Thick film pastes

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). 

Silver Thick Films. About half of the silver consumed in the United States for its electrical properties is used by the electronics industry. Of this amount some 40% is used for the preparation of thick-film pastes in circuit paths and capacitors. These are silk-screened onto ceramic or plastic circuit boards for multilayer circuit sandwich components. 

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 PEVD sample utilized in this investigation is a solid electrochemical cell with a ytterbia and yttria stabilized zirconia pellet (8%Yb303-6%Y303-Zr03) as the solid electrolyte to conduct oxygen anions from the source to the sink side. A commercially available Pt thick film paste was screen printed on the center of both surfaces of the solid electrolyte disk. Two Pt meshes, with spot welded Pt leads,... 

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. 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photoetching of metals or by screen-printing and firing of thick-film pastes. With recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

A cermet thick-film paste has four major ingredients an active element, an adhesion element, an organic binder, and a solvent or thinner. The combination of the organic binder and thinner are often referred to as the vehicle, since it acts as the transport mechanism of the active and adhesion elements to the substrate. When these constituents are mixed together and milled for a period of time, the result is a thick, viscous mixture suitable for screen printing. 

The ingredients of the thick-film paste are mixed together in proper proportions and milled on a three-roll mill for a sufficient period of time to ensure that they are thoroughly mixed and that no agglomeration exists. 

Thick-film pastes are dried prior to firing to evaporate the volatile solvents from the printed films. If the volatile solvents are allowed to enter the firing furnace, flash evaporation may occur, leaving pits or craters in the film. In addition, the byproducts of these materials may result in reduction of the oxides that comprise the fired film. Most solvents in thick-film pastes have boiling points in the range of 180-250°C. Because of the high surface area/volume of deposited films, drying at 80-160°C for a period of 10-30 min is adequate to remove most of the solvents from wet films. 

Walton, B., Thick film pastes and substrates. Hybrid Microelectronic Technology, pp. 41-52, Gordon Breach, New York, 1984. 

Screen printing is a method by which patterns of thick-film paste or solder paste are applied to a substrate as shown in Figure 5.1. 

Today, in the electronics industry, the primary mesh material is stainless steel, which adds an additional degree of control and precision over nylon in addition to added resistance to wear and stretching [9]. The crude hand methods of printing have evolved to sophisticated, microprocessor-controlled machines that are self-aligning, have the ability to measure the thickness of the film and also to adjust the printing parameters to compensate for variations in the properties of the thick-film paste [10-12]. 

This chapter deals primarily with the screen printing of thick-film paste on ceramic substrates. Information on stencils and solder printing is readily found in treatises dealing with the smface-mount technology. 

After the four major ingredients of the thick-film paste are selected, they are mixed together in proper proportions and milled on a three-roll mill for a sufficient period of time to ensure that they are thoroughly mixed and that no agglomeration exists. After the initial mixing, the paste is sometimes maintained on a slow-moving roller to ensure continual slow mixing and ensure that the phases do not separate. 

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. 

Cleanliness is essential. This cannot be overemphasized. A few particles of dust in a jar of thick-film paste will create agglomerates and may ruin the entire jar. Further, particles of dust and hair may bimi out during firing, leaving voids at are not visible after the printing process... 

Markstein, H.W., Thick Film Pastes Exhibit Steady Improvement, Electronic Packaging and Production, Vol. 37, No. 12, September 1997, pp. 64r-66, 68, 70. 

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%. 

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. 

Buzby, D. Dobie, A. (2008). Fine Line Screen Printing of Thick Film Pastes on Silicon Solar Cells. Proceedings of the 41 International Si/mposium on Microelectronics (IMAPS 2008), Rhode Island, USA, November 2008. 

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