TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The polyester-imide precursor has N,N-dimethylacetamide content of 0.5 wt.%, and intrinsic viscosity of 0.3 dL/g or more at 30degreesC. The polyester-imide film has dielectric constant of 3.3 or less, wire thermal expansion coefficient of 30 ppm/K or less, glass transition temperature of 300degreesC or more, and favorable film toughness. Preferred Definitions:    A=bivalent aromatic group or alicyclic group chosen from groups (1a,2a);and    B=divalent aromatic group or alicyclic group chosen from groups (1b,2b), however three-dimensional structure of cyclohexane ring in A and B is a chair type trans configuration.    EXAMPLE - 4,4'-Biphenol (in g) (3.724) was dissolved in mixed solvent of anhydrous N,N-dimethyl formamide (22 ml) and anhydrous pyridine (16 ml). Then, trimellitic acid anhydride chloride (8.4221) anhydrous N,N-dimethyl formamide solution (51 ml) was gradually added and stirred for several hours at room temperature. The reaction solution obtained was concentrated, water was added and deposit was obtained. The obtained crude product was recrystallized after vacuum drying at 200degreesC for 24 hours. The crystal separated was vacuum dried at 200degreesC for 24 hours and ester group-containing tetracarboxylic acid anhydride was obtained. P-phenylene diamine (1.0814) was dissolved in N,N-dimethyl acetamide (15 ml). Then, obtained tetracarboxylic acid dianhydride (5.3439) was gradually added, and the solution was stirred at room temperature for 24 hours to obtain transparent, uniform and viscous polyester-imide precursor solution. The precursor solution was neglected at room temperature for 1 month, precipitated and polyester-imide precursor having intrinsic viscosity of 1.12 dL/g at 30degreesC and N,N-dimethyl acetamide content of 0.5 wt.% was obtained. The polyimide precursor was applied on substrate, dried at 60degreesC for 2 hours and then at 250degreesC for 2 hours under reduced pressure, heat processed at 350degreesC for 1 hour and peeled from the substrate to obtain transparent polyester imide film with film thickness of 20 microns. The film had wire thermal expansion coefficient of 7.4 ppm/K, excellent dimensional stability, heat stability and favorable film toughness.