This article was originally featured in the Q1 2019 Edition of SPE Thermoforming Quarterly Magazine.
With continued population growth, urbanization, and real-time global connectivity, electronics has become one of the fastest growing industries in our modern society. Constant innovation in the fields of smartphones, tablets, laptops, gaming devices, video recorders and television, increases the need for packaging material to provide safe, easy to handle and lightweight material that keeps the product safe from Electro Static Discharge (ESD). North America is the headquarters for some of the leading consumer electronics markets. Products that were previously designed and assembled in North America but manufactured in Asia, are now returning to North America for production as a result of the increasing labor costs in Asia. It is very important for the part shipment and assembly lines for these expensive electronics to be ESD safe.WHAT IS ESD PACKAGING?
Let’s start with a definition of ESD. ESD is the sudden flow of electricity between two electrically-charged objects caused by contact, an electrical short, or dielectric breakdown. A buildup of static electricity can be caused by tribocharging or by electrostatic induction. So, what if a minor electric current (electrons) passes through our body to the electric part? What harm could that cause?
If we think a minor static electric current cannot be damaging then we are wrong – to put this into perspective, a static ESD of as low as 25 volts can damage a microelectronic part, whereas the static generated through ESD can be as high as 20,000 to 45,000 volts under certain conditions (material type, humidity, contact type, etc.). These major charges not only damage the electric part but can also damage the internal data by altering or erasing the data. In more extreme cases, these charges can also lead to fire or explosion when introduced to flammable liquids or gases in locations such as hospitals or gas stations.
OPTIONS FOR ESD PACKAGING
ESD packaging, or ESD packaging material is the material utilized to reduce the buildup of static electricity under certain or all conditions when used for highly sensitive electrical parts. There are three types of ESD packaging materials classified by how quickly electrons move through the material: antistatic material, dissipative material, and conductive material.
Antistatic Material: Antistatic material is used to prevent the buildup of static electricity caused by tribocharging. Antistatic material is the least expensive form of ESD material and is the most widely used ESD material not only in electrical applications but also for cosmetics and food packaging applications to preserve aesthetics and keep away dust. Antistatic material resistivity generally ranges between 1010 and 1012 ohms per square and initial electrostatic charges are suppressed.
Dissipative Material: Dissipative material is used for the protection of sensitive electrical parts where material directs and reduces the flow of electricity in a more slow and controlled manner. Dissipative material is the most ideal packaging material in the ESD range. Static dissipative resistivity generally ranges between 106 and 1012 ohms per square and possess low or no initial charges. This material prevents discharge to and from human contact.
Conductive Material: Conductive material is the material with the least electrical resistance where electrons can flow easily across the surface or through the material and pass on to the ground or to another conductive object that is in contact with the material. Both dissipative and conductive material are referred to as antistatic material in the industry. Conductive material resistivity generally ranges between 103 and 106 ohms per square with no initial charges providing the path for charge to bleed off. Conductive plastics material is used in packaging applications such as electronics packaging and storage, aerospace components, medical device, automotive and consumer electronics. Some of the benefits of conductive material include: 1) Light weight with up to 50% saving in weight compared to metals or painted/coated with surfactants, 2) Easy to handle, store, and transport, 3) Low cost, 4) Easy processability, and 5) reusability.
CARBON BLACK AND PROCESSABILITY
Most Plastic is insulative in nature so how is it made electrically conductive?There are two ways to make the plastic material conductive, 1) through the use of a carbon-based additive (carbon black, carbon nanotubes, graphene, etc.), and, 2) a metal-based additive (copper, nickel, stainless steel, silver, etc.). Depending on the conductivity and mechanical properties either one or a combination of both can be used in designing an ESD conductive material.
Metals are naturally conductive and increase the density of the material it is compounded with. Thus, Carbon black is the most widely used filler to make an electrically conductive polymer. Carbon Black is inexpensive and formed by burning hydrocarbons in a limited oxygen environment leaving fine residual carbon particles behind. All carbon blacks are conductive but it is very important to understand the dispersion of carbon black in the compound and its percolation threshold with adequate carbon black volumetric loading. Depending on how the carbon black was formed and dispersed, it gives flexibility to the compounder to meet its cost vs performance needs where sometimes lower carbon black content can lead to high electrical conductivity and vice versa. At the same time, it also affects the mechanical properties (tensile and impact properties) of the material.Compounding Carbon Black and later converting the pellets as a conductive compound or masterbatch to various thermoplastic resins requires a significant amount of shear for proper dispersion. Shear and residence time of the compounding can increase the electrical conductivity as a result of improved dispersion. However, eventually a “conductivity plateau” is reached and continued mixing can work negatively and reduce the electrical conductivity. Not only does it affect the electrical conductivity but it also affects the surface properties (surface roughness, surface defect) and to some extent the mechanical properties.
How do we know it’s an ESD Material and how do you test it?
There are two resistivity test methods used to measure the ESD properties of the material: surface resistivity and volume resistivity.
Surface Resistivity: Surface resistivity is the most common ESD measurement of a material and it is used for all materials that are intended to dissipate electrostatic charges. The surface resistance is measured using two heavily loaded electrodes with an ohmmeter connected in between two electrodes of the surface material being tested. It is very important to keep the testing material on an insulative table or floor to avoid any variance. Material needs to have good contact with the electrodes that are placed at a set distance as per ASTM D257 standards. Surface resistivity is measured in ohms/square.
Volume Resistivity: Volume resistivity is mostly used for conductive material to check the dispersion of conductive compound through the polymer. Volume resistivity is tested in a similar fashion to surface resistivity, however, electrodes are placed on opposite faces of a test sample to check on the conductivity as per ASTM D257 standards. Volume resistivity is measured in ohms-cm.
THIN GAUGE ESD PACKAGING MATERIAL FOR THERMOFORMING
Innovation in thermoplastics material with high mechanical properties, combined with carbon black masterbatch with improved electrical conductivity at lower loading rates, is moving brand owners towards thin gauge thermoformed material for their ESD packaging needs. Impact Plastics’ state-of-the-art extrusion lines have been designed especially for thin gauge applications and can produce thin gauge conductive sheet for use in the most challenging ESD applications for electrical, medical, automotive, and aerospace components. A recent project for an undisclosed aerospace application required an 0.018” (18-mil) product where gauge consistency as well as electrical conductivity were critical to the success of the project.The design for this custom thermoforming application called for a cavity approximately the size of a ballpoint pen or half the size of a Nano sim card. For such a precise thermoforming application it was important to process material that was engineered for both precision thin gauge extrusion as well as uniform carbon dispersion and electrical conductivity for maximum ESD protection. This was achieved through the use of our real-time online gauge monitoring device and offline surface resistance test meter.
To design the packaging material, it is very important to understand the final end-use application. Factors such as material cost, storage and environmental conditions (humidity), product ESD sensitivity, shelf life, aesthetics, primary or secondary packaging, type of packaging (disposable or reusable) and testing conditions can all affect the product recommended. Impact Plastics evaluates the packaging needs of each application to formulate a custom ESD protective packaging material that will keep electrical parts safe be it temporary or permanent. Our conductive HIPS, ABS, and PP provide excellent shielding properties along with all the mechanical and physical properties of the base material itself.
Check out the full edition of SPE's Q1, 2019 Thermoforming Quarterly Magazine!
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