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Post Fired Via Metallizations with Enhanced Functional Properties
Keywords: Via, sinter shrinkage, conductor
Manyfold multilayer assemblies obtain their final functionality and connection to the system by postfired via metallizations. The postfired vias must provide high electrical conductivity and in many cases the filling must be hermetically dense. Due to the substrate material and production process different requirements on a post fired via paste exist. In the drying and firing steps, the pastes must show nearly zero shrinkage to maintain via filling. Different substrate materials require different process temperatures for the paste solidification. Possibilities how to handle complex requirements were studied on the example of two different via fill pastes. For a highly conductive and gas tight via in a high frequency modul, inorganic additives were used to minimize the shrinkage of a silver powder based thick film paste. The influence of particle size variations of both the silver powder and the additive as well as the influence of the silver to additive ratio on the total volume shrinkage and the electrical conductivity were examined. In order to attain gas tightness, glass pastes of varying rheology were tested. For an ITO coated hetero junction solar cell, process temperatures of 200C must be undercut. Therefore, a number of silver powders having different particle shapes and sizes were dispersed in hardenable polymers. For both development targets, paste materials with very good performance could be obtained. The results will be discussed in terms of powder properties, microstructure evolution during firing and final film properties.
Markus Eberstein, Group Manager
Fraunhofer IKTS
Dresden, Saxony

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