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Institut für Struktur- und Funktionskeramik

Mechanical Reliability of LTCC Components for Automotive Applications

Low Temperature Co-fired Ceramics (LTCCs) are 3D micro-network of metal structures embedded within a glass-ceramic substrate, also known as Ceramic Printed Circuit Boards (see Fig. 1). They are used as high precision electronic devices ( e.g. mobile and automotive technologies). The co-sintering of ceramic substrates with metallic conducting paths ( Leiterbahnen) at low temperatures (i.e. 900 °C) enables the use of Ag and Cu as electrode material (see Fig. 2). In relation to typical polymer circuit boards (e.g. PCB), LTCCs are thermo-mechanically more stable (i.e. higher E-modulus, lower Thermal Expansion Coefficient) and can withstand higher acceleration forces (up to ca. 60g). The can be utilized in high temperature environments (until 180 °C), which facilitates the use of LTCC in systems that are to be both thermal- and mechanically loaded.

An important problem during the fabrication of LTCCs is the high rejection rates which are related to the formation of cracks in the glass-ceramic substrate (see Fig. 3) as well as delaminations between the ceramic sheets and metal electrodes. The identification of “ weak points” in the design of LTCCs has been the main goal of this project. For this task, new miniaturised testing methods have been developed (see Fig. 4) to determine the mechanical strength at different locations within the part. A new finding has been identified, namely the effect of humidity on the mechanical resistance of the LTCC substrate. This means that an LTCC in-service in locations with high humidity content may h half of the life time than a component utilized in dry environments. This know-how has been transferred to the companies involved in the project and is now daily practice in the LTCC production to control humidity and temperature.

 

Fig. 1: Radio Frequency module with a LTCC substrate.
Fig. 2: Cross-section of a typical LTCC component.
Fig. 3: Crack near a „vias“ connecting several levels inside the part.
Fig. 4: (Left) Localised strength measurement in regions of interest within the part. (Right) Fracture of a LTCC at a vias, identified as a “weak point” in the design.

A second outcome of the project has been the development of a design tool (based on FE-models) to analyse the mechanical stresses generated in certain locations of the part (i.e. near vias, between ceramic and metal electrodes) during sintering (see Fig. 5). With this model design strategies can be adopted to lower the mechanical loading of certain locations within the LTCCs, thus increasing the mechanical reliability (i.e. life time) of the part.

 

Fig. 5: FE-analysis of the stresses generating during sintering in a location with high metal content. The maximal stress corresponds to the location where cracks were detected.

The scientific achievement has been to clarify the damage mechanisms during the fabrication of LTCCs which lead to the formation of cracks in the part. In addition, the understanding of the effect of environmental conditions on mechanical strength of LTCCs can be also highlighted. From the technological viewpoint the generation of a parametric 2D FE model to describe the stresses during sintering, in special locations within the components, has helped the company partners (specially EPCOS) to improve their designs.

The project has been worked out in close cooperation with the company EPCOS (Deutschlandsberg) and the company CONTINENTAL (Regensburg, Germany) as well as with the Institut für Struktur- und Funktionskeramik in Leoben.

 

Relevant Publications

“Influence of internal architectures on the fracture response of LTCC components” , R. Bermejo, I. Kraleva, M. Antoni, P. Supancic and R. Morrell, Key Eng. Mater. 409, pp. 275-278 (2009)

Fracture Mechanisms of Structural and Functional Multilayer Ceramic Structures , R. Bermejo, L. Sestakova, H. Grünbichler, T. Lube, P. Supancic and R. Danzer, Key Engineering Materials, 465 pp. 41-46 (2011).

“Strength reliability of 3D low temperature co-fired multilayer ceramics under biaxial loading”, R. Bermejo, P. Supancic, I. Kraleva, F. Aldrian, R. Danzer, Journal of the European Ceramic Society, 31 [5] pp. 745-753 (2011)