Hybridization of additive manufacturing processes to build ceramic/metal parts: Example of HTCC

Abstract

Stereolithography is an additive manufacturing process, which makes it possible to fabricate useful complex 3D ceramic parts with a high dimensional resolution, a good surface roughness and properties close to those obtained by classical routes. Previous work concerning LTCC components, demonstrates that it is possible, by coupling the stereolithography with robocasting additive processes, to obtain multi-material components (e.g. ceramic / metal components). On the base of this previous work, the manufacturing of HTCC components using this innovative hybrid additive manufacturing process is described. Various complex and innovative geometries of HTCC alumina/tungsten components, in order to improve the characteristics of current circuits, are built and the mechanical and electrical properties characterized. Finally, hyper-frequency parameters of simulated HTCC complex micro strip resonators were compared to measured values on components manufactured by additive manufacturing.

Introduction

The "Temperature Cofired Ceramic" multilayer technologies appeared around the beginning of the 90 s. These processes consist in stacking ceramic dielectric sheets elaborated by tape casting, on which conductive patterns, resistances or capacities are printed by screen-printing. The different printed tapes are assembled and laminated to obtain multilayer circuits [1,2]. Then metallized holes through different layers, called vias, allow connections between different conductive patterns through the dielectric layers (Fig. 1).

After lamination, the green stack is submitted to different heat treatments in a controlled atmosphere to eliminate organic components (debinding) and densify both ceramic and metallic materials (co-sintering). Co-sintering can be carried out at high temperature (T > 1000 °C) for High Temperature Cofired Ceramics (HTCC) or at low temperature (T ≤ 1000 °C) for Low Temperature Cofired Ceramics (LTCC). The choice of the sintering temperature will mainly depends on the conditions of use of the component, in particular the temperature. For instance, HTCC parts are used in the power assembly for aerospace [1]. HTCC materials commonly used in industry are alumina (dielectric substrate) coupled to tungsten or molybdenum (metallic tracks and vias) [1].

The conventional manufacture of HTCC and LTCC parts, i.e. tape casting for the substrate and screen-printing for the tracks and vias [3], limits some characteristics of the components:

• The thickness of a ceramic layer must be typically larger than 150 microns in order to properly manipulating the tape-cast green sheet.

• The shape of the metallic conductive pattern is limited to 2D patterns linked by vertical vias between dielectric layers, which limits the complexity of the metallic network and then the electrical functions than can be integrated inside the TCC component [1].

• There is no possibility of developing lateral shielding.

• It is impossible to make slanted vias.

In this context, the present work proposes the use of an hybrid additive process, already used in a previous work concerning LTCC materials [4], to manufacture complex HTCC components impossible to obtain by conventional routes.

Indeed, as presented in the previous article concerning LTCC components [4], additive manufacturing processes make it possible to automate production, to build more complex 3D metal circuits, more complex ceramic packaging shapes and to decrease the thickness of each layer. Moreover, additive manufacturing processes gives access to specific architectures to improve properties or to introduce new functions with the deposition of the desired material on each voxel of the part during its construction. In addition, these processes allow the reduction of production costs because they do not require tooling and limit the loss of raw material.