Abstract: A friction stir welding method in which a first member to be welded and a second member to be welded and having a first step portion are welded by friction stir welding using a welding tool includes the steps of arranging the first member to be welded on a step supporting surface of the first step portion with a gap between the first member to be welded and a side surface of the first step portion, pushing the welding tool into the first member to be welded from a surface of the first member to be welded while rotating the welding tool and inserting the welding tool until reaching the step supporting surface of the second member to be welded and stirring the first member to be welded and the second member to be welded by rotating the welding tool to form a welding part.

Abstract: A semiconductor device according to the present invention includes a relay substrate provided on a plurality of semiconductor chips. The relay substrate includes an insulating plate in which a through hole is formed, a lower conductor provided on a lower surface of the insulating plate and having a first lower conductor and a second lower conductor, an upper conductor provided on an upper surface of the insulating plate, a connection part provided in the through hole and connecting the second lower conductor and the upper conductor together, and a protruding part which is a part of one of the first lower conductor and the upper conductor and protrudes outward from the insulating plate, the protruding part is connected to a first external electrode, and another of the first lower conductor and the upper conductor is connected to a second external electrode and is positioned inside the insulating plate.