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12 The PCB Magazine • April 2017 by G. Sidney Cox COX CONSULTING Abstract High-speed rigid boards have existed for many years, with fluoropolymers being the most common dielectric used. More recently, flexible circuit materials have been developed, and these new products use a variety of polymer (including fluoropolymers) and composite film approaches to allow high-speed flex circuits. This article will provide guidelines on how to compare the different options. The electrical benefits of the different polymers and construc- tions will be reviewed as well as the physical and flexible properties of different constructions. As with any new materials, the ease of processing is an important consideration, especially since some of these new products use thermoplastic adhesives or require high-temperature lamina- tion of bondplies and coverlays. Introduction High-speed rigid boards have existed for many years and continue to improve. Initially, most high-speed rigid boards used fluoropoly- mer dielectrics (fluorine-containing polymers like Teflon ® ). Now many new dielectrics have been developed for high-speed rigid boards, which has broadened both the material suppli- er base and the number of fabricators that can make high-speed rigid boards. Materials for high-speed flexible circuits are a much more recent development. This article will review the key material choices for making high-speed flexible circuits while also explain- ing why older flex materials were not a good choice for today's high-speed circuits. When talking about high-speed circuits, we are really talking about controlled impedance applications. This could be either microstrip or stripline designs. This paper will discuss flex- ible clads, as well as bondplies and coverlays. For controlled impedance circuits, the electrical properties of the clad and bondplies are criti- cal for striplines. The electrical properties of the clad and coverlay are critical for microstrips. Electrical Properties Older flexible circuit materials are not good for many high-speed applications because of the high dielectric loss (loss tangent). The di- electric constants for these older materials were very reasonable for high-speed (most were Material Choices for High-Speed Flexible Circuits FEATURE

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