In a conventional soldering method, an FPC-side electrode pad and a package-side electrode pad are closely joined together with a solder layer, and the soldered state after a joining process has not been easily confirmed visually. The present invention provides a solder joint structure including a side face electrode which is formed on each of the side faces of the end parts of an FPC board and a package or PCB board that are to be soldered, extending vertically relative to the faces constituting each of electrode pads on the boards, and which introduces solder. On the side face electrodes of the board end parts, a part of solder that is formed continuously from the solder joint portion is visible and the state of the solder joint between the electrode pads on the two boards can be confirmed. The efficiency of solder joint tests can be improved.