Printed wiring substrate

PRINTED WIRING ASSEMBLY DRAWING A document that shows the printed wiring base, the separately manufactured components which are to be added to the base, and any other information necessary to describe the joining of the parts to perform a specific function. PRINTED WIRING LAYOUT A sketch that depicts the printed wiring substrate, the physical size and location of electronic and mechanical components, and the routing of conductors that interconnect the electronic parts in sufficient detail to allow for the preparation of documentation and artwork. 

The general physical and chemical properties which are required of a printed wiring substrate are largely determined by the following factors ... 

Phenolics are the lowest cost resins used in printed wiring substrate fabrication, but in the USA, account for only 3% of the volume due to their replacement by higher performance epoxies. In Japan, phenolic resins represent over 50% of consumption in printed wiring due to the... 

The major disadvantage of phenolics is low insulation resistance. Phenolics generally exhibit greater water absorption and sensitivity of the electrical properties to humid environments than epoxy resins, although only the so-called low-loss grades are used for printed wiring substrates. For these reasons, the use of phenolics in laminate fabrication has been restricted to the lower cost paper-based reinforcements. 

The use of polytetrafluoroethylene or PTFE for printed wiring substrate fabrication is a relatively well-established practice brought about by the need for improved high-frequency materials. The critical properties in this field of application are a low dissipation factor, a uniform dielectric constant which exhibits little variation with frequency over a wide bandwidth up to 15 GHz, combined with environmental and temperature stabilities. These properties can be achieved in a number of teflon/glass or teflon/glass/ceramic composites based on both fabrics and random glass. The dissipation factor at 1 MHz is more than 100 times less than a typical epoxy FR4, as shown in Table 9.4. 

The most widely used plastic materials in the printed wiring industry are epoxy resins based on bisphenol A. Epoxy-glass fabric laminates account for approximately 80% of the consumption of printed wiring substrates in the USA. In particular, laminates with specifications equal to or better than NEMA grade GIO or FR4, which combine very good dimensional stability with excellent electrical properties, are suitable for the fabrication of high density wiring that is required for computers and telecommunications equipment. 

High temperature thermoplastics such as polysulphones and polyetherimides are newcomers to the printed wiring substrate arena and offer the potential for the production of relatively low-cost moulded substrates with excellent electrical characteristics and thermal stability. The low dissipation factors characteristic of these materials have generated considerable interest in their potential for use in... 

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