PRINTED WIRING METHOD

PURPOSE:To enable the wiring to attain low resistance, by impregnating inside a porous W thick film, which is densely sintered being surrounded by a W thick film by using normal tungsten paste, with Cu so that Cu impregnation is improved preventing a porous W thick film void from being filled up by a vitreous phase in mullite at the time of sintering, in a multilayer substrate of mullite. CONSTITUTION:Forming a through hole 1A to be opened in a green sheet 1 of mullite, normal W paste is applied to an internal surface of the through hole 1A and dried, thereafter the through hole is filled with impregnation W paste 2. Next the W paste 3 is printed on the surface of the green sheet 1 and dried, thereafter the impregnation W paste 2 is printed. The green sheet 1, finishing the printing, is laminated in several layers, sintered at 1350 deg.C or less and integrally formed obtaining a multi-layer substrate 1' forming thick film wiring patterns 2', 3' of composite structure. Next, the substrate 1', placing a Cu piece 4 on the pad of wiring pattern exposed on the surface, is heated to 1000 deg.C or less in a furnace being impregnated with Cu by thermal diffusion. Accordingly, printed wiring of low resistance is permitted to be surely performed for the mullite substrate.