Recently electronic and electric devices and transportation devices are becoming smaller and lighter in weight and have an increased performance, and this trend has emphasized the need for materials having an excellent heat resistance and moisture resistance.
Polyimide resins are universally known, as heat resistant resins, but since such resins are produced by a dehydration-condensation type reaction, the produced cured resin is liable to contain voids due to the water of condensation that occurs as a consequence of the reaction. These voids impair the reliability of the cured resin product. The polyimide itself is insoluble and unmeltable, and therefore, is not easily molded.
As polyimides having an improved moldability, such maleimide resins as bismaleimide and polymaleimide are known in the art. Bismaleimide, however, has a high melting point and therefore, has the disadvantage of a poor workability because the curing during the process of molding requires a protracted application of a high temperature generally exceeding 200.degree. C. (180.degree. to 350.degree. C. and 15 to 16 minutes). Bismaleimide is further disadvantageous in that it has a poor hydrophobicity and consequently, has a conspicuously poor reliability of the cured resin product. Polymaleimide has a curing temperature about 70.degree. to 80.degree. C. lower than that of bismaleimide, but the cured product of the polymaleimide has a disadvantage in that it is easily cracked and embrittled because of a high cross-link density and large residual strain.
The practice of using an epoxy resin having an excellent moldability, in the place of the maleimide resin, is also known in the art. Where the epoxy resin is used as a substratal resin, an amine, an acid anhydride, or a phenol is generally used in combination with the epoxy resin as a curing agent therefore, but the cured product obtained has an unsatisfactory heatproofing. The epoxy/phenol cure system, for example, has a glass transition point generally of from 140.degree. to 170.degree. C., and this glass transition point should be raised to a level above 190.degree. C., preferably above 200.degree. C.