Liquid thermosetting insulating resin composition and method for permanently filling holes in printed circuit board by the use thereof

A liquid thermosetting insulating resin composition and a method for permanently filling holes in a printed circuit board by the use of the composition are provided. The liquid thermosetting insulating resin composition comprises (A) an epoxy resin assuming a liquid state at room temperature, preferably in combination with (B) a phenolic resin assuming a liquid state at room temperature, (C) a curing catalyst, and (D) a filler, wherein the filler (D) contains a spherical filler and a ground filler. Preferably the spherical filler includes a spherical fine filler and a spherical coarse filler. In a method for permanently filling holes in the printed circuit board, the composition is applied to the board so as to fill the holes in the printed circuit board and precured by application of heat. Thereafter, the parts of the precured composition protruding from a surface defining the holes is removed by polishing and then the precured composition is further heated to cause final curing thereof.