Solution-processable CF 3 -substituted ductile polyimides with low coefficients of thermal expansion as novel coating-type protective layers in flexible printed circuit boards

Abstract

Abstract This work proposes practically useful novel coating-type cover layer materials. A copolyimide (PI) system derived from pyromellitic dianhydride (PMDA, 50 mol%), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, 50 mol%) with 2,2′-bis(trifluoromethyl)benzidine (TFMB) showed a low coefficient of thermal expansion (CTE) by thermal imidization. However, this system was not compatible to chemical imidization process because of gelation. A structural modification of this copolymer by 4,4′-(hexafluoroisopropylidene)dianiline (HFIDA) significantly improved the solubility, as a result, allowed for chemical imidization in a homogeneous state and the formation of a stable PI solution from less hygroscopic solvents such as triglyme (TriGL). The TriGL-cast PI film at an HFIDA content of 25 mol% displayed excellent combined properties; a low CTE (22.9 ppm K −1 ), a very high T g (327 °C), and a common level of tensile properties [modulus ( E ) = 3.6 GPa and the elongation at break ( e b max ) = 10.4%]. The PMDA(50);s-BPDA(50)/TFMB copolymer system was also modified with other ether-linked fluorinated diamines; i.e., 2-trifluoromethyl-4,4′-diaminodiphenylether (3FODA) and 4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (6FBAPB). At a 3FODA content of 25 mol%, the NMP-cast PI film displayed the lowest CTE (20.7 ppm K −1 ) among the PIs examined in this work. The modification using 6FBAPB was most effective to enhance the solubility. Thus, the structural modifications of PMDA(50);s-BPDA(50)/TFMB copolymer by these fluorinated diamines (HFIDA, 3FODA, and 6FBAPB) were an effective way to obtain almost satisfying target properties except for a high level of e b . A mechanism was proposed for explaining why the effect of the CF 3 -substituted ether-linked diamines on the toughness improvement was not so prominent. The approach simultaneously using some CF 3 -free ether-linked monomers enabled us to significantly enhance the film toughness ( e b max > 50%) while maintaining low CTE characteristics and good solution-processability. Thus, some of the PI systems can be promising candidates as novel coating-type cover layer materials.