Thick-Film Formulations

The basic features of inks or pastes printed and fired on the substrate are particles of metals and/or metal oxides, glass (metal oxides mixture), a binder, and a solvent to make the paste fluid in nature [24-26]. A metallic conductive component comprising one or more precious metals in finely divided powder form with powder sizes ranging from 1 to 10 pm in size. Structural shape and particle morphology are critical parameters that affect the desired electrical characteristics, and controlling these parameters ensures uniformity of the fired film properties. 

The organic binder, a nonvolatile organic, serves the purpose of holding the active elements and the adhesion elements in suspension until firing of the fihn takes place, and it also provides the paste with the desired fluid characteristics needed for screen printing. The organic binder, such as ethyl cellulose 

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The active phase for resistor formulation is the most complex of all thick-film formulations due to the large number of electrical and performance characteristics required. The most common active material used in air-fireable resistor systems is ruthenium, which can appear as RUO2 (ruthenium dioxide) or as BiRu207 (bismuth ruthenate). 

Thick-film formulations exhibit different types of dominant sintering mechanisms, depending on their composition. For example, thick-film conductors can sinter like sohd-state metal particles or spheres, while sintering of resistors and dielectrics is more complex and largely dependent on glass sintering behavior. 

Aluminum nitride is a nonoxide ceramic material. The GTE of AIN (4.5 ppm/°C) is substantially lower than that of alumina. Early attempts to use thick-film dielectric materials designed for alumina substrates on AIN were unsuccessful. Many of the glasses used in thick-film formulations designed for alumina substrates are chemically incompatible with AIN. Furthermore, the CTE differences between dielectrics designed for alumina and AIN substrates lead to cracking. Special thick-fihn dielectric formulations have been developed for use on AIN substrates. Table 8.12 shows properties of a typical thick-fihn dielectric on AIN substrates. " ... 

Other additives (i.e., rheology modifiers) are often added to thick film formulations to adjust paste rheology. The interaction between various ingredients is important in understanding paste rheological behavior. Since numerous ingredients are commonly used, statistical techniques must be employed to understand these interactions. 

Fundamentals of a method for developing models of mass transfer of low-molecular substances in non-reconstructing microheterogeneous membranes were formulated. The local properties of membranes differ in sorbability with respect to species and in the probability for a species to jump from one sorption site to another. Because of this, the permeability of a membrane depends on the amounts of different-type sites, their mutual arrangement, mutual influence of adjacent molecules, and the probabilities of jumps between different sites. The probabilities of occupation of different sorption sites are described by kinetic equations, which take into account the interactions between species. The atomic-molecular discrete and continuous models of mass transfer for thin and thick films are constructed. 

An important one is that deposition occurs on the anode, and it is known (5J that some metal is incorporated into the polymer film, presumably because oxidation of the substrate can compete with polymer deposition. Because of this problem of anodic metal dissolution, cathodic deposition of epoxy resins (9-10) was developed. A second disadvantage in using the polyamic acid is that the formulations used do not deposit an insulating film, and thus the film thickness is not self-limiting. Although this fact may allow quite thick films to be deposited, it also means that the film thickness is not uniform as it will depend on distance from the counter-electrode and other current density effects. 

In this formulation, the ratio of NVC to PTCEM is 1 1 in the mixture. One micron thick film was spin coated on a silicon wafer using chlorobenzene as the coating solvent. The film was vacuum dried for an hour at room temperature and coated on the top with PVA film. The film was vacuum baked for an hour and exposed with an e-beam. After e-beam exposure, the PDE film was dipped in deionized water for 30 seconds to remove the PVA layer, and baked at 120° C for 30 minutes. The patterns were visible after the bake. The development of patterns was conducted in a barrel etcher using O2 plasma. The temperature inside the barrel etcher during the development was maintained at <90 C. SEM micrographs of some plasma developed patterns are shown in Figures 2 and 3. 

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. 

Such an infinitely thick film, however, would not allow for a contact angle situation. Frumkin [15], in fact, concluded that for a nonzero contact angle to be possible, a continuous transition from adsorbed film to bulk liquid must not be possible. There is experimental evidence that equilibrium adsorbed films do not approach infinite thickness at P° [11, 15]. It is desirable, therefore, to consider at least qualitatively how a treatment might be formulated which would correctly predict a discontinuity between adsorbed film and adjacent bulk liquid in a contact angle situation. 

Irradiation. The liquid formulation was applied with a calibrated bar onto either a KBr crystal for infrared analysis, or a glass plate for hardness and gel fraction measurements. 10 to 20 pm thick films were exposed to the radiation of a 80 W/inch medium pressure mercury lamp, in the presence of air, at a passing speed of 60 m/min, which corresponds to a 0.1 s exposure at each pass. The incident light intensity (Iq) at the sample position was measured by radiometry (International Light IL-390) and... 

As intended by the adhesive formulation, no parasitic side reactions such as formation of urea, new uretdione, isocyanurate, biuret, or allophanate are detected for the bulk and for thick films on Al, Cu, and Au (dpu>l pm cf Figs. 6.3, 6.7). Even ultrathin PU films on Au correspond to bulk properties to a very good approximation (Fig. 6.8), which is a recommendation for films on Au as references with respect to effects in the interphase on different metals. 

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). 

Thick-film resistors are available with sheet resistance values in the range from 0.1 to 10 M Q/D. By blending different quantities of conductive material and an electrically insulating glass the resistivity is controlled. For a high sheet resistance formulation the ratio of conductor to glass would be about 70/30. 

In glass formulations used in thick-film inks alkali metal constituents are avoided. Why ... 

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