Internal composite layer for a multilayer circuit board

An internal composite layer for a multilayer circuit board, which is composed of an electrically insulating substrate with a copper layer formed thereon and a film of a coupling agent covering the copper layer is fabricated by treatment with a base such as sodium or potassium hydroxide or sodium methylate or sodium ethylate or lithium ethylate or the base and an accelerating agent to supply hydroxyl groups to a surface thereof. Subsequently, the treated copper layer is coated with the coupling agent, e.g., a silane, titanate or zirconate coupling agent. The coupling agent is chemically bonded to the treated copper layer by the reaction of the hydroxyl groups with the coupling agent, so that an adhesion strength between the copper layer and the coupling agent is increased. In addition, since it is not necessary for forming a copper oxide layer in the surface of the copper layer, the internal composite layer shows excellent acid resistance without causing a pink-ring which usually occur around a through-hole plating.