Technological Advancements for ESD Problems
Extensive testing has shown that the concept of alloying inherent dissipative polymers with standard thermoplastics is a significant
advance in ESD technology.
By Herman Van Wees and Gary Wilson
The association between electrostatic discharge (ESD) and plastics is still somewhat of a mystery to many designers, engineers and purchasing professionals who participate in the application arena. Every major plastic resin in its natural state is electrically insulative. The use of insulative materials in static-sensitive environments has long been known to create problems related to the build-up of static charges and attraction of particulate contamination that can lead to product damage. In fact, some studies show that 25-75 percent of all electronic component failures are related to problems arising from ESD, creating billions of dollars in losses per year as the result of assembly line rejects, warranty costs, service calls and liability claims.
The challenge is to create or modify plastics to allow for controlled dissipation of static charges in such a way that the original properties desired in a plastic material are not effected.
Electrostatic discharge
The European draft specification CECC 00015/1 defines electrostatic discharge as “a transfer of electrostatic charge between bodies at different electrostatic potentials caused by direct contact or induced by an electrostatic field.”
ESD events can be quite powerful; 10,000 – 26,000 volts have been reported from the simple removal of a circuit pack from a typical bubble pack film.1 However, even levels undetectable by touch, such as a few hundred volts, are enough to cause damage in many of today`s electronic devices. ESD can also be a problem in volatile environments where explosive gasses have built up or where the air is full of powdery materials.
Materials used in these environments must have the ability to transfer charge in a controlled fashion, at a rate sufficiently slow enough to avoid an arc, but fast enough to transfer the charge before a critical discharge voltage is attained in a reasonable period of time.
Effectively eliminating static build-up in plastics is a challenging proposition. Traditionally, two technologies have been used to render plastics dissipative:
1. Conductive fillers can be used to load the plastic, creating a composite with sufficient conductivity to dissipate the charge quickly.
2. Chemical antistats, applied topically or as migrating additives within the plastic, can attract enough atmospheric moisture to create a dissipative surface layer.
The first approach provides a permanent, highly dissipative solution, but suffers from a lack of colorability, difficulty in consistently reaching the dissipative range, and “hot spots” of conductivity which may lead to arcing problems.
Contamination
and corrosion
The chemical antistat approach has many drawbacks associated with it, including humidity dependence, lack of permanence, corrosion and contamination.1-8 In addition to these aesthetic and serious ESD performance problems associated with current methods of making plastics ESD safe, other problems that further limit the functionality of the finished goods produced exist. They include contamination control concerns and the possibility of promoting corrosion in metal leads.
Outgassing occurs when volatile materials, such as low molecular weight antistatic additives, migrate and volatilize from the surface of the plastic. This effect is accelerated at higher temperatures. Problems occur when these volatiles collect on sensitive parts. The extent of the problem varies depending upon the nature of the volatile material and the sensitivity of the application. Some volatile materials can cause corrosion, while others form thin contaminant layers on surfaces that interfere with product processing.
Sloughing is a problem typically associated with filled plastics. It is most evident in plastics that have been modified for ESD applications by incorporation of conductive carbon black. The sloughing effect is easily demonstrated by rubbing a carbon black loaded part on white paper; the abraded carbon particles leave a distinct black line. The release of these particles in a cleanroom environment is unacceptable.
Contact corrosivity must also be considered a concern. The electronics industry in particular uses a wide array of metal materials both in assembly and as part of the finished product. Plastic material choices can be affected by the propensity of some plastics to accelerate corrosion when they are brought into contact with metals.
Inherently dissipative polymer alloys
Recent work aimed at new methods of producing ESD plastics has led to the discovery of electrically active polymers that may solve both the ESD and contamination problems. The new approach involves the alloying of plastics with a new class of materials “inherently dissipative polymers (IDPs).” IDP refers to polymers with inherent conductivity properties in the dissipative range; defined by the Electronics Industry Association as 105-1012 ohms/sq. surface resistors. Blending of IDPs with conventional plastics will create a non-migrating dissipative network throughout the plastic, rendering it static-dissipative.13-15
These alloys can be processed by conventional molding and extrusion technology to produce finished parts, films or sheet products. The technology has already been introduced into business machine housings and internal parts, as well as in electronics packaging and handling. Commercial alloys have been produced using ABS, acrylic, polyester, nylon, and thermoplastic polyurethane elastomers.
Experimental
Materials. The following materials were used for alloy formation: BFGoodrich Stat-Rite® C-2300 polymer and general-purpose ABS injection molding grades.
Alloying. All blends were prepared by intensive melt mixing in a Berstorff 25-mm twin screw extruder. Test samples were prepared both by cast sheet extrusion, compression molding, or by injection molding. Some tests were done on pellets of pure Stat-Rite in its raw state.
Testing. Static decay testing was done on an ETS model 406C meter according to FTMS 101C Method 4046. All reported data was obtained using the 0 percent cut-off and a relative humidity of 15 percent. Resistivity testing employed an ETS model 803B probe in conjunction with a Keithley model 617 programmable electrometer using the meter`s internal 100 volt source.
All resistivity data was collected at 50 percent RH. All physical properties testing was done according to ASTM procedures as follows: tensile properties–ASTM D-412; hardness–shore A; and outgassing testing ASTM F 1227-89.
Results
Characterization of the IDP. The IDP functions in alloy systems by setting up a series of interconnected elongated domains that are capable of conducting ESD voltages in a controlled fashion. (See photo).
ESD Testing. Conclusive ESD testing involves evaluating the electrical properties of the material in various environments and aging cycles. Surface and volume resistivity [as defined by EIA-541]4, and static decay [as defined by FTMS 101C] were used to establish base line properties and reproducibility.
Loadings of the IDP were varied and optimal levels were identified that insured consistent performance in the dissipative range. Secondary testing was done to simulate potential use environments. These tests included studies done at low humidity, testing after heat aging, and testing after extended water or solvent contact. The results of an IDP and alloy system are shown in Tables 1 and 2.
Physical Properties. The Stat-Rite IDP is an elastomeric material that is added at levels between 15-25 percent. In rigid systems there is some loss in stiffness, usually less than 10 percent. In some cases, impact properties are increased while in other compounds, the impact was unchanged or reduced slightly. In well produced alloys, there is no significant change in elongation and mold shrinkage, allowing the modified plastics to be processed in existing tooling.
Outgassing Testing. Two methods were used to analyze outgassing. Both were done on the Stat-Rite C-2300 IDP in its pure form; additional work on the IDP in alloy form was similar.
The first test is an application-based test, where the IDP is heated in a closed container, a vacuum is drawn, and a semiconductor wafer is exposed to the effluent. Weight loss on the IDP sample is measured and weight gain on the semiconductor is measured. In all internal testing, as well as in more sensitive external testing, no collected volatile condensable material (CVCM) could be measured on the semiconductor.
In an analytical technique called GC Mass Spec, the results are confirmed; only air, carbon dioxide and water are given off at similar test conditions. The results of the testing are summarized in Table 3.
Corrosivity Testing. Contact corrosivity was used as a practical measure of the propensity of the pure Stat-Rite C-2300 material to accelerate corrosion in metals. The testing on the three test metals shows better results on the sections of the metal covered by the IDP than the sections that were left uncovered. Analytical tests done to look for known corrosive materials such as chlorine or sulfur, showed only trace quantities. The results of the test are shown in Table 4.
Conclusions
Many approaches have been tried to make insulative plastics perform as ESD materials. The ideal approach would be a permanently dissipative material that had no associated negative side effects. Because of the many problems encountered with current ESD approaches, a battery of tests is necessary to prove the value of materials. Extensive testing has shown that the concept of alloying inherently dissipative polymers with standard thermoplastics is a significant advance in ESD technology. This new approach exhibits none of the drawbacks associated with current conductive filled or chemical antistat filled plastics.
The IDP-based alloys are colorable, not dependent on humidity for performance, and are shown to be permanently static dissipative. Secondary testing has shown that use of IDPs does not contribute to outgassing, particulate contamination, or promote corrosion in metals. n
References
1. McFarland, W.Y., “The economic benefits of an effective electrostatic discharge awareness and control program–An empirical analysis,” An ESD Management Focus, The EOS/ESD Association, Rome, NY, p. 7-12.
2. Havens, Marvin R., “Understanding Pink Poly,” 1989 EOS/ESD Symposium Proceedings, New Orleans, LA, 1989, p. 95-100.
3. Cenelec Electronic Components Committee, Basic Specification: Protection of Electrostatic Sensitive Devices, CECC 00015I.
4. Electronics Industries Association, Washington, DC, Specification EIA-541.
5. Koyler, J.M. and D. Watson, “Electronic Packaging and Production,” May 1990, p. 72-75.
6. Koyler, J.M. and W.E. Anderson, “Permanence of the Antistatic Property of Commercial Antistatic Bags and Boxes,” EOS/ESD Symposium Proceedings, Orlando, FL, 1982, p.120-123.
7. Head, G.O., Drastic Losses of Conductivity in Antistatic Plastics,” EOS/ESD Symposium Proceedings, Orlando, FL, 1987, p. 36-40.
8. Anderson, J., J.R. Dalton and M. Smith, “Contaminated Antistatic Polyethylene,” EOS/ESD Symposium Proceedings, Orlando, FL, 1987, p. 36-40.
9. Topolski, A.S., “Incoming Inspection of Antistatic Packaging Materials,” EOS/ESD Symposium Proceedings, Las Vegas, NV, 1981, p. 65-74.
10. Mass, T.R., M. Woods, B. Lee, and K. Hicks, “ESD Polymer Alloys–A Novel Approach for Permanently Static Dissipative Thermoplastics”, EOS/ESD Symposium Proceedings, New Orleans, LA, 1989, p. 89-94.
11. Lee, B.L., “Permanently Electrostatic Dissipative Property via Polymer Blending,” ANTEC 1190 Symposium Proceedings, p. 409-411.
12. Marasch, M.J., “The Evolving Static Control Options for Thermoplastics,” SAMPE Proceedings, June 1990.
13. Mas, T.R., “An Alternative Approach for Producing Permanently Static Dissipative Polyethylene”, EOS/ESD Symposium Proceedings, Orlando, FL 1990, p. 237-244.
14. Mass, T.R. and T.E. Fahey, “ESD Polymer Alloys: Considerations for Electrical Property Optimization,” ANTEC 1991 Proceedings, p. 551-516.
15. Fahey, T.E. and G.F. Wilson, “Inherently Dissipative Polymer Films,” EOS/ESD Symposium Proceedings, Dallas, TX, 1992.
Gary Wilson is a senior research and development associate for BFGoodrich Specialty Chemicals. His major responsibility includes development of polymeric compounds utilizing Stat-Rite technology. Wilson joined BFGoodrich upon graduation from the University of Akron, where he received a BS in Chemistry. He has been with the company for 28 years.
Herman Van Wees is a research and development associate/supervisor for BFGoodrich Specialty Chemicals where he concentrates on the development of Stat-Rite technology. A native of The Netherlands, Van Wees holds a Master`s degree in chemistry from the University of Utrecht. He recently joined BFGoodrich from Nevamar.
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The electron micrograph shows the elongated domains that are left after the matrix plastic material is dissolved away.
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