With the development of new composites, a long-time aversion to using plastics in electronics packaging has been overcome. In the past, problems resulting from the buildup of electrostatic charges and electrostatic discharge (ESD) limited the use and acceptance of plastics for many applications.
Most unfilled thermoplastics are electrically insulating. These resins often carry inherent charges or generate a triboelectric charge via motion or friction. Extrusion-compounding electrically conductive additives with insulating resins creates hybrid thermoplastic composites. These composites have the capability to dissipate electrostatic charges via air or ground and resist tribocharge generation. Thermoplastic composites containing conductive additives span the surface- resistivity spectrum from antistatic to conductive to EMI shielding (surface resistivity ranges: 100 to 1012 W /sq). They are breaking down conventional thoughts about insulating thermoplastics and are finding applications in markets such as business machines and electronics that need static protection.
Plastics are classified as electrically insulating materials having surface resistivities ranging from 1014 to 1018 W/sq. These high resistivities prevent the free motion of static charges that cause ESD problems.
The static-dissipative thermoplastic composites commercially available today are heterogeneous mixtures of conductive additive and insulative thermoplastic base resin tailored to produce composites with surface resistivities across the resistivity spectrum from 100 to 1012 W/sq. The surface resistivity spectrum, as shown in Figure 1, can be divided into four classifications of material conductivity:
Antistatic Composites (109 to 1012 W/sq).
Dissipative Composites (106 to 109 W/sq).
Conductive Composites (102 to 105 W/sq).
Electrostatic Shielding Composites (100 to 102 W/sq).
Different types of conductive additives are used to produce composites within these ranges.
Antistatic composites are used when a part must not have an initial charge and does not experience high movement in application. These composites also prevent high leakage currents from being conducted between metal contacts on the surface of the part.
Static-dissipative composites are applicable when leakage requirements are less stringent. This range also is important where human contact is involved. The human body model describes humans to have a resistivity of about 106 W/sq.
Applications specify this resistivity range for two reasons:
To prevent the molded part from being more conductive (102 to 106 W/sq) than a human. This prohibits a human charge from being conducted to a sensitive device that is too conductive (100 to 105 W/sq).
To prevent conducting voltages to humans. A voltage-carrying assembly must be no more conductive than 106 W/sq (106 to 1012 W/sq).
In both cases, it is important that the application be 106 W/sq or greater to prevent conduction of high voltages to and from humans. These composites offer protection in an ESD situation by distributing the charge along the part surface and quickly dissipating it to the air or ground.
Historically, conductive composites have been the easiest resistivity range to produce, based on the percolation curves of stainless steel, carbon fiber, and carbon powder (Figure 2). Applications involving high-speed movement, such as paper-moving, often require conductive composites. These composites provide a grounding path for charge buildup and rapidly dissipate ESD voltages.
Electrostatic shielding composites block high ESD voltages from damaging electronic components and supply a grounding path for charge bleed-off. An EMI/RFI shielding material will attenuate or reduce the intensity of electromagnetic or radio frequency interference by either reflecting or absorbing these emissions.
Pitch and PAN (produced from the polymer polyacrylonitrile) fibers are the most commonly used reinforcements for ESD applications in the conductive composite range. Pitch carbon is produced from oil pitch, which can contain a variety of ionic and anionic contaminants. In some sources, high levels of free sulfur accelerate metallic corrosion, making pitch fibers undesirable for many electronic applications.
Some recent improvements in the production of pitch carbon have resulted in a product with significantly less contaminants. Pitch carbon fibers will improve mechanical properties, but not to the degree of PAN carbon fibers.
The raw materials must be very pure and contaminant free to complete the polymerization process. Furnace heating of polyacrylonitrile drives off the polymer component, leaving a conductive, partially graphitized carbon chain. These fibers provide higher strength/stiffness than pitch fibers and are low in contaminants.
Stainless steel fiber and nickel-coated carbon fiber are acceptable for ESD applications but typically are used when more conductive products in the EMI/RFI shielding range are required. These reinforcements help block high electrostatic voltages from damaging electronic components, shield electromagnetic interference/radio frequency interference, and provide an excellent grounding path for charge bleed-off.
Stainless steel fibers are thin filaments drawn down, typically to 8 microns. Nickel-coated carbon fibers have a layer of pure nickel on the surface of the PAN fiber. Both of these types of fibers have high aspect ratios that form a conductive matrix at lower reinforcement levels. Low-volume reinforcement levels of each of these fibers often are sufficient to provide EMI shielding.
Fillers are smaller particulate materials with aspect ratios that lie well below the critical level needed for reinforcement, often approaching 1:1. Common fillers used in ESD applications are carbon powder, metal powders, aluminum flake, and migratory and permanent antistatic polymers.
Conductive carbon blacks are the most common fillers used in static-dissipative composites. The carbon blacks are a special grade characterized by small particle size, high surface area, and low volatiles content.
Metal powders also are used to achieve conductivity in various resins. Unlike carbon powder, these additives are more spherical in shape and have low surface areas and high specific gravities. Since higher filler content is necessary, properties are sacrificed, and cost/in.3 can become prohibitive to their use.
Deciding which composites can achieve various resistivity ranges is tied into the percolation curve of surface resistivity vs volume content of conductive filler/reinforcement (Figure 2). Curves of surface resistivity vs content of stainless steel, carbon powder, and PAN carbon fiber exhibit similar characteristics with lower resistivity at higher filler levels.
The steepness of the curves makes it difficult to achieve tight resistivity ranges on the high end of the scale. Despite this, tight ranges are possible, especially with carbon powder grades that do not have wide variations in the filler aspect ratio. Some carbon fiber systems also attain tight resistivity ranges.
The most consistent fillers for composites in the antistatic range are migratory and permanent additives. Migratory systems tend to bloom to the surface and attract moisture to the part. These are nonpermanent and, after cleaning a part, require time to regenerate antistatic properties.
These systems often are highly dependent on relative humidity and can fail static-decay tests at low relative humidity. Permanent systems are based on nonmigratory conductive polymers that can compound with thermoplastic resins to create an antistatic composite. These conductive polymer systems are permanent and cannot be washed away like migratory antistatic additives. Unlike migratory systems, permanent systems are not dependent on relative humidity.
The increased acceptance of thermoplastic ESD composites has opened new doors to the electronics market. The possibilities for ESD composites go far beyond just the computer industry, and the challenge is to find new markets that can benefit from the use of these materials.
About the Author
Stephen L. Thompson has worked for LNP Engineering Plastics for three years and currently is the Stat-Kon® applications engineer. He is a graduate of Penn State with a degree in plastics engineering. LNP Engineering Plastics, 475 Creamery Way, Exton, PA 19341, (610) 363-4597.
Copyright 1998 Nelson Publishing Inc.