Electrical Electronic

With the diverse electrical properties of plastics, extensive use of plastics has been made since the first plastics was produced. Plastics permits the operation of many electrical and electronic devices worldwide. As it has been said many times most of the electrical/ electronic equipment and devices used and enjoyed today would not be practical, economical, and/or some even possibly exist without plastics. Plastics offer the designer a great degree of freedom in the design and particularly the fabrication of products requiring specific electrical properties and usually requiring special and accurately fabricated products. Their combination of mechanical and electrical properties makes them an ideal choice for everything from micro electronic components and fiber optics to large electrical equipment enclosures.

Development of many different polymers and plastic compounds (via additives, fillers, and reinforcements) continues to expand the use of plastics in electrical applications. By including fillers/additives, such as glass in plastics, electrical properties can considerably extend performances of many plastics (Fig. 4.55).

The electrical properties of plastics vary from being excellent insulators to being quite conductive in different environments. Depending on the application, plastics may be formulated and processed to exhibit a single property or a designed combination of electrical, mechanical, chemical,

Dielectric constant

% additives or fillers

thermal, optical, aging properties, and others. The chemical structure of polymers and the various additives they may incorporate provide compounds to meet many different performance requirements.

Plastic provides ideas for advancing electrical and electronic systems from conducting electricity to the telephone to electronic communication devices. Thousands of outstanding applications use plastics in electrical products. The users' and designers' imaginations have excelled in developing new plastic products.

Shielding Electrical Device

With the extensive use of plastics in devices such as computers, medical devices, and communication equipment the issue of electromagnetic compatibility (EMC) exists that in turn relate to electromagnetic interference (EMI) and radio-frequency interference (RFI).

EMC identifies types of electrical device's capability to function normally without interference by any electrical device. These devices are designed to minimize risks associated with reasonably foreseeable environmental conditions. They include magnetic fields, external electrical influences, electrostatic discharge, pressure, temperature, or variations in pressure and acceleration, and reciprocal interference with other devices normally used in investigations or treatment.

EMI or RFI as well as static charge is the interference related to accumulated electrostatic charge in a nonconductor. As electronic products become smaller and more powerful, there is a growing need for higher shielding levels to assure their performance and guard against failure. From the past 40 dB shielding, the 60 dB is becoming the normal higher value. There is EMI shielding-effectiveness (SE) that defines the ratio of the incident electrical field strength to the transmitted electrical field strength. Frequency range is from 30 MHz to 1.5 GHz (ASTM D 4935-89).

Many plastics are electrical insulators because they are nonconductive. They do not shield electronic signals generated by outside sources or prevent electromagnetic energy from being emitted from equipment housed in a plastic enclosure. Government regulations have been set up requiring shielding when the operating frequencies are greater than 10 kHz.

The plastic shielding material used may include the use of additives. Designs may include board-level shielding of circuit, bondable gaskets, and locating all electrical circuits in one location so only that section requires appropriate shielding. Designers of enclosures for electronic devices should be aware of changes in EMC that tend to continually develop worldwide.

Conductive plastics provide EMI/RFI shielding by absorbing electromagnetic energy (EME) and converting it into electrical or thermal energy. They also function by reflecting EME. This action ensures operational integrity and EMC with existing standards. Conductive plastics are generally designed to meet specific performance requirements (physical, mechanical, etc.) in addition to EMI/RFI or static control. Often these plastics have to perform structural functions, meet flammability or temperature standards, and provide wear or corrosive resistant surfaces, etc.

The usual plastics alone lack sufficient conductivity to shield EMI and RFI interference. Designers can reduce or eliminate sufficiendy electromagnetic emissions from plastic housings like those of medical devices and computers just by shielding the inner emission sources with metal shrouds in the so-called tin can method. The same effect can be obtained by designing electronics to keep emissions below standard limits or by incorporating shielding into the plastic housing itself. Designers will often employ all these strategies in a single design. What is most important is to attempt to locate all the shielding in a relatively small volume within the larger housing and then tin can it to provide a simplified solution rather than spreading it out.

Every electronic system has some level of electromagnetic radiation associated with it. If this level is strong enough to cause other equipment to malfunction, the radiating device will be considered a noise source and usually subjected to shielding regulations. This is especially true when EMI occurs within the normal frequencies of communication. When the electronic noise is sufficient to cause malfunctioning in equipment such as data processing systems, medical devices, flight instrumentation, traffic control, etc. the results could prove damaging and even life threatening. Reducing the emission of and susceptibility to EMI or radio frequency interference (RFI) to safe levels is thus the prime reason to shield medical devices (and other devices) in whatever type of housing exist, including plastic.

In addition to compounding additives for shielding, there is the technology of applying conductive coatings, such as vacuum systems or paint systems (sprays, etc.). Other methods include the use of conductive foils or molded conductive plastics, silver reduction, vacuum metalization, and cathode sputtering. Although zinc-arc spraying once accounted for about half the market, others have surpassed it. Other conductive coatings are also used. Unlike other shielding methods, conductive coatings are usually applied to the interiors of housings and do not require additional design efforts to achieve external aesthetic goals. All systems offer trade-offs in shielding performance, the physical properties of the plastics, ease in production, and cost.

Designers have to confirm the suitability of a material's shielding performance for each system through such conventional means as screen-room or open-field testing. Each approach to shielding should also be subjected to simulated environmental conditions, to determine the shield's behavior during storage, shipment, and exposure to humidity. Some times comparison of shielding materials becomes difficult. ASTM has a standard that defines the methods for stabilizing materials measurement, thus allowing relative measurements to be repeated in any laboratory. These procedures permit relative performance ranking, so that comparisons of materials can also be made.

Organizations involved in conducting and/or preparing specifications/ standards on the electrical properties on plastics include the Underwriters Laboratories (UL), American Society for Testing and Materials (ASTM), Canadian Standards Association (CSA), International Electrotechnical Commission (IEC), International Organization for Standardization (ISO), and American National Standards Institute (ANSI).

UL has a combination of methods for environmental conditioning and adhesion testing to evaluate various approaches to shielding and to determine the plastic types that are suitable. The primary concern is safety. Should a metalized plastic delaminate or chip off, an electrical short is formed that could cause a fire.

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