NOTE: This article has been translated from Spanish to English. It can be found in its original language in the 2014 Europe EMC Guide.
Tim Fornes, Ph.D.
Senior Staff Scientist, Chemical Research
Electromagnetic interference (EMI) is an unwanted disruption in the communication of an electrical device caused by foreign electromagnetic waves. Such foreign waves or radiation induce electrical currents that interfere with the device’s normal operation. Simply stated, EMI is the result of the intrusion between foreign and desired signals.
EMI can intermittently render a device inoperable. While this interruption can be benign, such as disturbing a cell phone conversation, it can have serious consequences for applications in the transportation industry, where, for example, just a brief interruption in the avionics system of an aircraft can be dangerous.
When designing electronic devices, manufacturers must conform to regional and global regulations to prevent electromagnetic interference. Since almost any device that has some electronic component is susceptible to foreign radiation, manufacturers have developed several circuit-grounding and device-shielding methods.
EMI Shielding Methods
Electrical device manufacturers have traditionally used metallic enclosures to safeguard against EMI. Metal enclosures, made from materials such as aluminum, steel, nickel, and nickel-iron alloys, deliver a relatively thick and exceptionally conductive boundary around the electronic device that reflects and absorbs disruptive signals. However, although metallic enclosures do a good job of minimizing EMI, they are heavier than desired for many electronic applications.
In the quest for lighter construction materials, many manufacturers are switching to thermoplastics. Thermoplastic enclosures are easier and quicker to manufacture. With metals, the enclosure’s shape must be stamped, a costly and time-consuming process which is not readily adaptable to intricate shapes. Thermoplastics, however, can be easily injection-molded into complex shapes at a high-throughput. This combination of lightweight design and production speed makes plastic enclosures less expensive to manufacture than metallic enclosures.
On the other hand, plastics are inherently electrically insulating, thereby offering negligible EMI protection. To overcome this problem, manufacturers often resort to using an electroless plating process or the plastics are coated with heavily-filled conductive coatings. This transforms the plastic part into an EMI shield.
Electroless plating is an intensive, multi-step, chemical process that ultimately deposits a pure, thin metal coating onto the plastic. While electroless plating does provide high shielding levels owing to its pure metallic coating, the process is environmentally unfriendly and time-consuming and, in turn, can be labor-intensive and expensive. Specifically, plastics are first swollen by solvent, and then etched with strong acids such as sulphuric and chromic acids. Following etching, the parts are rinsed, a catalyst is applied to the surface, and lastly, the metal, such as copper or nickel, is deposited onto the surface through a reduction reaction. This process, which can entail additional rinsing steps, has come under scrutiny lately because it uses hazardous chemicals that pose harm to the operator and the environment. Due to increasing environmental regulations, shield designers are resorting to alternative coating techniques for plastics.
In addition to safety and environmental issues, electroless plating can suffer from limited adhesion, especially around exposed edges or corners or where stresses in the substrate may be high. In such situations, the metal coating may flake off, thereby leading to holes through which electromagnetic waves can penetrate. This issue is compounded by the big mismatch in properties (e.g. modulus, elongation, and coefficient of thermal expansion) between the plastic substrate and metal coating.
As an alternative to metallic enclosures and electroless plating, many manufacturers are using dense, conductive coatings that are sprayed onto the thermoplastic enclosure. The coatings consist of a thermoplastic or thermosetting resin, such as polymethylmethacrylate and epoxy, respectively, that is highly loaded with metallic particles such as silver, copper, silver-copper, nickel, or hybrid combinations thereof. Although the coatings do provide high shielding effectiveness, their high densities can lead to sacrifices in properties such as adhesion, flexibility, weight, and/or cost.
Another possibility for making thermoplastics EMI-proof is filling the thermoplastic material with conductive particles. While this approach eliminates the need for a conductive coating after the plastic is injection molded, the resultant enclosure is still far less conductive than achieved by either electroless plating or via a dense conductive coating.
Highly Conductive Epoxy Coatings
Electronic manufacturers are seeking a conductive coating for thermoplastics that delivers the equivalent shielding performance of electroless plating without the safety, environmental, and cost issues inherent in the plating process. To meet this demand, new highly conductive epoxy coatings are entering the market. Moreover, these coatings overcome the tradeoff of shielding effectiveness and performance properties that are commonly seen with traditional high-density coatings. The new coatings, based on a novel combination of epoxy resin, curative, and conductive fillers, are capable of self-assembling into a unique structure during curing. This structure inherently is very conductive, yet still is largely polymeric in nature. Ultimately, this leads to a lightweight coating with very high levels of EMI shielding. Specifically, the self-assembling coatings offer 85+ dB of shielding at 25 micron (1 mil) thickness over a broad range of frequencies. Because of its polymeric nature, the coatings can also achieve higher levels of adhesion and flexibility. In addition, they are resistant to high temperature, humidity, and salty environments to which electronic applications are often exposed.
This efficient performance is especially significant for manufacturers who want to swap out their hazardous, electroless plating process with a coating that delivers at least 85 dB or higher at approximately the same thickness. Moreover, traditional coatings at the same density and thickness as the aforementioned self-assembling ones will not deliver more than 85 dB of EMI shielding. Given this, manufacturers are reluctant to apply thicker coatings owing to cost and/or weight restrictions.
The distinctive base chemistry of the coatings allows the material to be adapted into several useful product forms including coatings, adhesives and films. As a spray coating, it can be easily applied manually or with automated equipment. The spray handles just like paint and can be applied onto very complex thermoplastic shapes by using a high-volume, low-pressure (HVLP) spray gun. For precise control of thickness across a part, the coatings can be applied via robotic systems. Moreover, it has an indefinite shelf life at room temperature.
The room-temperature stability or latent nature of the coating enables the base chemistry to be manipulated into the form of a liquid-dispensable adhesive, thus permitting it to be used as a traditional adhesive but with EMI shielding properties.
In its film form, the base material can be sheeted into large pieces of film for sizeable applications. This form is particularly useful in the aerospace industry where it can be applied to large areas of an aircraft during assembly, offering protection from both EMI and lightning strikes. Lastly, depending on the mechanical requirements, the base material, whether in a spray, adhesive, or film form, can be formulated for rigid or flexible applications.
For repair procedures, the highly conductive epoxy coatings are available in a pressurized spray can. This simple delivery method allows the coating to be applied in field applications, without needing a spray gun, compressed air system or hose lines. For example, if an enclosure or part gets scuffed or nicked in the field, the damaged section can be touched up by simply spraying it. If an external section of an aircraft becomes damaged causing part of the epoxy coating to be removed, the repair technician can spray the coating on top of the damaged or repaired section.
With highly-conductive epoxy coatings, manufacturers have a material that is predominantly a polymer that behaves similar to a metal, over a very broad range of frequencies. Industry-wide applications have proven that these coatings typically yield more than a 50% cost savings over electroless plating of plastics and other highly-filled, high-performance acrylic systems. As manufacturers increasingly use lighter weight materials in their designs, highly-conductive coatings can provide efficient, economic EMI shielding that meets industry expectations.
ABOUT THE AUTHOR
Tim Fornes, Ph.D. is a Senior Staff Scientist in the Chemical Research Department of LORD Corporation. He is responsible for the design, creation, characterization, and modeling of novel polymer-based adhesives and coatings that have commercial relevance in the electronics, aerospace, and automotive industries. Fornes is the author or co-author of 19 peer-reviewed technical publications and is a co-inventor on six patents (two issued, four pending). He holds Chemical Engineering degrees from North Carolina State University (B.S.) and The University of Texas at Austin (M.S. and Ph.D.).