Terminal pads are arranged on a first surface of the substrate for an electronic component to receive terminals of the electronic component. An electrically-conductive film is formed on a second surface defined on the back of the first surface over the back of a mounting area for the electronic component. The mounting area is contoured along the outer periphery of the arrangement of the terminal pads. The ratio of the area of the electrically-conductive material to the area of the surface of the substrate over the back of the mounting area is set appropriate to the ratio of the area of the electrically-conductive material to the area of the surface of the substrate over the mounting area for each electronic component. This results in suppression of flexure of the printed wiring board during reflow.