Conductors, thick-film adhesion

W/m-K and 1.72 iQ cm. However, to be exact, the influence of the tiiick-film base layer needs to be taken into account. For a semiconductor mounted on a plated copper conductor over the thick film as shown in Figure 8.22a, the heat will flow from the semiconductor, through the die attach, tiirough the copper conductor, through the thick-film adhesion layer, and into the... 

For obtaining the required thick-film adhesion, oxides (fritless conductors) can also be used. Small amounts (0.1-1%) of chemically active oxides like CuO, CdO, or NiO, added to the paste, react at high temperatures with alumina substrates to form oxides like CuAlO, which provide better adhesion. A combination of glass and oxides also is used, but the amount of glass in this case is much lower than in the fritted systems. Adhesion is believed to be related to a combination of the above-mentioned effects. Table 8.1 shows several examples of inks used for gas sensor fabrication. 

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

The functional material in a thick-film conductor is a metal. The important metals and alloys used in thick-film conductors, together with some of their characteristics, are listed in Table 27.6. There are two mechanisms for achieving adhesion of the metal film to the substrate ... 

In this chapter we described tape casting. This process is used to make fiat sheets. Although this is a means of shaping and could have been described in the shaping chapter we described it here because it shares a common feature with the other thick film coating methods it uses a slurry. It also is the process most often used to make ceramic substrates for thick-film circuits. Ceramics are a major component of thick-film circuits. Even in thick-film conductors, ceramics are important in ensuring adhesion between the metal layer and the substrate (which is invariably also a ceramic). 

Screen printing of conductive andinsulative adhesives has been used for decades in the assembly ofhybridmicrocircuit and multichip modules. Since screen-printing processes are also used to deposit thick-film conductors, resistors, and dielectrics, the processes are fully compatible and cost effective in the production of hybrid microcircuits. Screen printing and stencil printing are both batch processes that are usually less expensive and... 

Thick-film copper conductor inks are not pure copper. The ink consists of a functional material of copper, a solvent, a temporary binder, and a permanent binder. The permanent binder tailors the CTE to that of the substrate. It also aids in the adhesion of either the substrate or the dielectric. The thick-film firing process in nitrogen bums out the solvent and temporary binders. This leaves just the copper and the permanent binder. In addition to tailoring the CTE, the permanent binder significantly reduces both the electrical and thermal conductivities of the conductor. A typical fired conductor thickness is 15-18 pm. If it were pure copper, it would have a sheet resistivity of 0.94-1.13 mQ/Q. The 9924 thick-film copper conductor material from El DuPont Electronics specifies a sheet resistivity of 1.9-4.8 mD/ for the same thickness. This results in the published resistivity of this thick-film material being only 23% of the resistivity of pure copper. 

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. 

T. T. Hitch, Adhesion Measurements on Thick Film Conductors, Adhesion Measurement of Thin Films, Thick Films, and Bulk Coatings, ASTM STP 640, K. L. Mittal (ed,), ASTM, pp. 

K. J. Williams, Adhesion Test Methods for Solderable Thick Film Conductors, Brazing and Soldering, no. 15 (autumn), pp. 10-18, 1988. 

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