Paul Muralt

Lamination of LTCC at low pressure and moderate temperature using screen-printed adhesives

In this work, we investigate a new method of low-pressure lamination of low-temperature cofired ceramic (LTCC) tapes, by pre-coating with specially-formulated adhesive layers. This allows fabrication of intricate structures that would be destroyed by high lamination pressures, yet preserves compatibility with standard processing operations, such as blanking, laser cutting, punching, via filling and screen printing. Techniques proposed previously are application of adhesive tapes, solvents or glues. These methods are applied after the tape processing steps and therefore require an extra operation in the processing chain, which may be unpractical, especially if the tape is very thin has been extensively cut. In our procedure, the adhesives are printed and dried on the blanked tape before any other operation, with the drying step also useful for pre-conditioning the tapes, which may then be stored as standard blanks. The adhesives are formulated to have low tack at room temperature, making them compatible with standard processing, but to become very soft at 50-60°C, allowing lamination at low pressures. Compatibility with different tape materials is investigated, and the requirements for adequate adhesive formulation are discussed.

An evaluation of thick-film silver die-attach as micro solid oxide fuel cells (μ-SOFCs) interconnection packaging

Micro solid oxide fuel cells (μ-SOFCs) based on MEMS fabrication processes have become an increasingly attractive option for portable power generation applications, due to their high power density and their compatibility with hydrocarbon fuels. For system packaging, the silicon-based fuel cell membrane module needs to be integrated together with other functional elements such as the gas processing unit and the post-combustor. Silicon wafer bonding techniques can be utilized to avoid thermo-mechanical stresses, however, the complete integration in silicon technology is very challenging; ceramic carriers combined with thick film technology are much more practical for total integration, but present other challenges. As ceramics may have a slight thermal mismatch with silicon, hermetic assembly by glass sealing can be problematic due to stress build-up. Moreover, the high necessary sealing temperatures are somewhat beyond the survival conditions of the current fuel cell modules. Therefore, a metallic thick-film silver die-attach approach is proposed as a potential bonding solution, due to its flexible fabrication requirements, the relatively low process temperatures and the high ductility and chemical stability of silver materials. Such a seal could potentially compensate the thermal mismatch between the silicon and the ceramic substrate as well as provide reasonable hermeticity in μ-SOFC operating conditions. In this work, we preliminarily evaluated several commercial thick-film silver products as interconnection materials in terms of mechanical strength, firing process and metallization approach, using shear testing and microstructure characterization. The failure mode of the die-attach was discussed in detail and related to their bonding mechanism. The influence of firing temperature and additional prior metallization on those thick-film bonds were also studied.