Conductors, thick-film properties

The aim of this work is to modify the viscosity of silver paste in order to get the required thickness and fine line printing of printed material on the substrate. As well known, controlling the properties of resulting conductor thick film paste is not a simple task, so in order to comply with required properties, the conductor paste need to do some adjustment in terms of its viscosity behavior. Viscosity can be lowered (by addition of the solvent) or increased (by addition of a thixotropic nonvolatile vehicle), although the latter will require re-milling of the paste. 

The conduction mechanisms in thick-film resistors are very complex and have not yet been well defined. Mechanisms with metallic, semiconductor, and insulator properties have all been identified. High ohmic value resistors tend to have more of the properties associated with semiconductors, whereas low ohmic values tend to have more of the properties associated with conductors ... 

It is common to refer to thick-film metallizations as "gold," or "silver," or "copper" — the conducting metal component in the paste. It is important to keep in mind that the typical conductors for ceramics are compositions of glasses, ceramic powders, and conducting metal particles. As a result, the conductivity of typical gold conductors is 30-50% that of bulk copper and that of typical silver conductors, 70-90% that of bulk copper. The conductivity of plated thick-film and DBC approach that of bulk copper. Table 2.1 summarizes the properties of typical conductors for ceramic application. 

Generally, the electrical connection between components and layers are given by the conductor paste such as silver, copper, gold, silver-palladium, silver-platinmn and etc. Compared to the other conductor materials, silver thick film has been used as the main conductive material for LTCC technology due to their excellent electrical properties, thermal... 

DuPont Electronic Materials Division and Hughes Aircraft introduced the first LTCC materials system at the 1983 ISHM Conference in Philadelphia. DuPont then marketed a complete line of tapes and conductor systems with which to build complete systems. Solderable conductors and screened resistor inks soon followed. The properties of these DuPont materials are shown in Table 1.13. It can be seen that their properties are similar to those of alumina and thick film dielectrics. 

The properties of thick-film conductors that are typically specified in order to determine formulation selection for a given application include the following ... 

TABLE 8.4 Typical Properties of Various Thick-Film Conductors... 

Test Pattern. The basic conductor properties can be measured using a single test pattern, as illustrated in Fig. 8.15. These include resistivity, print definition and film thickness, film density, solder leach resistance, wettability, adhesion, and wire bondability. Each property will be discussed individually with reference to Fig. 8.15. Many applications require functional use tests which usually require specific test patterns and even multilayer construction processes. Similarly, numerous applications require standard conductor tests on thick-fihn dielectrics instead of the bare substrate. 

Microstructural features of dielectric materials include the chemical composition and structure of the various phases as well as porosity and its distribution. These microstructural features, coupled with macroscopic defects such as pinholes, mesh marks, foreign matter, and interaction with conductor materials, determine dielectric properties. Very little information is published about composition of thick-film dielectrics, which are complex materials systems. The following section is a general discussion of dielectric microstructure development. 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

The use of films as electrodes makes possible numerous experiments that would be difficult or impractical to implement with the conventional bulk electrodes. Discussion here emphasises either thin (< 5 pm thick usually quite a bit thinner) or thick (> 5 pm usually quite a bit thicker) film electrode materials, consisting of a conductor, either a continuous or a spatially patterned film, most commonly deposited on a suitably prepared insulating substrate. Films consisting primarily of insulators are not considered here, except to the extent that they may be used to form patterned arrays or electrodes with special geometries. A view of applications and properties of film... 

The ability to reach ionic conductivities of the level of lOOS/cm in nanoscaled YSZ thin films is quite unique and is not possible to obtain in conventional materials as their diffusivity is limited by the lattice [7, 31]. The increase of the conductivity of epitaxial YSZ thin films offers new opportunity for oxygen conductors whose properties can be effectively controlled by the thickness and epitaxy level of these materials. 

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