Thick-Film Conductors

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. 

Frit bonding is the most common method, but there are several advantages to reactive bonding. First, very small amounts of additive are needed, which means that the electrical resistivity of the conductor is kept as low as possible. Second, the surface of the conductor is nearly pure metal, which enhances the attachment of thin Au or A1 wires during wire bonding. Some thick-film conductor compositions are termed mixed bonded and these contain a glass frit together with a mixture of oxides. 

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 ... 

TABLE 27.6 Metals and Alloys Used in Thick Film Conductors... 

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). 

Rank the thick-film conductors in Table 27.6 in terms of electrical conductivity starting with the most conductive. Is the trend consistent with their cost ... 

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... 

This structure is of particular interest as it employs a Pt metal wire and it can be easily adapted to the thick film methods Pt metal is added to thick film conductor pastes to aid solderability, thus a Pt-based conductor is very desirable. Most of the electrodes of the kind described above are generally reported to perform adequately , but little information is available on their longevity and operating mechanisms. 

Thick-film conductors must perform a variety of functions ... 

Frmctions of a thick-film conductor include conductor interconnections, soldered lead and device attachment, thick-film resistor terminations, crossover connections, capacitor electrodes, chip and die bonding, wire bonding, low-value resistors, and packaging of thick-film circuits. The factors considered in... 

Typical parameters of stainless steel mesh are given in Table 5.1. The most commonly used screen meshes are 80 mesh, used primarily for solder paste, 200 mesh used for thick-film conductors and resistors, 325 mesh, us for thick-film conductors, dielectrics, and resistors, and 400 mesh, used for fine-line (< 0.010 in.) thick-film conductors [14]. 

Standard thick-film conductor pastes are very important in the electronics industry. However, when applying these materials in patterns by screen... 

A copper thick-film conductor, 0.2 in. long and 0.01 in. wide, is fabricated using the standard screen-printing process and is shown in Figure 8.3. The ink manufacturer s data sheet specifies a resistivity of 1.9 to 4.8 mfl/Q for a 13-pm fired thickness [5]. Using the worst-case value of 4.8 mfl/Q, the resistance of the conductor is calculated using Equation 8.6. 

A 0.008 in. thick copper conductor with the same length and width as the thick-film conductor described above is fabricated using the direct bond copper (DEC) process. The resistance is calculated using Equation 8.4 and found to be 16.9 mO ... 

The firing process for thick-film copper is different from tirat of otirer thick-film conductors. Because copper readily oxidizes in tire presence of oxygen, the standard thick-film firing process must be modified to prevent tire oxidization. Instead of air flowing through the furnace, dry nitrogen with less than 10 ppm of oxygen is used [5]. 

Thick-film ink manufacturers do not publish thermal conductivity data for their conductor and via-fill inks. Work by Harshbarger [16] and Krum [17] has shown that the thermal conductivity of thick-film conductor inks is approximately 20% of that of pure metals. A first-order approximation of the thermal conductivity of thick-film conductor inks is to multiply the percentage of the published value of electrical conductivity (the reciprocal of resistivity) of the ink and the thermal conductivity of the pure metal in the ink. For copper thick film, as described in the above example, the electrical... 

In the etched pattern process as shown in Figure 8.20, thick-film conductor ink is blanket-coated onto a ceramic substrate using standard thick-film screening. After drying and firing, the pattern is etched using a photolithographic process. Line widths as fine as 0.001 in. are made with this process. 

The sheet resistivity of one vendor s [5] thick-film conductor ranges from 1.9 to 4.8 m i/Q. For this example, 2.0 mQ/Q will be used. 

To lower the electrical resistivity of a thick-film conductor, some manufacturers print multiple printings of the same copper conductor ink using toe same pattern. This process can provide a fired-ink thickness of 20 to 25 pm and a resistivity of approximately 1 to 2 mO/n. 

EHfferent conduction mechanisms of thick-film conductors and resistors. 

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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