Ceramic films surface finish

Three major aspects of surface finish are important to ceramic films and coatings. The first aspect is that of surface roughness of the deposited coating. For some applications, the roughness of the coating is not an issue and the user can accept... 

The ceramic surface finish is a function of the microgranular structure and the density of the ceramic-glass composite. Small grains, in as-fired ceramic structures, form a smooth surface, and are mainly used for thin-film or fine-line thick-film applications. The centerline average (CLA) is a measure of ceramic surface requirement, 0.381-1 pm (15 0 pin.) for thick-film surface requirement, and 0.127 pm (5 pin.) and less for thin-film surface requirement. It is important to realize that the glass inclusion degrades both the electrical and the thermal properties of the ceramic substrates. 

Surface Smoothness. The surface finish requirements for thick-film deposition (-10" cm) are not as stringent as those for thin-film deposition ( 10 cm). Consequently, either a glass or a glazed ceramic is required for thin-fihn deposition, whereas an as-fired ceramic substrate is satisfactory for thick-fihn deposition. 

Nearly all surface finishes and coatings, with the exception of ceramic types for high-temperature applications, are based on a polymer film of some sort. They account for the use of a lot of polymer, but determining just how much and which polymers is not easy because most formulations are proprietary, and production figures do not always separate polymer and nonpolymer components. Five traditional types of surfaces finishes, lacquers, oil paints, varnishes, enamels, and latex paints, will be discussed, along with the role the polymers play in the finish. 

Sample preparation for purposes of examination in the SEM is very simple. In order to avoid disruptive charges, electrically nonconductive ceramic materials must be coated with a film of gold, carbon, or platinum about 20 nm thick by a sputtering process. Etched and unetched sections, fracture surfaces, glazes, firing skins, powder compacts, and prefired and finished sintered products can then be examined directly in the scanning electron microscope. 

Nevertheless, preparation of the multilayer substrate surface is more complex and is usually carried out by the users instead of manufacturers of ceramic substrates. Simply applying a polymer layer onto the as-fired ceramic surface cannot lead to successful thin-film layers. The solvent trapped in microporosities in the conductor for vias and/or at the interface between conductor and ceramics may outgas during the reflow soldering when populating components on the finished thin-film substrate, resulting in poor adhesion of thin-film metal and dielectric at the position of vias. 

Peripherally leaded, surface mount packages use lead materials of Cu or an Fe-Ni alloy with a Ni solderable finish and an electroplated Pb n or Sn protective layer. Leadless chip devices have terminations comprised of a fired-on Ag thick film conductor that is overplated with a Ni or Cu solderable coating, followed with an electroplated Pb-Sn protective finish. Leadless ceramic chip components (LCCC) use castellated terminations with a thick-fihn Au finish. 

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