Synthesis and characterization of positive-type photosensitive silane-containing polyimide for cover layer materials application

A novel cover layer material for flexible printed circuit (FPC) applications is developed in this research. Cover layer film based on copolyimide was prepared from three monomers, i.e. 3,3',4,4'-biphenyl tetracarboxylic dianhydride (s-BPDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and 4,4'-oxydianiline (ODA), by a two steps polycondensation reaction in n-methyl pyrrolidone (NMP). The obtained copolyimide films were characterized at varied comonomer compositions for their thermal properties i.e. glass transition temperature (Tg), degradation temperature (Td) and coefficient of thermal expansion (CTE), as well as their tensile strength and dielectric constant since these properties are crucial for a successful use as a cover materials for FPC. A positive working photosensitive copolyimide based on poly(amic acid) (PAA) and nifedipine as a photoreactive compound has been developed. The PAA was prepared from s-BPDA/NTDA(70/30mol%)-ODA by a two steps polycondensation reaction in NMP. The obtained photosensitive copolyimide film (PSPI) exhibited high thermal stability with Tg of 360°C, Td up to 474°C and low dielectric constant of 3.01. The relatively low CTE of the resulting PSPI film was found to be closed to that of copper foil (i.e. 28.5 ppm/oC). Positive-type patterns could be obtained from nifedipine containing the PSPI with a sensitivity of 200 mJ/cm2. In addition, A positive type photosensitive silane-containing copolyimide film was improved adhesion property with peel strength of 0.80 N/mm with an addition of silane coupling agent at 0.5% by weight. The results revealed that the present PSPI system is a promising candidate as a novel cover layer material for flexible printed circuit board.