Printed wiring board having highly reliably via hole and process for forming via hole

The method of making a micro-via hole for electrically connecting a first copper foil as an outermost layer of a printed wiring board to a copper foil in a bottom of the micro-via hole with a carbon dioxide gas laser, the method comprising providing a coating or sheet of an organic substance containing 3 to 97% by volume of at least one member selected from the group having a melting point of at least 900° C. and a bond energy of at least 300 kJ/mol, a carbon powder and a metal powder on a copper foil as an outermost layer of a copper-clad laminate having at least two copper layers, irradiating the coating or sheet with a carbon dioxide gas laser at an output of 20 to 60 mJ/pulse, thereby removing a micro-via-hole-forming portion of at least the copper foil as the outermost layer, then irradiating micro-via-hole-forming portions of a resin layer or a resin layer and a copper layer with a carbon dioxide gas laser at an output of 5 to 35 mJ/pulse to make a micro-via hole which does not penetrate through the copper foil in a bottom of the micro-via hole, and electrically connecting the copper foil as the outermost layer and the copper foil in the bottom of the micro-via hole with a metal plating or an electrically conductive coating composition, wherein the coating or sheet of the organic substance is a coating formed of a coating composition containing a mixture of at least one of a metal compound powder, a carbon powder or a metal powder with a water-soluble resin, or a sheet which is formed by applying the coating composition on one surface of a thermoplastic film and has a total thickness of 30 to 200 μm.