Traditionally, metals have been the material of choice for EMI shielding. Today, however, the use of plastic materials for EMI shielding is increasing rapidly. Currently, the thermoplastic share of the shielding market in the United States is about 40%. In Europe, use is even higher because of the stringent regulations imposed by the European Community.
For example, high-performance optical encoders are susceptible to EMI if they are not shielded properly. In a move to replace metal housings, Quantum Devices adopted polycarbonate plastic impregnated with carbon fiber for its optical encoders.
As Quantum Devices and many other manufacturers have learned, plastic shielding has several advantages over metal. It is lighter weight, less expensive, and more resistant to corrosion. It also provides more design freedom for complex shapes, parts consolidation, and assembly options.
In their normal state, plastics are transparent to EMI. However, there are two basic ways to metallize a plastic to make it suitable as an EMI shielding material:
- Impregnate it with conductive metal or metallized fibers. This is the least expensive method.
- Apply a conductive coating.
Filling Plastics With Conductive Fibers
A number of conductive fiber suppliers, with the help of plastic compounders, have undertaken the development of high-aspect-ratio fibers suitable for EMI shielding. These special conductive fibers create a continuous network within the normally insulative plastic resin to provide effective shielding properties.
The conductive fibers usually are supplied as metal or metallized fiber in a chopped pellet form, ranging from approximately ¼² to 1². These pellets blend with precom-pounded resin pellets to give a ready-to-mold mixture called a cube blend. They also can be extrusion-compounded with resins into ready-to-mold homogeneous pellets called a compounded blend.
In both the cube blend and the compounded blend, the composition usually includes a matrix resin and a conductive modifier. The matrix resin consists of a thermoplastic resin with reinforcements, modifiers, or additives to impart particular physical properties to the final compound. The conductive modifier is chosen for its specific conductive and shielding properties and because it is compatible with the matrix resin.
Cube Blend
A cube blend feeds directly into an injection-molding machine. Its matrix resin component contains thermoplastic resin and possibly other additives such as a flame retardant, a wear modifier, a reinforcement, and pigments. This component is extrusion-compounded prior to blending with conductive fiber pellets. Since the conductive additive is not subjected to the shear of extrusion compounding, less shielding additive is needed to achieve the desired conductivity and shielding levels.
The primary advantage of the cube blend is lower cost. Conductive fiber pellets can easily be added during molding if conductivity or shielding performance must be improved.
Compounded Blend
The compounded blend will have all the same components as the cube blend but in one homogeneous pellet rather than a blend of two or more ingredients. Dispersion of the shielding additive generally is more complete with less effort during the injection process.
This blend will have more fiber length attrition, resulting in a reduction in conductance and shielding when compared to the same concentration in a cube blend. This can be remedied by increasing the amount of additive.
Metallic Additives
Metallic substances, including stainless steel, copper, nickel, and silver in fiber, flake, or particulate form, are typical additives. Metal-coated substrates such as glass or carbon fiber, minerals, or glass beads also supply shielding capability to thermoplastics.
The primary form of metallic additive for shielding composites is a fiber. Its high aspect ratio forms conductive pathways through the resin matrix at low concentration.
Stainless steel fiber is the most commonly used metallic additive. These 7- or 8-micron diameter stainless steel fibers are made from minute filaments woven into a strand, wetted with an appropriate sizing or thermoplastic resin, and cut into lengths of 4 to 7 mm. Copper, nickel, and silver fibers also are available. Metal flakes, powders, and particulate sometimes are used as shielding additives, but their aspect ratio is not good, and they have little use in commercial applications.
Metal-coated substrates are composites of nickel, copper, silver, or other metal blended with glass and carbon fibers, glass beads, mica, titanium dioxide, or other minerals. The primary commercial product in this group is nickel-coated carbon fiber (NCCF), usually supplied in a form similar to stainless steel.
Like minerals and glass beads, nickel- and silver-coated substrates sometimes are used for shielding additives, particularly where reinforcement properties are not required. Applications for these include seals and gaskets in elastomeric thermoplastics where elongation and elastic memory are desired and strength is not important.
Applying Conductive Coating
Several methods can be used to apply a conductive coating to plastic to make it suitable for shielding. Three of the most popular methods are vacuum metallization, electroless plating, and application of a metallic paint.
Vacuum metallization is the process where a layer of aluminum is deposited on the plastic inside a vacuum chamber.
Electroless plating is a multiple-stage chemical process. A thin layer of copper is applied for conductivity, followed by an overlay of nickel to inhibit corrosion. The nickel also provides a good surface for painting. Of all methods of coating, this offers the best EMI shielding; however, some plastics will not accept plating.
The third process consists of applying a metallic paint, generally with particles of silver, copper, or nickel dispersed in an acrylic, vinyl, epoxy, or urethane binder. For general shielding, nickel in an acrylic binder is most widely used. Most plastics accept and hold paint well, and automatic spraying systems apply coating with consistency.
Impregnation vs. Conductive Coating
A comparison of impregnation vs. conductive coating favors the impregnation method for several reasons:
- Shielding permanence is an integral property of the plastic that is impregnated. With a conductive coating, scratches may reduce the shielding effectiveness or thermal cycling can cause delamination or other adhesion problems.
- Impregnated plastic is relatively easy to recycle, but the sediment from decomposition of a conductive coating often presents disposal problems.
- Shielding a product’s contours with impregnated plastic is simpler and more reliable than using a metallic coating.
- Procurement lead times generally are shorter with impregnated plastic.
- Impregnated plastic is inherently corrosion-resistant. Copper coating is popular for applications requiring high conductivity but requires a protective topcoat to guard against corrosion.
- Impregnated plastic requires no special handling during molding, but a plastic base must be kept free of contaminants for the coating to adhere properly.
- The impregnated plastic is an EMI shield when it leaves the mold; the coating process requires additional care, including masking.
Shielding Capability and Cost
While the comparison is helpful, it does not address shielding capabilities. This is difficult because an impregnated plastic works primarily by absorption while the conductive coating does its job mostly by reflection.
With impregnated plastic shielding, most electrical conductivity is within the walls. There is little correlation between surface resistivity and shielding capability, but volume resistivity is a good measure of performance. On the other hand, with a plastic shield using conductive coating, the electrical conductivity is a surface property, and surface resistivity is the accepted measure of shielding.
To compare the techniques, it is necessary to test an impregnated plastic and a conductive coating on the same finished product. Such tests, performed by RTP and available on request, show no perceptible difference in attenuation or the frequencies that are covered.
To quantify the effectiveness of plastic shields of different compositions and thicknesses, RTP prepared shields with 5%, 10%, and 15% stainless steel fiber impregnation and thicknesses of 0.120², 0.090², and 0.075². For each shield, the attenuation was >39 dB from DC to 1.5 GHz, as shown in Figure 1.
The tests were carried one step further using nickel-coated carbon fiber. The test results in Figure 2 show similar readings for the moderately to heavily impregnated plastics but indicate that the lightly treated material had much poorer performance.
A cost analysis shows that, for a production run of 10,000 pieces, the impregnation shield is less than half the cost of coated shielding. For 100,000 pieces, the impregnated shield still has a cost advantage, although the margin is narrow at that volume.
About the Author
Larry Rupprecht is senior development engineer and manager of the conductive materials group at RTP Company. He has more than 20 years of experience in the formulation of electrically conductive plastics and a B.S. in chemistry from Winona State University. RTP Company, 580 E. Front St., Winona, MN 55987-0439, 507-454-6900.
Published by EE-Evaluation Engineering
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November 2000