Prepregs can be prepared by impregnating reinforcement materials such as glass, carbon or plastic (in the form of fibers or nonwoven or woven materials) with a solution of a polyepoxy resin and a polyamine as a hardener and subsequent drying. For the manufacture of storage-stable prepregs based on polyepoxy resins, 1,3,5-tris(3-amino-4-alkylphenyl-2,4,6-trioxo-hexahydrotriazine with a C.sub.1 - to C.sub.4 alkyl radical is used as the hardener. The new method leads to storage-stable prepregs and assures that composite materials, especially circuit boards, manufactured therefrom meet the requirements for these materials.

A process for manufacturing a prepreg exhibiting excellent formability and producing laminated boards and multiple layer circuit boards exhibiting high thickness precision is disclosed. The process of manufacturing the prepreg comprises (a) a step of impregnating a glass fiber substrate with a solvent, (b) a step of impregnating the solvent-impregnated glass fiber substrate with epoxy resin, (c) a step of heating the epoxy resin-impregnated glass fiber substrate, (d) a step of further impregnating the epoxy resin-impregnated glass fiber substrate, in which the epoxy resin has been cured, with the epoxy resin, and (e) a step of heating the lastly obtained epoxy resin-impregnated glass fiber substrate, wherein the epoxy resin reaction rate in the inner layer is 85% or more and the epoxy resin reaction rate in the outer layer is 60% or less.

The invention concerns a compressible prepreg based on flat substrates impregnated with thermosetting synthetic resins, characterized in that it comprises a first, C stage synthetic-resin layer directly deposited on the substrate and a second, B stage synthetic-resin layer deposited on the first synthetic-resin layer.

A method for coating cloth especially fiberglass sheets with a resin and resulting structure is provided. The coating is performed in two steps. In the first step, essentially all of the strands of the fiberglass are coated with the resin solvent mixture while maintaining the interstices or openings essentially free of the solvent mixture. This first coating is then partially cured to the extent that it will not redissolve in a second coating of the same resin solution. The coated fiberglass with partially cured resin thereon is then given a second coating of the same resin mixture which coats the first coating and fills in the interstices between the fibers. This second coating is then partially cured, which advances the cure of the first coating and results in an impregnated fiberglass cloth structure for use as sticker sheets. During lamination, the first coating acts like an impenetrable insulating sheet, preventing resin displacement and, therefore, preventing glass fiber contact with the conductive planes. The second coating is fluid enough to fill in spaces in the planes and to form the adhesive bond to cores and conductive layers.

A prepreg having substantially no excess resin while at the same time having an aggressive tack. Small discrete amounts of additional resin are selectively applied to a surface of the prepreg sheets in a regular array. The resin is applied at a sufficient viscosity that it does not sink into the prepreg fabric. The additional resin typically covers less than 25% of the surface and constitutes between 0.1% and 3% of the prepreg weight. The additional resin may be applied by a gravure roller having a surface etched with an appropriate array of indentations. A doctor blade partially defines a reservoir of resin in contact with a portion of the gravure roller and wipes the surface, thereby leaving resin only in the indentations. The doctor blade may be oscillated parallel to the roller axis to prevent accumulations of short fibers from impairing the wiping action.