Patent ID: 8431224

Claim:
A resin composition for forming an insulating layer of a printed wiring board, the resin composition comprising components A to E: (A) at least one epoxy resin, which is liquid at 25° C. and has an epoxy equivalent weight of 200 or less, and which is selected from the group consisting of bisphenol-A epoxy resin, bisphenol-F epoxy resin and bisphenol-AD epoxy resin, said component A provided in an amount of 3 to 20 wt % of the overall composition; (B) a polyvinyl acetal resin, said component B provided in an amount of 3 to 30 wt % of the overall composition; (C) a urethane resin, said component C provided in a positive amount of not more than 10 wt % of the overall composition; (D) an imidazole curing agent; and (E) a phosphorus-containing epoxy resin, said component E provided in an amount yielding a phosphorous atom content in the overall composition of 0.5 wt % to 3.0 wt %; wherein the resin composition resists moisture absorption when semi-cured, and a copper clad laminate manufactured with the resin composition exhibits a solder blister time of 600 seconds or more when the copper clad laminate is floated on a solder bath at 260° C.; wherein the copper clad laminate is manufactured by: preparing a resin varnish with the resin composition; coating a copper foil with the resin varnish; drying the coated copper foil to form a semi-cured resin layer; exposing the dried coated copper foil to a temperature of 30° C. and a relative humidity of 65% for 15 hours; and laminating the exposed coated copper foil to an inner layer core wiring board by hot-pressing.