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Design News, February 2013

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Medical single-faced, double-faced, or transfer the formulation. Also, as is the case in adhesive constructions — wound into many sensitive electronic applications, large rolls that are then converted to silicones cannot be used when silicone the exact specifications required for contamination is an issue during the the end application. manufacturing process. Advancements in capabilities of Rubber-based electrically conducPSA technologies are enabling smaller tive adhesives can be used in many electronic designs through thin, device applications, but are not the reliable bonds with highly desirable ideal choice for applications requiring functional benefits. The PSA's roll exposure to high temperatures. Beformat provides device manufacturcause rubber-based PSAs are typically ers ease of handling and application; formulated using synthetic rubbers such precise thickness control; a thin, as styrene-butadiene-styrene (SBS), styconsistent bond line while conforming rene-isoprene-styrene (SIS), tackifiers, The pressure-sensitive adhesive to irregular surfaces; and the capabiland plasticizers, the polymer becomes ity to be precisely die-cut into custom tape's roll format provides device too mobile at temperatures over 70C shapes and narrow widths for improv- manufacturers a thin bond line resulting in disruption of the conducand the capability to be precisely ing manufacturing efficiency with tive property. Rubber-based systems die-cut into custom shapes and mess-free processing. also present the potential for long-term narrow widths for improving As device manufacturers face the stability issues due to unsaturation in manufacturing efficiency with pressures of faster prototyping, quick the polymer backbone, although some mess-free processing.   changeovers, and the increased use of of these issues can be overcome with the automation, the advantages of PSAs addition of stabilizers. as an alternative material choice for While silicone and synthetic rubbers interconnects, grounding, and shielding applications, tradihave their own unique performance benefits, acrylic electrically tionally dominated by labor-intensive soldering operations, conductive adhesives are the most popular backbone choice for are becoming more evident. By combining the traditional many applications. The formulation flexibility emanates from benefits of the PSA format with additional desirable functhe choice of monomers available to build the polymer. Traditional capabilities, such as an electrically conductive adhesive, tional acrylic-based polymer PSAs employ a variety of functionsimplification of electronic device design and manufacturing al monomers to achieve specific adhesive properties. Functional is possible. groups such as acid, alcohol, or amines can also be used for subsequent crosslinking of the polymer. These modifications Designing Electrically Conductive PSAs enable the long-term reliability of the adhesive by restricting Achieving electrical conductivity in a traditionally insulapolymer chain movement during temperature cycling. tive polymer matrix is accomplished by adding conductive particles to the adhesive. A number of conductive particles Electrically Conductive Clean Adhesives are available for loading into an adhesive, including gold, With the advent of thin film metallic coatings on plastics such silver, nickel, copper, and tin. These conductive particles may as indium-tin oxide, silver, etc., device manufacturers must be prepared in solid metal formats, metal-coated polymer, or take precautions to select materials that do not contribute to metal-coated glass particles. The application and the level of the corrosion of these sensitive conductive coatings. Materials conductive reliability required will determine the type of con- making direct contact to these surfaces, such as adhesives, can ductive particle best suited for the matrix. Similarly, the level potentially cause corrosion, or outgas, which causes fogging of conductive reliability required also dictates the polymer and other performance issues inside sensitive electronic dematrix. With the advent of conductive carbon materials such vices. Any presence of corrosive components or residuals from as graphene and other nano materials, a new level of formula- the adhesive's polymerization process can contribute to these tion flexibility is available for achieving the desired conductive problems. properties. Acrylic polymers can be modified to prevent degradation of The process of designing an electrically conductive PSA bethese sensitive coatings. This is accomplished through the apgins with the polymer backbone of the adhesive. The polymer propriate choice of monomers and polymerizing the adhesive can be designed using silicones, acrylics, or synthetic rubbers. system to a very high conversion to create a low-outgassing formulation. While acrylic polymers offer the most formulation flexibility, the end application will determine the adhesive's chemistry. Electrically Conductive Structural Bonds Electrically conductive silicones are used in many applications where high and low temperature use is required (-60C to Traditionally, costly silver and gold-filled epoxies are used for bonding applications with very thin contact points. With the 250C). Silicones also offer some degree of moisture permeproper formulation, acrylic PSAs can be modified to provide ation and are often the preferred choice for device applicastructural bonds to potentially replace expensive silver and tions that require direct skin contact. However, silicones do gold epoxies. present the disadvantage of lower adhesive bond strengths These unique acrylic PSAs are cured with additional energy once additives, such as carbon or metal particles, are added to M2 med i c a l / a su ppl em e n t t o d e s i g n n e w s f e b r uary 2013 [www.designnews .com]

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