Thick film pastes applications

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

The thick-film design consists of four layers, to be separately screen printed and fired on a 1 in square alumina substrate (figure 14.9). Commercial formulations were used for electrodes, bridge trimming resistors, and passivation layers. The first attempted sensor layer was a commercial silver/palladium paste modified by the addition of palladium powder. Based on the performance of the first thick-film sensors, DuPont Electronics (Research Triangle Park, NC) specifically formulated a palladium-based thick-film paste for this application. 

The particle size distribution of a thick-film paste is a compromise between screenability and the properties of the fir film. For screenability, it is desired to have very small particles, but very small particle sizes in thick-film resistors produce parameters that are skewed and not suitable for most circuit applications. Larger particles will obviously be more difficult to screen and may actually block one or more screen openings. 

Rhodium alloys find most of their applications in electrical contacts for radio frequency circuits, precision potentiometers, and spark plug electrodes. The trend is toward lower usage in electronics, mainly because of price. Osmium has minor uses in electrical equipment, mainly in contacts. Ruthenium is used in electrical contacts and high-voltage relays up to 500°C because even RuO c is conductive. A ruthenate thick-film paste is used for printed circuit resistance elements. The ruthenate is converted to RuOa, which has very low resistance drift, 0.10%. 

Aluminum nitride has been demonstrated to be suitable for blank substrates for high power and high frequency applications. Substrates from many sources are available for thick or thin film metaUization. Thick film paste manufacturers now supply pastes formulated for aluminum nitride. The multilayer process described in the previous section has also been adapted for aluminum nitride. Both multilayer substrates and packages are available commercially. 

MAJOR PRODUCT APPLICATIONS conductive inks, pastes, coatings, adhesives, thick films, battery plates, electrical contacts, powder metallurgy, capacitor inks... 

More recently inks specifically developed for sensor applications have become available, for example Sn02 pastes incorporating Pt, Pd, and Sb dopants for the construction of semiconductor gas sensors. For biosensor applications, thick-film technology based on polymer films is extremely important, and special grades of polymer pastes (carbon, Ag, and Ag/AgCl) are becoming available... 

Thick-film circuits are single or multilayer structures produced by depositing a layer, or layers, of a specially formulated paste or ink onto a suitable substrate. Thick-film technology began in the early 1960s when DuPont introduced a thick-film resistor system for application in miniaturized circuits. IBM used thick-film materials in then-family of IBM/360 computers. Currently the worldwide market for thick-film circuits and devices is around 14 billion. Most thick-film circuits are still used in electronic applications such as in computers (Figure 27.14). 

As supplied, adhesives can be found in the form of low viscosity liquids, viscous pastes, thin or thick films, semisolids, or solids. Before application to a substrate, an adhesive need not be sticky or otherwise particularly adherent. A distinct exception is the pressure-sensitive adhesive (PSA), which is inherently tacky when first made. Such an adhesive is applied as a thin film with or without a backing, the combination of the adhesive and the backing defining an adhesive tape. The PSA remains throughout its useful lifetime essentially the same material it was when first made. All other forms and types of adhesives undergo a transformation which is central to their function as an adhesive. This transformation is usually carried out through imposition of time, heat, or radiation, either actively or passively. 

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. 

Thick-film resistors can be processed with a tolerance of about 25%. Laser trimming increases the resistance value. Therefore, a resistor is designed to a lower value than desired and will be trimmed to its target value later on. Besides the resistance value required, the power dissipation density is required to design a thick-film resistor. The power dissipation density (Pdensiiy in mW/mm ) is a paste property, which is specified in the data sheet. It is typically related to a 50% trim cut (maximum allowable trim length) and application on prefired alumina. For a stable resistor, the minimum area Ag is determined by the maximmn circuit power dissipation requirement, as in Equation 9.3 ... 

A silver paste conductor with an acryUc resin matrix is the most popular conductor material as the major thick-film conductor material for flexible circuits. It can provide very flexible conductor layers via a simple screen-printing process. Copper-based and carbon-based paste materials have been developed as the low-cost materials, but their conductivity is very low and unstable, and therefore their applicable areas are limited. 

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