Advances in Conducting Polymers Research
It is likely that the new applications will have specialty niche markets, unlike the massive present market of commodity polymers. The economic factor. These products will be sold by function, not weight. Organic polymers play a crucial role as insulating materials in electronics.
Advances in inherently conducting polymers
The most visible applications are in silicon chip encapsulation and in dielectric layers for printed circuit boards PCBs. The polymer employed is usually an epoxy novolac that is highly loaded with silica powder to reduce the coefficient of thermal expansion. Differences in thermal expansion between chip and encapsulant create large stresses on cooling from mold temperatures and as the temperature of the assembly is cycled in testing and in use. Encapsulation is mainly for mechanical and chemical protection of the chip and the lead frame and thus facilitates handling for automatic assembly.
Materials and processes have been developed to a high degree of sophistication. High mechanical strength is achieved with the smallest external dimensions. Printed circuit boards are layered structures of patterned copper connection paths "wires" placed on a polymer substrate. Polymers employed include epoxies, polyesters, fluoropolymers, and other materials, but glass-reinforced epoxies usually bisphenol-A based are by far the most widely used. Metal patterns are defined photolithographically and plated to the desired thickness, and the layers are then piled up and cured in a press.
Circuits with more than 40 copper layers signal, power, and ground have been produced commercially. Connection to the inner layers is made through ''via" holes that are copper plated. One super-computer was marketed in which all of the electronics was placed on a single multilayer circuit board. The materials and process control requirements are challenging, and the functional end-product is worth a great deal.
MCMs represent the leading edge of interconnection technology, and they are used when the time of transit of signals from chip to chip is an important limitation on the processing speed of the electronic system.
The speed of light is the ultimate barrier, and consequently it is essential to employ dielectrics that have the lowest practical dielectric permittivity. This is an area in which polymers offer substantial advantages over inorganic dielectrics. Practically any twentieth-century gadget you can think of, from the cheapest clock-radio to the most expensive mainframe computer, has its electronic guts mounted on printed circuit boards.
These "boards"—actually fiberglass cloth impregnated with a brominated epoxy polymer resin—got their name because the electronic components on them are wired together by thin copper ribbons deposited directly onto the boards, like ink on paper. The idea that bulky, plastic-clad copper wires could be replaced by ribbons of bare metal on an insulating background was one of the fundamental breakthroughs of the electronics revolution of the s.
Printed circuit board substrates are an example of a "composite material"—a multicomponent material that performs better than the sum of the properties of its individual components.
The chemical structures of such a material's components, and their relative proportions, can be tailored to provide just the right set of properties for a given application. In this case, the material has to be not only lightweight and strong but also an electrical insulator, which rules out the use of metal sheets. The material must also be fracture-resistant, so that it can be cut to shape or drilled without cracking.
And the material must be thermally stable—some of the newest, high-technology computer chips give off a lot of heat. The board has to handle such a hot spot without melting. The board also has to be flame retardant, so that an electrical short does not become a conflagration that wipes out a lot of expensive hardware.
In this composite material, the glass-fiber cloth gives the board its lightweight strength, while the brominated epoxy resin eventually becomes a rigid, three-dimensional network that gives the board the necessary stiffness, fracture resistance, and other properties. The manufacturing process starts with a roll of glass-fiber cloth. Carefully adjusted tension rollers feed the cloth at a precisely determined rate through a bath of the resin, which has been dissolved in a solvent.
The resin-impregnated cloth then wends its way over other rollers and through a series of ovens to evaporate the solvent. The heat and a catalyst also ''cure" the resin—promoting the chemical reactions that harden it into a tough, durable solid.
Several layers of partially cured cloth can be laminated together before further curing to make an even stronger circuit board. Finally, the cured board, now as stiff as its namesake, is sawn up into the individual circuit boards. Circuit board substrate materials have evolved over the years. New epoxies are now being used to improve dimensional control. Alternative polymer matrices are used for applications demanding high-temperature performance.
Polymers are also being used for the reinforcing fibers themselves.
Conducting Polymers Research Papers - tendpostremoni.ga
Printed circuit boards, the key interconnection medium for electronics, depend critically on polymers and their composites. By far the most research and development on materials for MCM dielectric layers has gone into polyimides, and most existing applications are based on polymers of this family. Great strides have been made in achieving the demanding property mix required through careful tailoring of the monomer chemistry.
Improved adhesion, lower dielectric constant, reduced sensitivity to moisture, higher thermal stability, and other properties have been improved greatly.
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The in-plane coefficient of thermal expansion was reduced and adjusted to the range of silicon, metals, and ceramics. Most major electronics companies manufacture MCMs based on polyimides. In spite of the extent of commitment to polyimides, it has proved difficult to achieve all the desired properties in a given composition. Other polymer dielectrics are in use, and new materials are under consideration. For example, commercial MCMs are manufactured by one electronics systems provider based on a proprietary epoxy-acrylate-triazine polymer that is photodefinable.
In spite of the large experience base with the polyimide materials, the newer polymers have advantages and offer attractive alternatives. All of the candidates are glassy polymers. The dielectric constants may be compared as follows:. In the final analysis, the choice of materials will be based on the sum of property advantages and processing practicality. Polymers offer the lowest dielectric constants and the thinnest "wires. Lithographic processes and associated technologies have advanced to the point that semiconductor device cells and conductor lines i.
This is owing to the fact that the propagation of signals through the wiring on the chip and in the module is becoming the dominant limitation on processor cycle time. The velocity of pulse propagation in these structures is inversely proportional to the square root of the dielectric constant of the medium. Hence, reductions in the dielectric constant translate directly into improvements in processor cycle time, in part because of the speed of propagation.
In addition, the distance between signal lines is dictated by noise issues or "cross-talk" that results from induced current in conductors adjacent to active signal lines. A reduction of the. Performance demands on polymers incorporated as permanent parts of the chip structure are even more stringent than the requirements for MCMs and PCBs. Insulating materials in chip applications must be able to withstand the very high temperatures associated with the processes used to deposit metal lines and to join chips to modules. At a minimum, they must withstand soldering temperatures without any degradation or outgassing.
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They must have thermal expansion coefficients that are closely matched to that of silicon. Silica meets all of the requirements extremely well, and this would continue to be the material of choice were its dielectric constant not so high. While much attention has been given to polymers with very low permittivities, there is an increasing need for high-permittivity polymers in capacitor applications.
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Clearly, organic polymers currently play a critical role as insulators in electronic devices and systems. Continued success in the development of new generations of these critical dielectric materials depends on close interactions between the microelectronics and the chemical communities, a relationship that is not in evidence in the United States. New partnerships are needed if we are to maintain competitiveness in this vital industry.
Organic materials are generally insulators or, in other words, poor conductors of electricity compared with metals and semiconductors. Electrical conductivity in metals and semiconductors arises from the delocalized electrons of the system, and they are best described by "band theory.
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