Poly(epoxy imide)s were prepared by a reaction between a hydroxyl-group-containing soluble copolyimide and commercial epoxy resins at 220 degreesC for 2 h. Poly(epoxy imide) thin films exhibited higher thermal stability and lower dielectric constants than a commercial flip-chip package material (U300). The thermal stabilities of the poly(epoxy imide)s were 1.4-2.0 times higher than that of U300. The thermal stability increased with increasing crosslink density and with decreasing bulky CF3 groups (which were easily decomposable). The dielectric constants of the poly(epoxy imide)s were 1.1-1.3 times lower than that of U300, and this is highly desirable for the microelectronic packaging industry. The dielectric constant dramatically decreased when bulky CF3 groups were added and when the functionalities of epoxy resins decreased. The residual stresses, slopes in the cooling curves, and glass-transition temperatures of the poly(epoxy imide)s were measured with a thin-film stress analyzer. Low residual stresses and slopes in the cooling curves were achieved with a higher crosslink density. However, in the presence of bulky CF3 groups, the copolyimide backbone structure did not affect the residual stress values. (C) 2004 Wiley Periodicals, Inc.