Glass, inorganic dielectric properties

Aromatic polyimides have glass transition temperatures in excess of 400 °C, excellent toughness and elongation properties and dielectric constants comparable to that of inorganic dielectrics, about 3.5. An important feature relative to these applications is their ability to planarize the topography when spun on as the soluble precursor polyamic acid. The subsequent intramolecular condensation reaction to form the heterocyclic imide is typically a thermal "curing" process. 

The important feature of LC polyesters is a low coeffident of thermal expansion a < 1 x 10 grad that is comparable with the value for inorganic glasses (5 x 10 grad ) and significantly less in comparison with ordinary polymers (1 x 10 grad" ). These properties have led to the use of Vectra LC polymers in many electronic applications such as sockets, switches, bobbins, connectors, chip carriers, and sensors. Vectra LC polymers have replaced stainless steel in medical applications. The LC polyesters are particularly attractive for miaowave frequency electronics due to low relative dielectric constants, low dissipation factors, and the commerdal availability of laminates. 

A variety of low-dielectric, low-loss resin systems are available for high-speed circuit apph-cations. These include polytetrafluoroethylene (FTFE or Teflon ), cyanate ester, epoxy blends, and allylated polyphenylene ether (APPE). Likewise, a few different reinforcements and fillers are available that can be used to modify the electrical properties of the base material. Although E-glass is stm the most commonly used fiberglass reinforcement, it should be noted that others are available. In addition, inorganic fillers are sometimes used to modify electrical properties as well. Table 9.6 provides electrical property data on some of the available fiberglass materials. Table 9.7 provides data on some of the base material composites available. 

The properties of inorganic materials with dielectric constants less than 12 are described in this section under the headings of Ceramics, Glasses, Micas, Substrates, Glass-Ceramics, and Thick Films. Since micas and glasses are also used as dielectrics, the properties associated with capacitor applications are discussed in the section on Dielectrics. 

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