A composite structure of high frequency low dielectric film and the application in flexible printed circuit (FPC)

Abstract

This study has prepared a high frequency bond-ply (HFBP) film in three-layer composite structure. The HFBP film contain two low dielectric adhesive layer (AD) and a core layer in the middle, which is polyimide (PI) film. HFBP film can be fabricated as a thick film which is 100μm or more easily if needed. This HFBP film can be used as a bonding sheet in copper clad laminate (CCL), and also can be used with copper foil for the preparation of high frequency single/double-sided substrate. In HFBP film, the low dielectric adhesive layer which is adding a modified low dielectric thermosetting resin have low dielectric constant (Dk), low dissipation factor (Df) and enough peel strength with the surface of the copper foil or CCL. And the core layer provides good mechanical strength and excellent resistivity to whole structure. Subsequently, we would compare HFBP film with liquid crystal polymer (LCP) in physical, thermal, electrical properties and operability. LCP film is a kind of thermal plasticity material that has been used for a very wide range at the field of high frequency printed circuit board (PCB) industry. We would discuss about different thickness of low dielectric adhesive and core layer affecting to Dk, Df, operability, solder resistance, peel strength, elongation, tensile strength...etc. A new series of different structure bond-ply were successfully fabricated, the Dk of the structure is approximately 2.80 in 10 GHz, the Df is approximately 0.006 in 10 GHz, the peel strength is greater than 1.2 kgf/cm. Compared with LCP film, the HFBP film can be laminated at lower temperature, which the quickly lamination condition is 175 ~ 185°C and the typical lamination condition is 160 ~ 170°C. Lamination condition of LCP is higher than 280C in typical lamination condition. So HFBP film has better performance in the operability and thickness tolerance than LCP film. When flexible printed circuit (FPC) is used for a high frequency signal transmission, less thickness tolerance is easier to adjust the impedance design. HFBP film also has better electrical properties than regular bond-ply. So, we think the HFBP film is more suitable for application in FPC industry.