During flame soldering, a fuel gas/oxygen flame is used. Unlike gas welding, the surface of the joint partners is not melted but only heated to just above the melting point of the filler material. The filler, which is usually a wire, is added whilst being continuously heated and melted. A small gap should be maintained between the joint partners into which the filler can flow (capillary effect). A fluxing agent improves the wetting between the components and the filler.
Depending on the working temperature there is a difference between soft-, hard-, and high-temperature soldering.