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Improved Standards for Specifying Static Control MaterialsDavid E. SwensonAlthough static control materials themselves have changed little in the past 10 years, the standards for them have evolved and should assist designers in developing a conscientious static control program.
Standards and Specifications Today, users of static control materials have the benefit of industry standards to guide selection. Static control material suppliers also benefit from these standards, because the standards help alleviate disagreements about test methods, performance attributes, and material specifications. In 1990, the industry had no ESD-related handling practice documents that enjoyed widespread acceptance. In the ensuing 10 years, various groups developed such handling practices, which are gaining in acceptance. Readers should become familiar with three documents that will affect the industry for many years. These documents are
Each document contains tables that list almost every conceivable static control product or material along with a suggested range of performance suited for the intended application. The specifications for static control products and materials involve an electrical property or a physical characteristic that imparts a static control property. Static control material types include static dissipative, static conductive, static shielding, static discharge shielding, and low charging (formerly known as antistatic). The definition (by electrical or physical characteristic) of each material depends on the application. For example, a static-dissipative floor material has a different resistance range than a static-dissipative packaging material. Also, test methods vary among applications. Specifiers of static control materials must be aware of the different properties and test methods to ensure that they use products and materials that meet not only the intended application but also the handling practice specifications referenced in their ESD control program plan. It is important to note the new term, low charging, which has replaced the word antistatic. Many of the handling practices documents have made this change because the term antistatic has been misused and misapplied. IEC originated discussion about eliminating the term antistatic, and the phrase low charging was agreed upon by all member countries in IEC Technical Committee 101, Electrostatics. The new phrase more accurately reflects the physical property sought in the design of interacting surfaces. In other words, this interaction should result in low static-charge generation between surfaces that contact and separate. Older U.S. standards, such as EIA 541, the premier packaging material standard, defined antistatic exactly the same way. Unfortunately, antistatic is often used in a generic sense to describe the full range of static control materials and products, which results in a great deal of confusion between specifiers and suppliers. Table I shows the currently accepted static control material types, definitions, and test methods.
Test Methods In the past 10 years, the standards development groups, especially the ESD Association, have made much progress in creating and refining test methods for static control materials and products. In 1990, a controversy surrounded the use of ASTM D257 for measuring the surface resistivity of static-dissipative materials. Today, ANSI/ESD S11.11 is widely used for this purpose, although the results are expressed simply as ohms rather than ohms per square (see articles on this subject in past issues of Compliance Engineering by Niels Jonassen). International standards such as IEC 61340-4-1 also reference the methodology described in S11.11. These newer standards eliminate the variables in the ASTM method that caused confusion for the dissipative range of materials. Historically, more papers have been presented on problems with using ASTM D257 for the dissipative range of materials than on almost any other static control subject. Please note, however, that ASTM D257 is still recommended and used for its intended range—insulating materials. ANSI/ESD S11.31 measures static discharge shielding of bags. This test method or a similar updated variant will be used in the upcoming revision of EIA 541 to replace the static shielding test (V-ZAP—capacitive probe test) cited in Appendix E (1988). The newer ESD Association method eliminates significant variables associated with making a differential voltage measurement as described in the EIA document. Because the information obtained by each test method is useful for ensuring quality assurance and assessing material design, packaging-material suppliers may opt to use both methods. Both methods use a capacitive probe inserted into the package under test (bag or pouch). The EIA version uses two voltage probes that monitor the voltage on the top and bottom plates of the capacitive sensor. The observed voltage values are added together and the sum is reported as the peak voltage observed inside the package. The newer test places a 500-W resistor across the plates of the capacitive sensor. A current probe measures the induced current flowing through the resistor. Generally, a computer program is used to calculate the resulting energy by integrating the area under the observed current waveform. Charge generation testing has always been controversial, and nothing is different in the year 2000. No methods are recognized industry-wide for determining whether something has a low charge-generating propensity. The best solution is to create a test that simulates the end use of the proposed static control material in conjunction with the items or materials it will contact. Many papers have been presented on this subject in the EOS/ESD symposiums over the years. Interested readers are encouraged to look at proceedings of the EOS/ESD symposiums from 1990 to the present for more information. No values are shown in Table I for the charge generation level associated with the term low charging, because each application must have its own definition. The end-user community is beginning to recognize resistance to ground as the appropriate measurement for most items used in the typical static control work environment. Installed floors, work surfaces, chairs, shelving units, carts, people, and any other conductive or dissipative item may be evaluated for static control performance by measuring resistance to ground. Resistance between two points is a measurement used to compare materials during the selection process and to evaluate items like garments, chairs, wrist-strap components, shoes, etc. Some element of care is needed to select the appropriate test method. Of course, to avoid disagreement, it is best to use industry-accepted tests. An old test method, called static decay, is still widely referenced even though it is controversial. Although static decay can be used to give an indication of the charge-draining characteristic of materials, some experts argue that resistance measurements can provide the same information. In some cases, though, resistance measurements alone may not predict how an item may react when electrically charged and then brought into contact with ground. Static-decay testing may provide information about the dynamic nature of charge mobility on an item that resistance tests (with defined voltage and current) cannot provide. The main argument in the industry relates to how the item under evaluation receives the charge. The methods generally discussed include dc (power supply contact charging), triboelectric charging (contact and separation with some other material), and corona discharge (noncontact charging). The charge-decay measurement of an item or material sample is relatively straightforward using a noncontacting voltmeter, electric-field meter, or other such device, along with some sort of timer. Please note, however, that charge-decay testing is really only meaningful on homogeneous materials. Because the most conductive path or layer in the material controls the result, testing composite materials or laminated structures results in information that may be difficult to interpret. Performance Requirements To address static control performance logically, the electronics industry might be segmented as general electronic assembly, semiconductor manufacturing, disk drive manufacturing, and flat-panel display manufacturing. Although many would (perhaps rightfully) argue that this list is an oversimplification of the complex world of electronics, it provides a reasonable breakdown to discuss the important static control material properties within these broad industry segments. Other physical properties can also often affect the acceptability of a given item or material within these broader segments. Interestingly enough, static control properties and performance specifications have not changed much over the past 10 years, but actual products and materials have changed dramatically. This change is due to the other physical- and chemical-property requirements demanded by end-users. All of these industry segments now have requirements for cleanliness, ionic content, outgassing, and water vapor transmission. These other requirements, rarely discussed just 10 years ago, are driving material and product changes far more than static control property considerations. The difficulty for the static control material designer is how to obtain or keep the static control property and maintain cleanliness at the same time. It is important to understand that one of the reasons the demand for cleanliness is currently so great is because of the ease (relatively speaking) of detecting infinitesimally small amounts of anything on a surface. Can 20 ng of sodium or nitrate ion per square centimeter on a packaging material really cause a problem? This almost-magical detection ability is great for forensic science, but is demanding that a wristband strap material contain no common ionic levels above a few micrograms really necessary? If that is the case, then how can the people who must wear the wristbands be allowed in a work area? People are obviously far more contaminating than a wristband. It would be helpful to the industry overall to see some scientifically sound proof, presented in an appropriate peer-reviewed forum, that demonstrates that the cleanliness demanded by many manufacturers is truly needed. A material design triangle for the industry appears in Figure 1. Pick any two attributes, and the third will not be (readily) available. Although this statement is obviously a bit cynical, it is depicted to make a point about material properties; that is, it is difficult to do everything for everyone without some give-and-take regarding specifications.
Recent Advances in Materials A major advancement is the commercialization of materials that meet the definition of intrinsically conducting polymers (ICPs). These polymer-based materials are similar to semiconductors in that a stable matrix has been modified with a dopant additive that allows conduction through the (insulating) polymeric matrix. This process, of course, is significantly different from bulk loading of conductive powder, fiber, or crystalline materials into a polymer during the melt phase to create a bulk conducting material. Continued ICP development will result in new, cost-competitive materials that are stable and far more useful than the materials available in the 1980s and early 1990s. Of particular note is that this ICP class of materials can maintain a clean surface. Many of the new materials are useful for a variety of applications. Coatings and surface treatments to modify the electrical or triboelectric (charge-generating) properties of material surfaces have been used for more than 30 years. The need to provide surface electrical resistance, lubricity, and other modifications to reduce charge generation will continue. Some segments of the electronics industry now require that materials provide low ionic content, low outgassing, and reduced nonvolatile residue. Factoring in these requirements will remain a challenge to materials suppliers. It is not well understood in the electronics industry that reducing static-charge generation between any two contacting surfaces requires some sharing of ionic species (chemicals) between the surfaces. These concerns apply to all sorts of static control products and materials, including work surfaces, flooring, garments, grounding devices, packaging and containers, chairs, ionizer systems, and anything else that enters the work environment. Metallized surfaces for the purpose of static shielding or static-discharge shielding are widely recognized and used systems. However, adding the requirement of low water-vapor transmission (WVTR) affects the transparency of packaging materials. Most conventional and reasonably priced moisture barrier films contain a heavy aluminum layer that results in an opaque package. Although with special materials it is possible to provide low WVTR in a clear film, the cost is prohibitive for all but the most sophisticated applications. If static discharge shielding is also required in the clear film, the cost rises substantially. Low outgassing is a consideration for all materials that enter a cleanroom. So, reducing the volatile components from items entering a cleanroom is a logical cleanliness requirement, but it also has cost and static control performance implications. Many additives that provide low charge-generation characteristics or static-dissipative properties possess considerable outgassing levels that cannot be avoided. Naturally, some newer technologies may be able to show reduced or, in some cases, acceptable levels of outgassing for many applications. However, as discussed earlier, it is time to produce proof that many of the cleanliness-related specifications are really necessary. This article cannot provide sufficient detail on the many recent advances in static control materials. Many suppliers offer a wide selection of materials, so it is valuable to review the latest materials available. In addition, industry forums such as the EOS/ESD Symposium present a number of papers on materials (see http://www.esda.org for this year's symposium details). Conclusion Handling electronic parts requires an electrostatic protected area (EPA). This concept is universally present in the handling practices documents recently prepared by the Electronic Industries Alliance, the International Electrotechnical Commission, and the Electrostatic Discharge Association. The definitions of static control material properties and their specifications are fairly well harmonized in the handling practice documents. The small variations that do exist should be relatively easy to work around in a well-thought-out static control plan. Cleanliness issues require careful, thoughtful, and reasonable consideration when determining materials specifications. It must be understood that, in most cases, static control attributes for many products are provided at the expense of cleanliness. Some new materials claim to have low contamination levels while maintaining static control properties in the static-dissipative or static-conductive ranges. Materials properties should be investigated based on realistic and necessary application requirements. David E. Swenson is a technical service specialist for 3M Electronic Handling and Protection Division (Austin, TX). He currently serves on the ESD Association's board of directors and executive committee in the position of president emeritus and represents the United States to the International Electrotechnical Commission (IEC) Technical Committee (TC)-101, Electrostatics. |
What is different about static control materials now compared to 1990? Standards and specifications exist today that did not just 10 years ago. These standards and specifications are a result of better understanding of what is needed for maintaining static control. During this same period, specifiers of static control materials have demanded higher performance at a lower total in-use cost. Equally important, advances in the disk drive industry have led to a level of process and product ESD sensitivity that has taxed the performance ability of static control materials. Each of these considerations has affected the formulation, processing, and application of static control materials.
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Figure 1. Material design triangle. When selecting materials, choosing two attributes often means the third will not be possible.