carried out at a temperature higher than 170 u00b0 c preferably higher than 180 u00b0 c when the pressing process is carried out at a temperature lower than 170 u00b0 c the lower temperature does not promote the metal atom diffusion process between the metal particles of the electric conductive paste as a result even if the via hole has a low electric resistance at initial period it shows the break down or an increase in the electrical resistivity during the reliability test the necessary pressing pressure varies with the kind of the metal particles used the lower pressure is allowable with the electric conductive particles having a higher ductility such as gold or silver on the other hand a higher pressure is required for the electric conductive powders hardly deformed such as copper or its alloys a copper plate having a thickness of 0 5 mm is previously degreased and etched lightly with an aqueous solution of sulfuric acid and hydrogen peroxide after water washing the copper plate is electrically plated with silver film by a conventional electric plating method the plating solution comprises a non cyanide type commercially available the used anode is a silver plate and the current density is 0 5 a dm 2 the resultant silver plating film has a thickness of 500 angstroms and is colored silver white at the copper surface two copper plates having silver plating film formed thereon are washed with water dried and then overlapped to each other the overlapped copper plates.