Abstract: Process of impregnating a layer substrate having openings which is substantially defined by opposing two main surfaces with a resin composition in the form of liquid, and the process is characterized in that one of the main surfaces and the resin composition are kept in such a contact condition that the substrate is located on the resin composition. A prepreg and a laminate are also provided which are produced by the use of the substrate which has been prepared by the above process.

Abstract: An addition type imide resin prepolymer is obtained by a reaction of unsaturated bisimide of 1.0 mol to diamine of 0.25 to 0.43 mol and an addition of polyamine having three or more benzene rings. The prepolymer is thereby made to contain substantially no unreacted diamine component which affects human body of operators, and to have remarkably excellent heat resistance and adhesive properties, while effective to restrain a production of any component of a molecular weight exceeding 15,000.

Abstract: Superior polyimide prepolymers particularly suitable for fabrication of multi-layer printed boards are produced by reacting unsaturated bisimides with diamines to consist essentially of 41-55% of unreacted reactants, 39-54% of components having a molecular weight of between 400 and 15,000, and 3.0-6.8% of components having a molecular weight of more than 15,000. The resulting prepolymers enable the preparation of their solution having enough working life for successfully impregnating a given substrate therewith in the preparation of prepregs.