A Guide to Selecting Electrostatic-Dissipative Plastics

Many components for automated semiconductor manufacturing and testing equipment now are made of plastic materials. In addition to providing excellent chemical resistance, low metallic ion impurities, and low cost, plastics can protect against electrostatic discharges. But, how do you select the best material for resistance to ESD?

Many high-performance plastics have excellent electrical insulating capabilities, moderate to high heat resistance, and low coefficient of thermal expansion. But unless they are modified for ESD protection, components made from all of these materials actually can generate electrostatic charges. This can occur if they come in contact with moving devices.

Electrostatic-Dissipative Plastics

Several test methods can determine the ability of a material to dissipate or prevent the accumulation of static charges. For years, materials have been specified based on ASTM D-257, although the specification really is intended only for insulative materials and not for electrostatic dissipative or conductive materials. Nonetheless, materials often are specified based on their surface or volume resistivity by D-257.

The surface-resistance value can be converted, if desired, to a surface resistivity number by multiplying by 10. However, as pointed out in EOS/ESD Association Standard S11.11, this conversion may not be valid for materials that are laminated, plated, or metallized with conductive materials. In 1993, EOS/ESD Standard S11.11 was issued to provide the industry with a more relevant test method for measuring the surface resistance of static-dissipative planer materials.1

The commonly accepted range for an electrostatic-dissipative material is 104 to 1011 W resistance (105 to 1012 W /sq surface resistivity). While you might select a product with a low resistance to provide extra assurance that a charge will be dissipated quickly and completely, this may not always be the best choice.

For example, nests and sockets used in IC test fixtures must be insulative enough to minimize the crosstalk between adjacent leads, but conductive enough to prevent static buildup which can occur when electronic devices are fed through a test handler (Figure 1). For these applications, a resistance of 109 to 1011 W (1010 to 1012 W /sq surface resistivity) usually is the target.

Other applications often use a more conductive material. However, if a charge is dissipated too quickly, it can be harmful in some applications by lessening the degree of protection from charged device model events.

Likewise, the static-decay rate often is the criterion for ESD protection. MIL-B-81705C and FTMS 4046 generally are the accepted protocols for measuring the time required for a surface charge to dissipate. Although there may be general agreement between surface resistance and the static-decay rate, there is no quantitative formula for predicting one from the other. A maximum time of 2 s is standard for dissipating a 5,000-V potential applied to a surface if the material is to be considered electrostatic dissipative.

Environmental Considerations

Environmental factors are very important in selecting the optimum electrostatic-dissipative plastic. These factors are temperature (continuous and excursions), chemical (concentration, time, and temperatures), wear, and humidity.

With regard to temperature, there are electrostatic-dissipative plastics for continuous use at temperatures lower than 90°C, such as carbon-black-filled polyolefins, through temperatures of 260°C, such as electrostatic-dissipative Torlon PAI. Material costs generally increase with temperature capability.

Humidity can be a factor in several ways. The plastic or an additive in the plastic may be hygroscopic. Some, typically lower-end, materials are made electrostatic dissipative by compounding with a hygroscopic agent. These additives migrate, or bloom, to the surface and absorb a layer of water.

Obviously, this mechanism requires a threshold level of relative humidity (RH) for effectiveness. Surface resistance can vary depending on the RH. EOS/ESD S11.11 requires conditioning the samples at 12 ±3% RH, thus eliminating many of these materials.

Humidity can be a factor for hygroscopic plastics, such as nylon, which can absorb moisture and increase in dimensions. In some cases, dimensional stability may not matter; but for applications requiring very tight tolerances, it could make the material unacceptable.

In a dynamic application, wear is another consideration. Sloughing is the release of particles when a material is abraded. For some electrostatic-dissipative materials, such as carbon-black or graphite-filled products, these particles can be highly conductive. Carbon-fiber-filled plastics usually do not slough, nor are plastics made electrostatic dissipative by incorporating partially conductive organic additives, such as inherently dissipative polymers (IdPs).

Purity and Cleanliness

Ionic impurities and condensable materials often are areas of critical concern. Besides abrasion, contamination from components can affect the wafer processing in several ways. Liquids and chemicals can leach soluble constituents from the electrostatic-dissipative material, and plasma etching or ablation can release even the insoluble impurities. High temperatures and vacuum can result in the vaporization and redeposition of volatile components.

The ionic impurities may be measured in ppm or ppb by either leaching under conditions simulating the use environment or by the total digestion of the sample. The latter is much more severe and will indicate higher levels of impurities. It also is more predictive of the performance in an etch chamber where contamination by ions such as sodium, iron, calcium, or magnesium is detrimental to the process.

In addition to data from the resin suppliers for each type of impurity, NASA publishes a compendium of outgassing data (ASTM E-595 and F-1227) for thousands of materials.2 It is available on-line (http://arioch.gsfc.nasa.gov/313/).

Mechanical and Thermal Properties

Since electrostatic-dissipative plastics are available in a wide range of materials, their mechanical properties cover a broad spectrum. Glass or carbon fibers also can be added to make electrostatic-dissipative plastics stiffer and stronger to support higher loads.

Creep behavior must be considered, especially for seals or packings that might be subjected to a high continuous pressure. Unfilled PTFE (Teflon™) can creep and lose its tight seal. The addition of fillers to PTFE improves the performance several-fold. Creep data at a wide range of temperatures is available from material suppliers.

The heat-deflection temperature (ASTM D-648) often is used to obtain an indication of the maximum temperature at which a material exhibits most of its structural strength. This test measures a short-term thermal-softening effect.

The UL 746B can determine the material’s continuous-use temperature (CUTR). CUTR is a measurement of the long-term, irreversible loss of physical properties through oxidation and thermal degradation.

Table 1 shows the range of choices that currently are available in electrostatic-dissipative thermoplastic stock shapes.

Means of Fabrication

The design, size, and complexity of a part play a significant role in your choice of the ideal material and method of fabrication. If a part is to be mass produced in tens of thousands of identical, relatively small pieces, then injection molding may be the best choice.

Often in the semiconductor industry, technology developed in-house will reduce the number of parts to a few thousand or less. The production quantity and the desired speed of turn-around from design to finished part or the need to manage part inventory on a make-to-order basis often make machining from stock shapes the cost-effective solution.

Machining allows greater design freedom for complex parts and tighter tolerances than are possible with injection molding. Larger parts, like retaining rings, often are machined at a much lower cost per part than would be possible by other means. To optimize flexibility and yields, the stock shapes can be supplied in the form of plate, rod, or tubular bar, each in a wide range of sizes.

Summary

Many factors beyond surface-resistance properties must be considered when selecting an electrostatic-dissipative plastic for a mechanical part in a semiconductor application. Other equally important physical and mechanical data must be taken into account and matched to the requirements for the part in service.

References

1. Halperin, S.A., “The Difference Between Surface Resistance and Surface Resistivity,” EE-Evaluation Engineering, June 1996, pp. 46-57.

NOTE: This article can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Archives and use the key word search.

2. Campbell, W.A. and Scialdone, J.J., “Outgassing Data for Selecting Spacecraft Materials,” NASA Reference Publication 1124, Revision 2, 1993.

About the Author

Richard Campbell is the manager of product development at DSM Engineering Plastic Products. During his career, he has received 20 U.S. patents. Dr. Campbell earned a B.A. degree in chemistry and a Ph.D. in polymer science and engineering from the University of Massachusetts. DSM Engineering Plastic Products, P.O. Box 14235, Reading, PA 19612-4235, (610) 320-6600.

Table 1

ESD Material

(base resin and ESD technology)

Surface Resistance

Heat-
Deflection

Temperature

Continuous-
Use
Temperature,

Flexural Modulus

(psi)

Total
Material
Loss

Collected
Volatile
Condensable
Material

Semitron ESD 225

(Acetal + IdP)

1010 W

107°C

82°C

190,000

1.00%

0.05%

Semitron ESD 410

(PEI + CF)

104 to 106 W

210°C

170°C

850,000

0.46%

0.00%

Developmental ESD PES (PES + CF)

108 to 1010 W

210°C

170°C

550,000

0.20%

0.00%

Semitron ESD 500

(PTFE + proprietary)

1010 to 1012 W

260°C

260°C

350,000

0.00%

0.00%

Developmental

ESD Torlon

(PAI + CF)

1010 to 1012 W

260°C

260°C

800,000

0.95%

0.00%

Copyright 1997 Nelson Publishing Inc.

September 1997

Sponsored Recommendations

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!