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Prägeverfahren bieten eine interessante Alternative zur Strukturierung keramischer Folien und Laminate. Im Gegensatz zur etablierten Stanz- oder Laserbearbeitung von Einzelfolien sind damit einerseits Strukturtiefen realisierbar, die nicht einem Vielfachen der Einzelfoliendicke entsprechen, andererseits können Strukturierungsgrade erzielt werden, bei denen perforierte Einzelfolien nicht mehr handhabbar sind. Heißprägen grüner Folien und Laminate erfolgt bei Temperaturen um 130 °C mit Metallstempeln. Die plastische Verformung erfolgt zeitabhängig. Dabei sind die Fließeigenschaften des polymeren Binders der Grünfolie maßgeblich für die Konturtreue der Prägung. Randeinzug und Gründichtegradienten um den Eindruck können zu Defekten im Sintergefüge führen. Je nach Sinterverfahren muss die Schwindung der geprägten Struktur berücksichtigt werden, eine Kombination dieses Verfahrens mit zero-shrinkage Techniken ist aber möglich. Sinterprägen stellt eine Erweiterung der Drucksintertechnologie für glaskeramische Komposite dar. Dabei wird ein Pressstempel aus glasartigem Kohlenstoff während des Brandes in die erweichte Glasphase des Komposits gedrückt. Die Abformung erfolgt durch viskosen Fluss der Glasphase. Mit Kenntnis des Dichte- und Viskositätsverlaufs des Werkstoffs kann das Verfahren so gestaltet werden, dass die Pressform konturtreu abgeformt wird. Mittels Sinterprägen lassen sich auch erhabene Strukturen realisieren, wobei diese häufig eine erhöhte Porosität aufweisen.
Advancements in pressure-assisted sintering technology for low temperature co-fired ceramics (LTCC)
(2016)
Steadily increasing demands on design and dimensional accuracy of ceramic multilayer modules, as well as the processing of new materials, require continuous improvements of manufacturing technology, especially thermal processes. The capabilities of pressure-assisted sintering (PAS) for the manufacturing of highly integrated low temperature co-fired ceramics (LTCC) multilayer have been considerably extended in the last years by procedural and device-related advancements. A lambda probe has been integrated in a sintering press prototype to monitor and control the process atmosphere. Thereby, the development of oxygen partial pressure during binder burnout of real modules can be observed. On the other hand, the oxygen partial pressure can be regulated during densification, for example to prevent diffusion of silver from circuit paths into the surrounding LTCC. Thin-film capable surfaces can be produced without post-processing by using setter plates made of glass-like carbon in an advanced PAS process under nitrogen. A newly developed advancement of this approach enables in-situ hot-embossing of LTCC during PAS by using structured glass-like carbon molds. The prototype press is further extended by a sensitive displacement transducer for monitoring the thickness shrinkage of real modules with an edge length of up to 8 inch.
A novel process to structure the surfaces of low temperature co-fired ceramics (LTCC) is presented. Lowered and raised structures are formed by hot-embossing with glass-like carbon molds during pressure-assisted sintering. Molding is driven by viscous flow of the LTCC glassy phase above the glass transition temperature. For accurate molding of embossments on the LTCC surface, proper filling of cavities in the glass-like carbon mold is necessary. Therefore, de-airing of the mold cavity has to be assured. Two strategies have been investigated: (i) hot-embossing at 850 °C after termination of LTCC shrinkage with de-airing through vent holes in the mold; and (ii) hot-embossing of open porous LTCC at 775 °C with dense molds, de-airing through pore channels in the LTCC, and subsequent densification by further heating to 850 °C. Circular embossments with 10 mm diameter were molded on a commercially available LTCC (Ceramtape GC, CeramTec GmbH, Marktredwitz, Germany). The sintered height was measured using optical profilometry. Image processing was used to evaluate porosity distributions in the sintered structures. The influence of embossing temperature on LTCC viscosity and mold filling behavior is discussed. Successful molding of 47 µm high raised grids and characters by hot embossing with 0.41 MPa at 775 °C and further heating to 850 °C under constant load is demonstrated. Thereby, the high potential of hot-embossing for precise structuring of LTCC surfaces is illustrated.
Glass-ceramic composite multilayer structures - modeling of densification and shrinkage mismatch
(2016)
Low temperature co-fired ceramics (LTCC) are glass-ceramic composites that are processed by tape casting and multilayer technology to create multilayer circuit boards and electronic packages for high frequency applications, microsystems, and sensors. Pressure-assisted sintering (PAS) enables the densification of LTCC without lateral shrinkage, and thereby the manufacturing of miniaturized modules with minimal shrinkage tolerances. Co-firing of LTCC with other functional ceramics facilitates the implementation of innovative module concepts. The master sintering curve model is a useful tool to supplement the empirical optimization of such sintering processes. Two applications of the model are presented. Firstly, an algorithm is deduced that allows the simulation of shrinkage mismatch in a combined multilayer structure. Based on this simulation, an optimized process for the co-firing of LTCC and ferrite is derived. Secondly, the pressure-assisted sintering of LTCC is modeled. A comprehensive description of the densification behavior of LTCC during PAS is thereby possible.