As any ESD packaging engineer or electronics purchasing agent will tell you, many different types of packaging technologies and materials can protect your product from environmental and ESD contamination. The package can be a flexible metallized bag, an antistatic bag, a rigid molded case, or a thermoformed clam shell.
Thermoformed packaging offers the lowest cost route to a custom-designed package for electronic components. It also provides a stronger package than flexible bags. By choosing from the multitude of thermoformable materials available, you can optimize the material’s cost vs ESD performance, mechanical properties, and contamination characteristics. These options allow you to design the most cost-efficient package for your needs.
Polyester (PETG), PVC, polystyrene, polyacrylics, and other plastics are surface-coated with quaternary ammonium salts, amidoamines, or salts of octanoic acid to impart nonpermanent ESD properties. The antistat combines with water vapor in the air to form a simple positive-negative circuit that allows the movement of electrons to dissipate a static charge.
These treated plastics have surface resistivities of 109 to 1010W/sq at 50% relative humidity (RH). Since these materials use moisture in their dissipation mechanism, the true test of their functionality is whether they remain static-dissipative at relative humidities as low as 12% to 15%. For example, EOS/ESD S11.11-1993 Surface Resistance Measurement of Static-Dissipative Planar Material measures the surface resistance of the material at both 50% and 12% RH.
Another criteria used widely to qualify this type of packaging material is the polycarbonate compatibility test in EIA 564-1992. It determines if the dissipative material can damage polycarbonate substrates, and it can give you some understanding of whether the coating can damage certain parts contained in the thermoformed package. As a result, materials evaluated by these tests may meet your packaging requirements.
Chemically, these coatings contain ionic species that could transfer from the sheet to both electronic devices and worksurfaces. Because this technology is not permanently bonded to the base resin, migration of the treatment is a definite possibility.
Special handling procedures may be needed for packages manufactured from this material. Excessive handling and abrasion as well as contact with solvents, even water, can remove the treatment and, consequently, its static-dissipative properties. Since this technology can lose its effectiveness over time, it is important to verify the ESD characteristics of the material periodically.
Selecting a thermoformer with experience designing and manufacturing packages with this technology is important. Improper thermoforming will degrade the antistat’s capability to function.
Topical coating of plastic does not affect the optical clarity or mechanical properties of the base resin. This technology offers the lowest cost-per-pound of all of the choices that we will discuss. If proper precautions are taken when choosing antistat-treated products, they can be an appropriate, cost-effective choice for certain one-way packaging applications.
Permanent Static-Dissipative Filled Plastics
Permanent static-dissipative properties can be imparted to any plastic by mixing an electrically conductive or dissipative material into the plastic when it is molten. Carbon black, powdered metals, mixed-metal oxides, and inherently dissipative/conductive polymers are used to achieve surface resistances ranging from 103 to 1010W.
Carbon black is a semimetallic, microscopic, fibrous material mixed with plastics to impart surface resistances ranging from conductive (30 W) to static-dissipative (104 to 1010W).
Carbon-loaded plastics accommodate package designs ranging from static- dissipative to EMI/RFI shielding. Carbon loading can enhance the chemical resistance of the plastic. Conversely, higher carbon levels result in harder, more brittle materials that are difficult to thermoform.
To achieve conductivity, carbon loading can be as high as 10% to 15%. As a result, these materials are only available in opaque black, which prevents bar-code reading and visual inspection of components in the package. Additionally, the material is difficult to color or tint.
Carbon loading offers a relatively inexpensive means to achieve permanent conductive properties in a plastic material. Because the overall density of the material is less, shipping costs for this type of packaging are lower than with materials that use metals and metal-oxide fillers.
Because of the fibrous nature of carbon black, all plastics treated this way will slough conductive particles, a concern in clean-room operations. The conductive particles released from the material will contaminate the manufacturing environment as well as the electronic components. The costs of premature part failures due to particle contamination can be almost as high as the failures attributed to ESD.
Powdered metals such as nickel, copper, silver, and aluminum are used to load plastics to make them conductive. These metals make highly conductive products used almost exclusively for EMI/RFI shielding applications.
Even though the loading of these materials typically is less than 10%, the density of the material increases significantly. An increase in the material density raises the cost for two reasons. First, the number of packages that can be made from a pound of material is decreased. Second, the additional weight of each package increases shipping costs, which will add up over the life of a reusable package.
Since metal-loaded plastic generally cannot be made dissipative, the value of metal-loaded plastics can only be used in EMI/RFI shielding applications. This is where the material’s higher cost, higher package densities, and high conductivities can be justified.
Overall, powdered metal loading enjoys the same advantages and suffers the same disadvantages as carbon black loading, with the additional disadvantage of being high in cost.
Mixed-metal oxides, such as indium tin oxide, antimony tin oxide, silicon dioxide/tin oxide, and titanium dioxide/tin oxide, are examples of semiconductors that impart static-dissipative properties to plastics. These materials are not as conductive as powdered metals or carbon black. Primarily, they are used to provide permanent static-dissipative properties ranging from 103 to 1010W. Depending on the choice of mixed- metal oxide, you can obtain optical clarities ranging from very clear to opaque.
As with carbon black and powdered metals, mixing these materials into the plastic can reduce its tensile strength and make it brittle. By balancing the loading level and the glass transition temperature of the plastic, you can make semiconductor loaded sheet material that has good thermoformability.
Mixed-metal oxide loading can range up to 50%, depending on the type of mixed-metal oxide used. The absolute conductivities of the different mixed-metal oxides vary greatly as to siemens/grams, which dictate the loading efficiency of the individual mixed-metal oxide in the plastic.
Mixed-metal oxides also vary greatly in price. For example, indium tin oxide is comparably priced to powdered silver, and titanium dioxide/tin oxide is nearer to powdered aluminum.
A polymer alloy is a mixture of a nonconductive plastic and an inherently conductive or dissipative polymer. These alloys are permanently static-dissipative.
Polymer alloys are a reasonable alternative to carbon-loaded plastics because they do not slough conductive particles, and they can be colored. Unlike antistat-treated materials, this dissipation technology does not migrate or outgas and will remain active after repeated washings and reuse.
The alloying process typically decreases the strength of the base polymer. In turn, this reduces the depth of draw available for a given package design. In some cases, the material must be thicker to produce comparable mechanical properties found in other technologies. This may mean that more material is needed to manufacture a package with the same mechanical protection of a stronger material, negating any price-per-pound advantage. This is especially true for PETG.
Permanent coatings are a form of ink or paint applied on the surface of plastics that can be thermoformed. This type of coating must be stretchable and thermostable and maintain its bond to the plastic as well as its electrostatic properties.
Four types of permanent coatings are commercially available for static-dissipative, thermoformable plastics: a carbon black loaded coating, a coating containing polyanaline, a coating containing an antimony doped tin oxide, and an ultraclear semiconductor coating. Currently, all of these coatings are applied to PETG sheet.
Because permanent coatings are applied on the surface of the plastic, they do not change the mechanical properties of the base material. As a result, the thermoformability of the plastic is unchanged. Maintaining the electrostatic properties of the surface is directly related to the stability, stretchability, and continuity of the coating.
Of the four coating types commercially available, carbon-loaded coatings suffer from conductive particle sloughing that manifests itself as the crayon effect. As discussed in carbon-loaded plastics, this type of contamination is problematic for some clean-room applications.
The carbon black coating is applied to the plastic so light can pass through single sheets of the material, allowing only shadows to be seen. The polyanaline-coated material is a semitransparent dark green color, making it clear enough to identify the configuration of the part inside, but colors are difficult to recognize.
The antimony tin oxide is a white translucent plastic similar in appearance to PETG/inherently dissipative polymer alloy sheet. In some cases, it is difficult to determine whether the translucent package contains parts without opening it.
The ultraclear semiconductor coating allows you to visually inspect the parts without opening the package. In fact, the noncontact bar-code reading capability is unique to this permanently dissipative material. This feature gives you the flexibility to identify and inventory multiple parts within one package without confusion or risk of damage. In addition, the ultraclear material can be tinted, and the others cannot. This permits you to color-code packaging without losing clarity.
With all four of these materials, accurate identification of parts inside the protective package is impossible without labeling the outside or opening the package.
Opening the package is fundamentally counterproductive to the purpose of protection. Once the package is opened, the part is exposed to the environment. Even under controlled conditions, the risk of damage due to ESD or contaminants increases significantly.
All four product types are water washable, reusable, and humidity independent.
Polyanaline, however, is pH-sensitive, and its static-dissipative properties may be lost under certain washing conditions. All four products have some degree of resistance to isopropyl alcohol, with the ultraclear semiconductor coating having the best chemical resistance.
Resistance to incidental solvent or detergent contact is important, since these chemicals are present in many manufacturing environments. Contact with these chemicals could cause immediate package failure or degradation over time. In turn, this could lead to parts failure or contamination of the manufacturing area.
All four types of permanently coated plastic are priced higher than their nonpermanent, antistat-treated counterparts. But, all four generally are less costly than filled plastics. Table 1 summarizes the general performance characteristics of commercially viable PETG-based packaging materials.
All packaging material must be tested to determine whether it meets your company’s ESD and contamination requirements, such as resistivity, ion content, outgassing, and particulation. The packaging material must provide adequate strength and rigidity to protect the parts from mechanical and structural damage. Ultimately, the decision to select a material must not be based on the material alone. The selection of a manufacturer that has a reputation of quality, technical/customer service, on-time delivery, and fair pricing should not be overlooked.
About the Authors
Robert L. Benson is the product manager for ESD coatings at Specialty Coatings. He holds a B.S. in chemistry from Purdue University. Specialty Coatings, 3500 Delta Lane, Elk Grove, IL 60007, (847) 766-3555, e-mail: [email protected].
Suneer V. Patel is the product development/technical service manager at Pacur Co. Mr. Patel has a B.S. in textile chemistry from Clemson University and an M.S. in polymers from Georgia Institute of Technology. Pacur Co., 3555 Moser St., Oshkosh, WI 54901, (920) 236-2888.
Ultraclear Mixed- Metal Oxide
Carbon Black Coated
Outgassing ASTM D595
105 to 106
103 to 105
109 to 1010
103 to 108
103 to 107
108 to 1010
Strength of Material
Copyright 1998 Nelson Publishing Inc.