A blanking plate is attached to the bottom surfaces of a split board and a remaining board, which are separated from one bare board. After depositing solder paste on a back electrode of the split board, a solder ball is attached on the solder paste. The split board is then heated to melt the solder ball. The molten solder flows along the back electrode and an edge electrode into a through-hole. A portion of the molten solder swells out of the bottom surface of the bare board, and solidifies and bonds to the back electrode and the edge electrode.