ESD Resource LibraryTHE CHALLENGE OF STATIC ELECTRICITY
Intrinsic static-dissipative polymers can minimizestatic contamination in production processes.
By James R. Skinner
ESD Resource LibraryStatic electricity, earlier a scientific curiosity, is acquiring increasing importance as a source of industrial contamination.
As industry achieves higher degrees of miniaturization and more precise, repeatable production control, improved contamination management is essential for manufacturers to remain profitable via consistently high quality and high yields. Particulates are no longer the only problem. Static electricity is emerging as a significant contaminant and industrial concern.
The aerospace industry faced the task of creating new military aircraft made partly from materials able to dissipate electrical energy. 'Stealth' fighters and bombers were built from new plastic compounds that absorbed or conducted very low levels of electromagnetic energy--i.e. radar signals--used to track them, without compromising airframe performance and life.
From these innovations, recently declassified, a new family of injection-molding plastics has evolved. These new materials incorporate the intrinsic ability to dissipate static electrical charges, so that such charges can neither estroy sensitive products nor generate forces that can attract particulates. The new materials: ‘intrinsic static dissipative’ (ISD) polymers.
Static electricity has been studied for millennia. Electrostatic forces were observed 2,000 years before either batteries or transformers existed-- 300 years before Christ, Theophrastus found that amber rubbed with dry materials, attracted straw and feathers. The Greek word elektron for amber was used 1,800 years later by Queen Elizabeth I's physician William Gilbert, to form the new neo-Latin term via electrica he coined; the word later almost reacquired its original form: 'electron.'
The list of scientists who studied static electricity is long and illustrious--Ampere and Priestley, Franklin and Farraday, Coulomb and Volts among them. Their laws that describe electrical behavior remain definitive. In 1733 , C.F. de Cisternay du Fay defined two types of static charge: 'resinous' electricity due to excess electrons, e.g. in amber and 'electron deficiency' electricity, noted in glass. A piece of amber as small as a pencil eraser may acquire >100 billion excess electrons in <0.1 second, though the charging current may be less than one ten-millionth of an ampere.
In the final decade of the 20th century, static electricity is becoming recognized as a source of industrial problems, e.g. in its propensity to attract particulates or discharge into ultrasensitive electronic materials or devices and thus destroy them.
As recently as the '80s, dimensions and tolerances used by high technology industries were well understood and controlled at high levels of repeatability in most production processes. Contamination--primarily particulate--was generally manageable. Today microelectronics features are shrinking from tenths of a micron (a millionth of a meter) to Angstrom widths. As a result, power dissipation in electronics and computer equipment is continually declining, down to microamp levels, as designers strive to reduce physical size, weight and power needs, especially when easy portability is required and batteries may be a forcing function.
Examples abound: gigabyte magnetic and optical disk drives with dimensions as precise as integrated circuits must work reliably and error-free for tens of thousands of hours. Printer ink-jet cartridges and fuel injectors, with tiny orifices, are similarly critical. Toiletries applied to human skin and medicines intended to be ingested must be scrupulously clean. Medical devices of all kinds need to be surgically sterile from manufacture to end use. Such products--the list is growing daily--demand strict contamination control throughout every production process.
Until recently the term 'contamination' was used primarily in reference to physical particles--dust or 'dirt'--required to be filtered from ambient atmospheres. Today it is being applied to many other effects, including factors to which attention has not been dedicated previously. They include but are by no means limited to electromagnetic interference (EMI), even outgassing of materials used in products or processes. Static electricity is one such contaminant, as newly defined.
Static electricity, as discovered by Theophrastus, may be generated whenever dissimilar materials move or abrade. Thus the modern production environment, featuring (for example) conveyor systems that enable advanced levels of automation to be achieved, may generate potentially damaging static charges. For the purposes of this paper, plastics used in conveyors are the principal concern. New advances are described here that enable plastic conveyor components to dissipate static effectively, economically and long term, vs. undesirable insulation or conduction.
High levels of insulation or conduction are equally undesirable in production processes. Yet materials and products exhibiting these characteristics have been used historically, either because the work in progress (WIP) or the production environment (conveyor systems, as one typical example) inherently incorporated one or other of these attributes.
Insulators such as plastics are rarely perfect but inherently include measurable conductance. They can build large electrical charges that create initial dirt-attracting forces and naturally seek a conductive discharge path. In an insulating material, the decay or discharge rate due to its minute residual conductance might be as long as an hour, a time during which the material would retain the charge and thus the attracted dirt.
Conductors such as metal are rarely perfect but inherently retain measurable insulation. In a highly conductive material, the decay rate of an electrostatic charge can be measured in nanoseconds. In conductors, a static electrical charge bleeds off virtually instaneously at extremely high voltage, potentially damaging parts or materials through which it is conducted.
The accompanying figure defines the electrical behavior of insulating and conducting materials in terms of surface resistivity, in accordance with the U.S. Electronics Industry Association criteria. The information shown comes from EIA Interim Standard 5A. Though later specification have been proposed and some of them adopted, these data reflect current art with respect to conducting, semiconducting (dissipating) and insulating characteristics in materials.
Static-dissipative properties are essential because they are semiconductors, with surface resistivities falling in the range shown on the chart. These intermediate levels of conductivity offer excellent ESD protection for high-technology products, because they permit controlled discharge rates.
The recently released EOS/ESD S11.11-1993 specification requires static-dissipative surfaces to exhibit surface resistance within the range of 104 to 1011 ohms. A static decay rate or 5,000 VDC to zero in less than two seconds is also required, as specified in U.S. Federal Test Method 101C per EIA Standard 541. IDP Materials shown below, meet these criteria.
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| Figure 1. Conduction, semiconduction and insulation resistivity ranges per EIA Interim Standard 541. Note: surface resistance is measured in ohms and is an order of magnitude lower for an equal value compared with resistivity measured in ohms/square. |
Starting with the Industrial Revolution, metal was the primary material used in high-volume production processes. It was widely available at low cost, and world industries learned to use it. But in the 20th century, especially in the final two or three decades, intensive worldwide basic and applied R&D discovered an increasing array of alternatives to metal, for example plastics.
Plastics used in production processes have established important advantages over earlier materials and methods that were dirty, unreliable and noisy. Earlier (typically metal) systems suffered from high cost because of limited life and were often inflexible because of their innate design and construction.
Use of plastics vs. metal in conveyor systems stemmed from a need to address these and other problems. Result: creation of new conveyor designs from many manufacturers, in which the principal parts, and elements in contact with work in progress, were made of plastics rather than metal. This material replacement--plastics vs. metal --demonstrated several important advances:
For most manufacturers of relatively low-technology products these benefits were sufficient over the past two decades to make plastic-based conveyors essential in modern production systems. In the '90s high-technology product manufacturers started to use them and initially achieved satisfactory operating results. Plastics-based conveyors could be used advantageously in hyper-clean (Class 100 or cleaner) environments essential to the manufacture or advanced electronics products and systems.
But as product dimensions and tolerances started to shrink below micron levels, difficulties arose for those high-technology manufacturers. The inherent insulation characteristics of plastic conveyor components started to cause problems. Electrostatic discharge (ESD) became an increasingly worrying factor.
Most kinds of plastics are inherently insulators. Plastic materials can develop a static electrical charge when they move through air or come into contact with other materials. Such charges may reach 106 volts or more. Two specific difficulties arise for manufacturers who specify standard plastic conveyors and thus face possible electrostatic contamination:
Two over-all approaches were developed initially to reduce the effects of electrostatic charge: chemicals (incorporated into or deposited onto the plastic material) or the use of carbon or metal fillers in the material.
Anti-static agents as basic ingredients in the plastic polymer can control static. These agents are typically incorporated as integral compounds of plastic polymers when molding powders or pellets are manufactured. They migrate slowly to the surface of, for example, a molded component or sheet of plastic, increasing its surface conductivity. But over time they become depleted from within the polymer; their ability to dissipate static charges deteriorates until they become ineffective.
A second approach is to apply a chemical anti-static agent to the plastic surfaces. Like the integral compounds described above, most such agents require the presence of moisture to become conductive, calling for relatively high ambient humidities (>50%). These topically applied anti-static agents are easily worn off or they may be removed from the plastic surface, either with water or common solvents--both widely used to clean plastic components or all kinds.
Because both these methods are innately impermanent, industry is questioning their use for components or systems expected to perform without attention for long periods--for example >104 hours. In addition, chemical anti-static agents reportedly cause or contribute to metal corrosion within production systems or work in progress, e.g. in electronic devices, printed-circuit boards or PCB-mounted electro-mechanical components. They can also cause fungus or bacteria to grow on the work in progress, or in the manufacturing equipment itself.
Another way to increase surface and/or volume conductivity is to incorporate conductive carbon or metal fillers into the molding powder or pellets. But carbon- or metal-filled thermoplastic polymers do not bind the carbon or metal permanently. Over prolonged use, they slough off the carbon or metal materials, compromising particulate cleanliness of the manufacturing process and work in progress. In addition, fillers render transparent plastic materials opaque, limiting product identification within the package as often required for production tracking.
In a contamination-control world that is becoming continually more demanding, all these concerns must be addressed effectively. Costly failures are the alternative.
Intrinsic dissipative materials for use in molding plastic conveyor links were developed from aerospace technology. Static-free autoclaving bags were needed in which to build composite aircraft wings. This unique technology was acquired by the author, resulting in a family of materials that addressed and solved static issues. The principal goal was to control static dissipation.
Dissipative materials are inherently semiconductors. They strike a balance between insulation and conduction, to permit continuous, controlled (i.e. safe) discharge of charged electrons inevitably generated when dissimilar objects move, abrade or are adjacent to each other. Such discharge must be controlled to less than about two seconds but must not be instantaneous.
Static-dissipative polymers thus achieve the essential middle ground between the two undesirable characteristics of insulation and conduction as discussed and illustrated earlier. These new, intrinsically static-dissipative (ISD) plastic polymers have evolved from aerospace applications for use in industry: in conveyors and packaging used in manufacturing, processing or transporting computer disk drives, semiconductor integrated circuits (ICs), fuel injectors, ink-jet printer cartridges, toiletries and medical/surgical equipment.
These ISD materials represent the state of the plastic-polymer art for solving the problems of electrical discharge and dirt attraction in manufacturing processes. They can be modified or 'doped' to alter their valence ring, thus reducing the number of electrons in the molecule. The electron 'holes' provide an electrical path in the plastic material through which voltage may flow They can readily be compounded and chemically coupled with certain other compatible polymers, to make materials that can be extruded, injection molded or blow molded as required in the specific workplace application.
The resulting compound can be tailored to deliver a static dissipative surface in a continuous-phase mixture. The intrinsic dissipative properties do not depend upon ambient humidity to achieve electrical conductivity, and do not wear off or wash off.
The use of plastics in conveyors evolved and grew partly because of the willingness and ability of engineering, marketing and management staffs, both at conveyor manufacturers and at their customers' sites, to embrace unprecedented change as a way to progress technically. The result has been the continual growth of plastics conveyors in key applications worldwide, and the achievement of continually growing revenues and profits in work situations where these desiderata were becoming increasingly compromised by new forms or contamination.
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