Electrically conductive adhesives charge forward
As electronic devices become increasingly smaller and more complex, reducing the size of component real estate is becoming more important. Electrically conductive pressure-sensitive adhesives (PSAs) enable smaller electronic designs through thin, effective bonds because they not only bond components together, but also provide the added functionality of supplying pathways for electrical current. By eliminating the need for other conductive elements, electrically conductive PSAs present options for simplifying electronic device design and manufacturing. Some applications where electrically conductive PSAs are well-suited are: electrical interconnections for low current circuits; splicing conductive materials; bonding electromagnetic interference gaskets; ground plane assembly; and static control for printed circuit boards and assembly.
The days of “glue” simply bonding electrical components are long gone. The adhesive systems of today have evolved into sophisticated polymer formulations to deliver reliable bonds with added functionality. For example, electrically conductive PSAs are highly tailored polymer formulations loaded with materials to create conductive properties in the adhesive. The adhesive is then coated onto a carrier/substrate that may also contribute to the tape’s conductive properties, in single-faced or double-faced constructions. PSAs may also take the form of a self-wound transfer adhesive.
Creating conductivity
When uniformly dispersed, conductive particles create pathways within the adhesive matrix to make contact from one surface to another. The conductive fillers may be comprised of a number of materials, such as: nickel, silver, and carbon, or a combination of these. The adhesive matrix may be formulated from silicone, acrylic, or rubber polymers to ensure the maximum flexibility and compatibility with metal, film, and potentially low surface-energy substrates.
Adhesives Research, a leader in the development of electrically conductive adhesive technology, has been designing, developing, and manufacturing leading-edge, electrically conductive adhesive products for more than 20 years. Our electrically conductive PSAs are based on a patented homogeneous carbon-based adhesive technology that delivers exceptionally stable bonds to electrical contact points, even under extreme stress. The bonds are particularly reliable because the conductive particles within the adhesive form strong carbon chains for good point-to-point conductivity. The chains are flexible to provide movement with the adhesive as it expands or contracts against bonded substrates during temperature changes. This flexibility results in uninterrupted electrical contact for reliable electrical interconnections.
Depending upon the application, a number of adhesive or film formulations can be created in combination with the carbon particles to achieve the desired outcome:
- Carbon-filled adhesive matrix
- Carbon- and metal-filled adhesive matrix
- Carbon and silver compound adhesive
- Adhesive-embedded composites for X-Y vs. Z conductivity control
- Conductive films and laminates
The carrier or substrate can also play an important role in the conductive performance of the adhesive system, or play a role in addressing specific design challenges, like the metal foil or fabric substrates used in EMI shielding gaskets and seals. A number of specialized substrate materials are available, ranging from plastic films to galvanized foils and metallized fabrics. Today, design engineers have more material choices for conductive substrates than ever; including metal-coated polyesters, metal-plated mesh fiber, and carbon mesh.
Performance is key
In recent years, advancements in conductive PSAs enabled formulators to tailor the adhesive’s physical properties for resistivity, conductivity, and a number of environmental stresses, including shock and moisture resistance. For example, the Adhesives Research (AR) conductive technology can achieve Z-axis resistances from a few milli-ohms to a few ohms reliably, and can be produced in thicknesses ranging from 25 to 100 microns. This technology is also proven for use in fine pitch connections where X-Y isolation is critical, and provides reliable connections down to 300 x 300 microns.
As the applications for electronic devices broaden, the adhesives in these are also required to withstand a wider range of harsh environmental conditions, including significant humidity and temperature fluctuations. AR’s technology demonstrates stable bonding performance through temperature cycling (-20°C to 85°C) and humidity up to 95% Relative Humidity. The adhesive system can also be tailored to provide a degree of removability that is important for parts requiring re-work during the manufacturing process.
Design engineers should consider the process and handling benefits of PSAs when selecting an adhesive for their product designs. PSAs form instant bonds and provide reliable electrical contact without mechanical pressure or cure times. Because these adhesive systems are manufactured in a continuous web format, rolls can be manufactured into wide roll formats, or slit to specified widths, lengths, or sheets, and further converted into die-cut components. The manufactured PSA rolls are easy to handle and process, with no messy clean-up required.
Applying the technology
Electrically conductive PSAs are sought out for a number of applications, including electrical interconnection and assembly, EMI/RFI shielding and grounding, and static dissipation and control in a wide range of electronic, medical, and pharmaceutical applications. Some examples where Adhesives Research’s electrically conductive technology is used include:
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Several areas where we are seeing growing interest for conductive adhesives are in thin-film photovoltaic modules (PV) for solar energy, and in more portable medical electronics. Interestingly enough, one PSA product, ARclad® 90038, provides versatile functionality in these two applications, as discussed in more detail below.
Electrically conductive bus bars for photovoltaic modules
In thin-film PV modules, the electrical current generated in the module’s semiconductor is extracted by contacts on the front and rear of the cell. Widely spaced thin metal strips, often referred to as fingers, transport the electrical current generated within the cells to a larger metal bus bar strip. The purpose of the bus bar is to efficiently gather the electrical current from the individual fingers and transport the current from the solar module to a nearby junction box.
ARclad® 90038 is a 2.4-mil, highly conductive PSA tape featuring a 1-oz tin-plated copper foil backing and demonstrates excellent conductivity and corrosion resistance for bus bar applications. Rated UL 510, this tape features Adhesive Research’s patented homogenous conductive adhesive technology that forms exceptionally stable, conductive bonds to electrical contact points even under extreme environmental stress. The adhesive-coated foil tape construction offers a clean, easy-to-use format for efficient application, unlike copper ribbons and conductive adhesives or epoxies that require a two-step bonding process.
EMI shielding in medical electronics
Advancements in electronics technology are quickly bridging the gap from consumer products to medical products in the form of medical electronic equipment designed for faster diagnosis, improvement of patient quality of life, and new drug-based therapies. One trend is the movement toward compact and portable monitoring, display, and testing equipment that is more accurate and versatile for improved bedside treatment. Such equipment includes blood glucose monitoring systems, insulin pumps, defibrillators, and neurological stimulators. Implantable devices are also incorporating the latest technological advancements for sophisticated and targeted therapies ranging from drug delivery and pain management to the treatment of neurological disorders.
As more of these highly complex devices become commonplace in hospital, medical office, and home care settings, the importance of assuring the electromagnetic compatibility and safety of these devices is essential. The problem is further compounded now that an increasing number of devices are relying on wireless communications to transmit biodata. Inadequately protected devices or components may potentially be at risk for electromagnetic interference (EMI), causing an undesirable response, malfunction, or degradation of device performance. EMI problems with medical devices can be very complex, not only from the technical standpoint but also from the view of public health issues and solutions.
For shielding applications, ARclad® 90038 provides shielding effectiveness ranges from 80 to 95 dB in the 1 to 18-GHz frequency range. The tin-plated backing of this pressure-sensitive EMI shielding tape can be easily soldered and is resistant to corrosion and oxidation. It is supported by a polyester release liner that is easy to die-cut and apply.
Conclusion
Electrically conductive adhesives combine bonding performance with the added benefit of conductivity — both are often desirable features now that electronics are becoming smaller and more complex. Standard electrically conductive adhesive products are available, but Adhesives Research finds that most design projects begin with a base technology that is then customized for each application. Some aspects of the adhesive system that can be tailored by application include: adhesive performance properties and formulations, finished thicknesses (25 to 150 microns), and substrate and release liner material selections. The conductive properties of the adhesive itself can be tailored to range from 1 milli-ohm to 10,000, depending upon the needs of the application. Development work continues to push the technology envelope for conductive adhesives, including the combination of these with conductive films, coatings, and inks for added conductive capabilities.
About the author
Deepak Hariharan is the Electronics Business Manager for Adhesives Research, Inc. Hariharan has a PhD in Chemical Engineering from Purdue University (West Lafayette, IN) and a degree in Chemical Engineering from the Indian Institute of Technology (Bombay, India), as well as an MBA from the University of Maryland (College Park, MD).
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