Thick film resistor paste

Table 4-1 Inorganic conductive materials in thick film resistor paste and their resistance specifications.

This section describes the characteristics and issues associated with the material ruthenium oxide/glass that is widely used in thick film resistor paste. 

Thick-film resistor pastes are composed of a combination of glass Mt, metal and oxides. Resistor pastes are available in values ranging from 0.2 ohm/sq to 10 M ohm/sq. Firing temperatures 600-850°C. Temperature coefficient of resistance can vary from 40-150 ppm/C. These pastes are used in microcircuits, voltage dividers, resistor networks, chip resistors and potentiometers. 

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). 

The functional phase in thick-film resistors is a mixture of electrically conducting (or semiconducting) ceramic powders such as ruthenium dioxide (RUO2), bismuth ruthenate (Bi2Ru207), lead ruthenate (Pb2Ru206), and Ag-Pd-PdO mixtures for use in air-fired pastes and tantalum nitride (TaN) for nitrogen-fired pastes. The resistance of thick-film resistors is specified in terms of sheet resistance, which has units of ohms/square (Q/D). 

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. 

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 ... 

The general behavior of thick-film resistors (TER) during HVP trimming depends on the resistance of the paste used ... 

Thick-fihn materials must be formulated to adhere to AIN. The lead oxides prevalent in thick-film pastes that are designed for alumina and beryllia oxidize AIN rapidly, causing blistering and a loss of adhesion. Thick-film resistor materials are primarily based on RuOj and MnOj. 

Micwstructure Development. Thick-film resistors are prepared by formulating pastes consisting of submicrometer size conductive phase particles and micrometer size glass particles. Since conductive particles are of submicrometer size, complete mixing of conductive phase particles and glass powder is difficult to achieve because the smaller... 

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). 

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. 

There are three basic classes of thick-film material (conductors, resistors, and dielectrics) and all have at least one component that is a ceramic. They are supplied in the form of a paste or ink, which contains the following ... 

The term thick film (T-F) technology is accepted to mean that field of microelectronics in which specially-formed pastes are applied and fired on to a ceramic substrate in a defined pattern and sequence to produce a set of individual components such as resistors and capacitors, or a complete functional circuit (19). Figure 11.3 shows a flow diagram of a standard thick film process. 

For the resistive layer in general purpose resistors thick film technology is normally used. In most cases the contacts are made by dipping a conductive paste. Sometimes evaporated or sputtered contacts are used. To ensure good soldering the contacts are covered with a solder or tin layer. Standard SMD resistors have a tolerance of 5% and a TCR of 200 ppm/°C. With special thick-fihn techniques and appropriate contacts, tolerances of 1% and a TCR of 50 ppm/°C can be made. 

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]. 

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. 

D. P. H. Smith and J. C. Anderson, Electrical Conduction in Thick Film Paste Resistors, Thin Solid Films, vol. 71, p. 79, 1980. 

PASTES. Conductor, resistor, dielectric, seal glass, polymer and soldering compositions are available in paste or ink form. They are used to produce hybrid circuits, networks and ceramic capacitors. The materials are often called thick film compositions. 

Gold thick film conductors not only provide printed wiring in devices, but also resistor terminations, electrodes, and solderable pads to facilitate the packaging and assembly of circuits. However, to simplify the terminology used in this chapter, the generic term electrode will be used to describe the final printed and heat-treated paste. 

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