u00b0 c at a relative humidity rh higher than 60 preferably higher than 70 more preferably higher than 75 for five to 60 minutes occasionally 5 to 15 minutes the starting metal clad laminate to be subjected to the aging treatment according to the present invention may be produced by any known method namely a fibrous substrate such as woven or nonwoven fabrics or paper like sheets made of glass fiber or asbestos natural or synthetic fibers and papers made of cellulosic fibers such as kraft pulp or cotton linter pulp is impregnated with a curable resin and a plurality of impregnated substrates are stacked a cladding metal foil is applied on one or both sides of the stack and the resultant assemblage is cured the above steps may be carried out either batchwise or continuously examples of curable resins which may be used for impregnating the substrate include condensation polymerization resins such as phenol resins urea resins and melamine resins addition polymerization resins such as epoxy resins and radical polymerization resins such as unsaturated polyester resins vinyl ester resins and diallyl phthalate resins they may be of the thermosetting type or those which cure upon irradiation of an ionizing ray such as uv light electron ray and gamma ray resins which are normally solid at room temperature are used as a resin varnish in a suitable.