Which of the following types of solder should not be used for radio and electronic applications?
Why Solder contains a flux core that cleans the metal so the joint wets properly. Acid flux is designed for plumbing and sheet metal, and the acid residue keeps corroding copper traces, wires and component leads long after the joint cools, eventually causing high resistance or open connections. Electronics work uses rosin flux, which leaves a non-corrosive residue that is safe to leave on the board.
Watch out Lead-tin and tin-copper describe the metal alloy, not the flux, and both are common electronics solders; rosin-core is exactly what you should be using.
Acid is for pipes, rosin is for radios.