A semiconductor device includes a semiconductor chip, a wiring on the chip, an insulating film coating the wiring and having an opening partially exposing the wiring, a Ti/W film on a portion of the wiring facing the opening, a Cu layer on the Ti/W film and the wiring's exposed portion, and having a peripheral edge protruding away from the opening more than Ti/W film's peripheral edge in parallel to an insulating film surface, and a solder ball bonded to the Cu layer. The protrusion of the Cu layer's peripheral edge with respect to the Ti/W film's peripheral edge is greater than the Ti/W film's thickness. The Ti/W film's surface doesn't vertically surpass the Cu layer's upper surface in the opening's center. A Cu layer/solder ball interface is arc-shaped on both sides of the Cu layer's upper surface in a cross section taken perpendicularly to the Cu layer's upper surface.