Thick-film conductor, functions

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 ... 

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

Since conductors with a bonding function are the surface layer, forming a thin film on the top of a thick film conductor using plating, sputtering and the like is a technique widely used to improve bonding characteristics. 

Materials classified as thick-film conductors play a major role in hybrid microelectronics, electronic packaging, components, displays, and photovoltaic apphcalions. Generally speaking, the functionality of thick film conductors is analogous to wiring in predecessor technologies. 

The constituents of polymeric thick film conductors exhibit variations to the above description. These type conductors typically consist of a silver, carbon (C), or Ag and C mixture functional phase dispersed in a thermoplastic or thermosetting polymer and solvent. In the case of polymeric thick film conductors, the polymer serves as an insulative binder and... 

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. 

Thick-film copper conductor inks are not pure copper. The ink consists of a functional material of copper, a solvent, a temporary binder, and a permanent binder. The permanent binder tailors the CTE to that of the substrate. It also aids in the adhesion of either the substrate or the dielectric. The thick-film firing process in nitrogen bums out the solvent and temporary binders. This leaves just the copper and the permanent binder. In addition to tailoring the CTE, the permanent binder significantly reduces both the electrical and thermal conductivities of the conductor. A typical fired conductor thickness is 15-18 pm. If it were pure copper, it would have a sheet resistivity of 0.94-1.13 mQ/Q. The 9924 thick-film copper conductor material from El DuPont Electronics specifies a sheet resistivity of 1.9-4.8 mD/ for the same thickness. This results in the published resistivity of this thick-film material being only 23% of the resistivity of pure copper. 

Test Pattern. The basic conductor properties can be measured using a single test pattern, as illustrated in Fig. 8.15. These include resistivity, print definition and film thickness, film density, solder leach resistance, wettability, adhesion, and wire bondability. Each property will be discussed individually with reference to Fig. 8.15. Many applications require functional use tests which usually require specific test patterns and even multilayer construction processes. Similarly, numerous applications require standard conductor tests on thick-fihn dielectrics instead of the bare substrate. 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

Pearson s theoretical treatment was based on a linear stability analysis of the type described in Section 10.4 in connection with jet stability to small disturbances and paralleled Rayleigh s analysis for buoyancy driven instability. He assumed an infinite homogeneous liquid film of uniform thickness h whose lower surface is in contact with a rigid heat conductor at a fixed temperature and whose upper surface is free. Gravity is neglected (Ra = 0) and a linear temperature distribution across the film is assumed, with the high temperature at the lower surface. The surface tension is a function of temperature alone, and the rate of heat loss from the free surface is also a function of temperature only. 

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