Which of the following types of solder should not be used for radio and electronic applications?

Acid-core solder contains a highly aggressive, corrosive flux designed specifically for plumbing and heavy sheet metal work. This strong acid cleans oxidized copper pipes effectively but is disastrous for delicate electronics. If used on a radio or printed circuit board, the acid residue remains active long after the joint cools. Over time, it will literally eat away at the copper traces, component leads, and wiring, eventually causing the circuit to fail. Furthermore, acid flux is slightly conductive, which can create unintended electrical shorts across the board. For radio and electronic applications, you should always use rosin-core solder. Rosin flux gently cleans the joint during heating but leaves behind a residue that is non-corrosive and electrically insulating.