When electrodes of a BGA plated by electroless Ni plating are soldered with solder balls of a lead-free solder, peeling of soldered joints readily takes place under an external impact. When a BGA electrode plated by electroless Ni plating is soldered with a lead-free solder to which 0.03 - 0.1 mass percent of is added, the growth of brittle SnNi intermetallic compounds formed on the portion being soldered and a layer on the electroless Ni plating surface is suppressed, resulting in an increased bonding strength.