Adhesion of poly(4,4 '-oxydiphenylene pyromellitimide) to copper metal using a polymeric primer: Effects of miscibility and polyimide precursor origin

Abstract

Poly(amic acid) (PAA) and poly(amic diethyl ester) (PAE) precursors of poly(4,4'-oxydiphenylene pyromellitimide) (PMDA-ODA PI) were synthesized. Miscibility of these precursors with poly(arylene ether benzimidazole) (PAEBI), which is known to exhibit a strong cohesive adhesion to copper metal, was investigated in N-methyl-2-pyrrolidone (NMP) as well as in the condensed and the imidized state. PAEBI is completely miscible with the PAA precursor in NMP and also in the condensed state. This miscibility might result from complex formal of imidazole groups of PAEBI and carboxylic acid groups of precursor. Furthermore, the imidized blend films are optically transparent. However, aggregation of polyimide chains in the films is evident from X-ray diffraction patterns. In contrast, the miscibility of PAEBI and PAE precursor in NMP is limited up to a concentration of 11.06-15.9 wt.-%. They were almost fully immiscible in the condensed state so that phase separation took place during soft-baking. This suggests that there are relatively weak interactions of the imidazole groups of PAEBI with both ester and amide linkages in the precursor, leading to phase separation in the blend. The phase separation occurred further during thermal imidization. The phase separated domains were in a size of greater than or equal to 2.3 mu m, depending on the blend composition. Only the 10/90 and the 90/10 blend films were observed to be optically transparent. The difference in the miscibility of PAEBI with the polyimide precursors is correlated with the adhesion strength of PI/PAEBI/copper joints: higher miscibility results in higher peel strength.