What is the primary use of solder?

• A. Joining two metal surfaces.
• B. Painting metal surfaces.
• C. Coating surfaces with zinc.
• D. Polishing metal.

Answer: (A)

Soldering is used to create a joint between metal surfaces by filling the gap with a filler metal that melts at a low temperature. When heated, the solder becomes liquid and flows into the joint, aided by capillary action, wets the surfaces, and, as it cools, forms a solid metallurgical bond. This makes it ideal for linking components in electronics and plumbing where a reliable, conductive connection is needed without applying excessive heat to the parts. Flux is often used to clean oxides and improve wetting so the joint bonds smoothly. The other options are different processes: painting, zinc coating, or polishing do not join metal surfaces through a molten filler metal.