solutions e.g. in the area of electronic application with polymer PCBs (Printed Circuit Boards), thick and thin film hybrid technology. LTCC has been available for many years. The technology has enjoyed a significant market growth in recent years, particularly in microwave applications and automotive electronics. But, to expand the application fields of LTCC, innovations in material development and ceramic processing as well as integration of process control methods in the LTCC technology are constantly necessary. One innovative research direction of advanced LTCC are “two-material” multilayer. Advanced LTCC by “two-material” multilayer challenge and chance Precondition for a successful production of “two-material” multilayer is the development of well adapted materials, respectively tapes. This task is very ambiguous, because both processing parameters (lamination, debindering, sintering) and material properties (high-temperature reactivity, thermal expansion.....) have to be matched. LTCC offers a broad variety of possibilities to tailor microstructure and phase composition. The microstructure of most LTCC materials consists of both glassy and crystalline phases. But the amount can strongly vary. Fig. 1 illustrates, that four types of LTCC materials with different glass amounts are known. Each type shows a characteristic sintering mechanism. Most of the current commercial LTCC materials possess a glass amount higher than 50 vol.-%. They can be classified either in “Glass Ceramic Composites (GCC)” (e.g. Du Pont 951, Heraeus CT 700) or in “Glass Ceramics (GC)” (e.g. Ferro A6) [2]. In the last years also “glass-free” LTCC was introduced, e.g. crystalline phases in the system Bi-Ti-Si-O. But LTCC can also be produced with a low glass amount (about 10 vol.-%), so-called “Glass Bonded Ceramics (GBC)”. In this case glasses with very low viscosity and high reactivity against the crystalline phase are necessary, to allow for solution-reprecipitation process below 950 °C. The multitude of LTCC material concepts expands the chances developing proper tapes for a “two-material” multilayer. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1181-1184 doi:10.4028/www.scientific.net/KEM.264-268.1181

Effects of Dispersed Al2O3 Particles on Sintering of LTCC

The sintering of Low Temperature Co-fired Ceramics prepared from alumoborosilicate glass- and Al2O3 powders of similar small particle size was studied by dilatometry, heating microscopy, microstructure analysis, glass- and effective viscosity measurements. The steric effect of Al3O3 inclusions was studied using a “non-reactive” model composite. With increasing Al3O3 volume fraction ( Φ ≤ 0.45), sintering decelerates and its final stage shifts to higher temperature. The attainable shrinkage is reduced as Al2O3 particle clusters bearing residual pores become more frequent. The kinetics of sintering could be described formally superposing the weighed contributions of differentially sized and randomly composed glass-crystal particle clusters and assuming a sintering rate controlled by the effective matrix viscosity, which increases with Φ and with progressive wetting of Al2O3 particles during densification. The “reactive” model composite shows significant dissolution of Al2O3 into the glass, which has two opposed effects on sintering: reducing Φ and increasing glass viscosity. For the present case ( Φ = 0.25), the latter effect dominates and sintering is retarded by Al2O3 dissolution. Crystallization of wollastonite starts after full densification. Dissolution of Al2O3 was found to promote the subsequent growth of anorthite.

Kinetic Modeling of LTCC Shrinkage : Effect of Alumina Content

We studied sintering of LTCC-type glass matrix composites (GMCs) consisting of small glass and alumina particles of equal size. Primarily, crystals act as rigid inclusions, decelerating the densification rate. In later stages, they also dissolve, partially increasing the viscosity. Release of alumina finally induces crystallization of alumosilicates, which enables post-firing stability. To study both effects, two model GMCs were prepared: an α-Al 2 O 3 + barium alumoborosilicate glass (BABS)-GMC, which shows neither significant dissolution nor crystallization, and an α-Al 2 O 3 + calcium alumoborosilicate glass (CABS)-GMC, which dissolves readily and promotes crystallization. The kinetics of shrinkage for both GMC were modeled by utilizing Frenkel theory for the early stage and Mackenzie-Shuttleworth theory for the late stage, assuming that sintering is superimposed by the weighted contributions of triparticle glass-crystal clusters, their random occurrence (ideal mixing), and a shrinkage rate controlled by the GMC effective viscosity. In agreement with modeling, the experimental results showed that the shrinkage rate of BABS-GMC decreases progressively for crystal volume fractions Φ > 0.15. The attainable shrinkage is reduced by up to 8% for Φ = 0.45. For the CABS-GMC with Φ = 0.25, a reduction of Φ to 0.20 was evident due to partial α-Al 2 O 3 dissolution. This effect was found able to increase the sintering temperature by ∼50-60 K.

Rate Controlled Debindering of Glass Ceramic Composites

Green compacts of ceramics, glass ceramic composites and sinter glass ceramics contain different amounts of organic materials added as pressing aids or binders. Before sintering, these organics have to burn out completely. In oxidising atmospheres, the debindering process is mostly exothermic and therefore difficult to control. This uncontrolled heat production due to locally enhanced debindering and respective gas release may cause damages in the green compact microstructure. Therefore, debindering is usually operated with very low heating rates (< 3 K/min) which requires long processing times of many hours. In this paper, we will show that it is possible to reduce the processing time for debindering dramatically by using the decomposition rate of the organic binder, detected by the weight loss of the sample, as a control factor of the furnace.