A Composite Structure of Low Dielectric Substrate for the Sub 6GHz and the Application in Flexible Printed Circuit (FPC)

Abstract

This study has prepared a three layer composite substrate (TLCS) for 5th generation millimeter Wave (5G mmWave) flexible printed circuit (FPC) application which has dielectric layer and double sided copper foil. The dielectric layer of TLCS is consist of thermosetting modified polyimide (mPI) and low loss polyimide film (LLF), both component can provide good electrical properties. In this paper, we discuss about the electrical properties, mechanical properties and operability of TLCS. TLCS has tensile strength over 150MPa, elongation over 30%, elastic modulus between 4.5 to 5.5 GPa, CTE about 8 to 10 ppm/°C, solder resistance as high as 330°C, Df value between 0.003 to 0.004 at 15 GHz and better operability than LCP substrate does. TLCS has good peel strength over 0.7 kgf/cm with low roughness copper foil which causes lower conductor loss. TLCS has similar insertion loss with LCP substrate at same thickness. And LCP is limited at processing to fabricate thick film over 100μm which is easy for TLCS to access. Comparing with TLCS which has 150μm thickness dielectric layer, insertion loss of 100μm thickness LCP substrate has significant gap especially 5G mmWave frequency over 30GHz. Although TLCS has higher moisture absorption than LCP substrate, TLCS which has 125μm thickness dielectric layer or more still has lower insertion loss after get moistened than commercial 100μm thickness LCP substrate meaning full of reliability. Comparing with LCP substrate, TLCS is better at peel strength, operability and no need for high temperature lamination. Peel Strength of LCP substrate is too weak to use low-roughness copper foil, so that the LCP substrate can only use a copper foil having a higher roughness, however this will causes an increase in transmission loss due to conductor loss. Comparing with LCP substrate, TLCS has some advantages at the application of fine pitch design and low transmission loss in high frequency by these properties. And TLCS can freely adjust thickness to over 150 μm for thick dielectric layer design and it is good at impedance control. From the above mentioned, we think TLCS is more suitable for FPC industry in 5G mmWave region than LCP substrate.