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

Solder requires a chemical agent called 'flux' to clean the metal surfaces and allow the molten solder to flow and bond properly. Acid-core solder contains a highly aggressive, corrosive flux designed specifically for plumbing and heavy sheet metal work. If acid-core solder is used on delicate electronics or circuit boards, the acidic flux residue will remain on the joint. Over time, it will literally eat away at the copper traces, component leads, and wiring, eventually causing the circuit to fail completely. For radio and electronics work, you must always use rosin-core solder (or modern 'no-clean' electronic flux). Rosin is active when heated to clean the joint but becomes inert and non-corrosive once it cools down, leaving the circuit safe from chemical damage.