Electroless Cu plating on poly(ethylene 2,6-naphthalate) with plasma surface modification and its application to high-frequency flexible printed circuits

Abstract

Poly(ethylene 2,6-naphthalate) (PEN) is a material characterized by its low dielectric loss tangent and high transparency, making it an attractive dielectric film for flexible circuits in wearable devices. To create flexible circuits, it is essential to deposit a metal layer on the PEN film. One method for forming the Cu layer on the PEN film involves laminating Cu foil with an adhesive. Before lamination, the Cu surface is intentionally roughened to enhance the adhesion strength of the adhesive resin. However, this roughened Cu surface can lead to increased signal transmission loss at high frequencies due to the skin effect. To address this issue, we propose a process for electroless Cu plating on the PEN film without roughening the Cu surface. Our approach utilizes plasma surface modification followed by a silane coupling treatment on the PEN surface to facilitate electroless Cu plating. This paper also discusses the characteristics of a bandpass filter operating in the 5 GHz range, constructed from electroless plated Cu on the PEN film.