Conductors, thick-film 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). 

MCM-C interconnect substrates are produced from either low-temperature cofired ceramic (LTCC) or high-temperature cofired ceramic (HTCC). Either process can produce multilayer substrates having high numbers of conductor layers (up to 100), although for most applications 2-20 layers are sufficient. The fabrication of MCM-C involves thick-film processes that have wider lines and spacings (5-20 mils) than MCM-D, but are lower in cost. 

For many power systems, hundreds of amperes of current may be flowing through a substrate in a relatively small area. To prevent severe losses in the conductors, the metallization must be very thick and low in resistivity. One approach to this problem is direct bond copper (DBC), which was developed by General Electric in the mid-1970s. Unlike thick-film or thin-film conductors, DBC can be purchased with metal thicknesses up to 0.65 mm (25 mil). Combined with the low resistivity of copper (0.12 mD/n) and a high thermal conductivity substrate such as AIN, this approach creates nearly the ideal substrate for this type of application. 

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 materials for AIN substrates have been in use for over a decade. Earlier, considerable effort was made on treating the surface of AIN to make it more bondable to thick-film materials. Recently, more attention has concentrated on making conductor materials more compatible with AIN substrates and overcoming difficulties with conventional thick-film materials that work well on alumina substrates but either blister or adhere poorly to AIN substrates. This development was also driven by enviroiunental concern for the use of high-thermal-conductivity beryllium oxide (BeO) ceramic, where many manufacturers felt compelled to switch to alternative substrates. Typical data for popular substrates used in high-dissipation applications are tabulated in Table 2.6. 

A variation on the above photoetching process uses thick-film inks that are photosensitive. This eliminates the need for the photoresist application and removal steps. Although photosensitive conductors and dielectrics for use with gold and silver are available, similar materials for use with copper are not. 

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. 

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

Base Metal Conductors. Copper conductors have gained some acceptance over the past decade because of their relatively low metal cost, low resistivity, good adhesion on AljOj substrates, excellent solder leach resistance, and low migration tendency. Advances in compatible thick film dielectric formulations have resulted in significant use of Cu in multilayer interconnect boards, primarily for military applications. Also, uses of Cu conductor materials have included power hybrid and microwave-related applications. Their applications in more complex systems and networks have been limited by the availability of state-of-the-art nitrogen-firable resistor systems. However, there are additional factors that complicate the widespread usage of this versatile material. 

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. 

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