Material Choices for High-Speed Flexible Circuits


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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 constructions 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 lamination 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 fluoropolymer dielectrics (fluorine-containing polymers like Teflon®). Now many new dielectrics have been developed for high-speed rigid boards, which has broadened both the material supplier 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 explaining 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 flexible clads, as well as bondplies and coverlays. For controlled impedance circuits, the electrical properties of the clad and bondplies are critical for striplines. The electrical properties of the clad and coverlay are critical for microstrips. 

To read the full version of this article which appeared in the April 2017 issue of The PCB Magazine, click here.

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