A solder alloy, a solder paste, a solder ball, a solder preform, a solder joint, a vehicle-mounted electronic circuit, an ECU electronic circuit, a vehicle-mounted electronic circuit device, and an ECU electronic circuit device which have a liquidus-line temperature and a solidus-line temperature falling within predetermined temperature ranges, and have excellent heat conductivity and excellent heat cycle resistance. The solder alloy has an alloy composition of, by mass %, Ag: 3.0 to 3.8%, Cu: 0.1 to 1.0%, Bi: more than 0% and 0.9% or less, Sb: 1.0 to 7.9%, Fe: 0.020 to 0.040%, Co: 0.001 to 0.020%, with the balance being Sn. The solder alloy may further contain, by mass %, at least one of Ge, Ga, As, Pd, Mn, In, Zn, Zr, and Mg: 0.1% or less in total.