Resistors, thick-film compositions

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. 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resias remain an iategral part after cuting. Owiag to the relatively low (120—165°C) processiag temperatures, both plastic and ceramic substrates can be used, lea ding to overall low costs ia materials and fabrication. A common conductive composition for flexible membrane switches ia touch keyboards uses fine silver particles ia a thermoplastic or thermoset polymeric biader. 

The first resistor ink system with a wide range of sheet resistivities was developed in 1958 by J. D Andrea. This palladium and silver system (PdO/ Ag) had a high firing temperature influence on the sheet resistivity, caused by the complicated chemical-dynamical process. The wide range of resistances that this composition could achieve was one main reason for the rapid growth of thick-film technology since then. 

As previously mentioned, the glass composition is also critical to compatibUity of thick-fihn conductors with other thick film materials such as resistors and dielectrics. This requires additional considerations when the formulation is designed. Figure 8.11 shows conduc-tor/resistor incompatibilities at the interface resulting in cosmetic as well as performance-related issues. Glass chemistry selection is also very dependent on the firing atmosphere, as discussed earlier in the section on Cu conductors. 

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. 

Statistical loading curve models have been proposed that take into account the segregated nature of thick-film resistors. A systematic microstmcture development study has been conducted by Vest for RuOj model thick-film resistors. Vest proposed a statistical loading curve to explain the sheet resistivity variation as a function of volume fraction of the conductive phase and particle sizes of both the conductive phase and glass. Several theoretical models have heen developed to explain the variation of resistivity as a function of volume fraction of the conductive phase. A comprehensive model to explain the sheet resistivity variation in thick-film resistors in terms of composition and physicochemical properties of powders over the entire resistance range has not heen developed to date. 

S. M. Chitale and R. W. Vest, Critical Relationships Between Particle Size, Composition, and Microstructure in Thick Film Resistors, IEEE Trans. CHMT, vol. 11, no. 4, pp. 604-609, 1988. 

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