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March 2017 • The PCB Magazine 63 STRETCHING BEYOND FLEX stretchable polymer technology that may ad- dress many of the challenges with other stretch- able materials. This resin is currently being used in a number of developmental electronic material embodiments including a stretchable film which exhibits some unique and attractive properties. The material has good elongation properties (stretching up to 170%) and the cur- rent version of the film has a stress relaxation value in excess of 60% at 50% elongation, which is attractive for epidermal devices where skin comfort is a design priority (Figure 1). The stress relaxation properties may be tailored to meet specific application requirements. Additionally, the film also exhibits very low hysteresis—well under 1%, which means that it incurs practi- cally no permanent deformation after elonga- tion (Figure 2). This same film has a high surface energy which allows it to bond well with many other materials without priming or surface treatment. Both thermoplastic and thermosetting resin sys- tems have been successfully tested for bonding and encapsulation. Because the film is supplied on a polyester liner, it can be laminated with pressure sensitive adhesives and die cut using conventional roll-to-roll converting processes. Encapsulated constructions have been made by laminating B-staged film over circuits, compo- nents and other features. The material can be selectively ablated using conventional lasers. It also has a high temperature resistance. The film does not melt or degrade even after floating in solder at 260°C. Polymer decomposition oc- curs around 325°C. This combination of elastic properties, temperature resistance and material compatibility opens up entirely new avenues for circuit metallization, assembly and end use applications. Stretchable Conductors— a Substantial Challenge Even as promising stretchable thermoset substrate materials arrive on the scene, the creation of stretchable circuitry that combines desired attributes such high conductivity, high elasticity, high solderability and high durabil- ity remain an industry challenge. In the short term, it appears that design engineers will be forced to accept trade-offs as they evaluate the market-ready approaches for creating the next generation of stretchable circuits. One obvious choice for creating stretchable circuit traces is to use thermosetting elastomers to formulate conductive pastes analogous to the flexible and stretchable thermoplastic pastes reviewed previously. This is an attractive ap- proach for many end use applications because the fairly stretchable circuitry can be manufac- tured with conventional processes. While using Figure 1: Graph illustrating the stress relaxation properties of Panasonic's developmental stretch- able thermosetting film. Figure 2: Graph illustrating the ultra-low hyster- esis characteristic of Panasonic's developmental stretchable thermosetting film.