Copolyimide thin films, which has the low level stress and stress relaxation induced by water sorption, were characterized for potential application as encapsulant, stress-relief buffer, and interlayer dielectrics. The polyimides studied are poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(p-phenylene 4,4'-hexafluoroisopropylidene diphthalimide) (6FDA-PDA) and their random copolyimides with various composition. These copolyimide films exhibited good combinations of physical and mechanical properties with low thermal expansion coefficient (TEC) and residual stress behavior by appropriately selecting the ratios of the acid dianhydride component. For these copolyimides, residual stress increased in the range of -5 to 50 MPa, whereas stress relaxation induced by water uptake decreased in the range of 20.5-4.5 MPa at 30 degreesC, 100% relative humidity with increasing 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) composition, respectively. The degree of in-plane chain orientation was directly correlated to the residual stress and stress relaxation coefficient (S) in the film, which is an in-plane characteristic. However, the stress relaxation caused by the water uptake was significantly influenced by packing order induced by chain mobility, which might occur during thermal imidization. Overall, the candidate for the low level stress buffer application from the PMDA/6FDA-PDA copolyimide was both the 90/10 (= PMDA/6FDA in molar ratio) and 70/30 copolyimides.