Copolyimide thin film, which has low-level stress and stress relaxation induced by water sorption, was characterized for potential applications as an encapsulant, a stress-relief buffer, and in interlayer dielectrics. The polyimides examined were poly(p-phenylene pyromellitimide) (PMDA-PDA) and poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) as well as their random copolyimides with various compositions. These copolyimide films exhibited good combinations of physical and mechanical properties with low thermal expansion coefficients, residual stress, and moisture-induced stress-relaxation behavior by appropriately selecting the ratios of the dianhydride component. For these polyimides, the residual stress increased in the range of -8.1-7.5 MPa, whereas stress relaxation induced by water uptake decreased in the range of 10.3-4.7 MPa with increasing BPDA contents, respectively. The major factor in determining the magnitude of the stress behavior induced by both the thermal mismatch and water uptake in films should be the morphological factors such as chain rigidity, chain orientation, crystallinity, and microvoids. Their morphological structures were examined by wide angle X-ray diffraction and a prism coupler, and the thermal properties were measured using a dynamic mechanical thermal analyzer as well as thermomechanical analysis. Overall, the candidate for the low level stress buffer application from the PMDA/BPDA-PDA copolyimide was the 30/70 (= PMDA/BPDA in molar ratio) copolyimide. This copolyimide showed no residual stress after curing at 400 degreesC and relatively insensitive stress relaxation to ambient humidity. (C) 2001 John Wiley & Sons, Inc.