Organo-Soluble Low CTE Polyimides and their Applications to Photosensitive Cover Layer Materials in Flexible Printed Circuit Boards

Abstract

A novel cover layer material for flexible printed circuit (FPC) applications was developed in this work. A high molecular weight of polyimide composed of four monomers, pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 2,2'-bis(trifluoromethyl)benzidine (TFMB) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (BisApAf) was prepared by the one-pot polycondensation in N-methyl-2-pyrrolidone (NMP) without precipitation and gelation. The polyimide (PI) was highly soluble in various common organic solvents. Interestingly, the PI film formed by simple solution-casting of the PI solution had a low CTE value close to that of copper foil, therefore almost no curling of the PI/copper laminate was observed when the PI film was directly formed on a copper foil. The PI film also exhibited other combined properties, including a low CTE, a high Tg, high thermal stability, low water absorption, comparatively good transparency, the highest levels of non-flammability, good insulation properties and sufficient film flexibility. In addition, fine positive-tone patterns could be obtained from DNQ-containing PI with a sensitivity of 238 mJ cm-2. The results revealed that the present PI system is a promising candidate as a novel cover layer material.