A long-chain branched copolyester (i.e. p-hydroxybenzoic acid (HBA)/2-hydroxy-6-naphthoic acid (HNA)/1,1,1-tris(4-hydroxyphenyl)ethane (THPE) copolyester) and its extrusion film with superior physical properties to commercial Ticona A950 and Vecstar CTZ have been manufactured by the design of appropriate prescriptions, T-Die extrusion, and thermal treatment of crystal transformation. In order to investigate the fabricating feasibility for flexible copper clad laminate (FCCL) of fourth/fifth generation long-term evolution (4G/5G LTE), we have also agglutinated lab-made copolyester extrusion film with the copper foil by hot compression. Experimental results manifest that lab-made copolyester extrusion film is a highly potential FCCL substrate of 4G/5G LTE because its thickness, dielectric constant (D-k), dielectric loss (D-f), hygroscopicity, yellowness index (YI), antistatic capability, flammability, melting temperature (T-m), coefficient of thermal expansion (CTE), and peel strength to copper foil are 50 mu m, 2.90, 0.00140, 0.04%, 2.7, 6.5 x 10(9) Omega/square UL-94 V0, 305 degrees C, 24.8 ppm/degrees C, and 29.9 lb/in, respectively.