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PCB-Mar2017

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56 The PCB Magazine • March 2017 FEATURE by Andy Behr PANASONIC ELECTRONIC MATERIALS Emerging end-use electronic applications are driving dramatic innovations in circuit board and interconnection technology. New form factors, functionality and durability re- quirements are challenging the status quo for the PCB industry and pushing design, material and process development to the limit. Incipi- ent devices like wearables, epidermal monitors, embedded sensors, smart labels, human ma- chine interfaces (HMIs), conformable antennas, flexible displays and in-mold electronics (IME) require a combination of circuit stretchability and toughness that isn't achievable with con- ventional circuit manufacturing technologies. Device manufacturers are seeking alterna- tive methods for creating wiring patterns and interconnecting components that are more conformable, elastic and durable than currently available. To set the stage for a discussion on stretchable circuit technology, this article de- scribes two classes of polymers commonly used for manufacturing circuit boards and outlines the developmental arc of two fundamental PCB materials, conductive circuits and organic sub- strates. The article concludes by reviewing state- of-the art, commercially available stretchable substrate and conductor technologies, as well as new materials and processes that are being researched. Thermoplastic and Thermosetting Polymers Broadly speaking, the polymers used for manufacturing electronics may be divided into two classes: thermoplastic and thermoset. The division is based on the degree of chemi- cal cross-linking between the constituent mol- ecules and the temperature-related mechanical properties this cross-linking (or lack thereof) imparts. Depending on the resin system, both classes of polymers can exhibit a wide variety of properties. For example, both thermoplastic and thermoset resins can run the gamut of hardness physiognomies from rigid and brittle to flexible and bendable to elastomeric and stretchable. Thermoplastic polymers typically have a low degree of intermolecular cross-linking. The long chain polymer molecules are tightly "tangled" at temperatures below the glass transition (Tg), resulting in a solid or "glassy" material. These same molecules loosen up and untangle at el-

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