In soldering, the mating parts are joined by a filler material, or solder. The melting temperature of the solder is lower than that of the component materials. As a result, only the solder is melted.
The mating parts are merely warmed. Once melted, the solder flows into the gap between the parts and bonds with the surface of the workpiece (diffusion bond). Soldering a joint requires access to only one side of the joint. The thin gap between the components functions like a capillary, drawing the liquid solder into the joint.
The soldered joint is only as strong as the solder material. In a related process called brazing, solders made of copper and zinc can produce joints that are as strong as those achieved during welding. The surface of the solder seam is smooth and clean, forming a nicely curved transition to the workpiece. Since solder seams do not require finishing, they are often used in the automotive industry for making body parts such as trunk lids or car roofs. No processes area required prior to painting except a simple cleaning.
Other applications can be found in mixed constructions. Components made of dissimilar materials often cannot be welded or, if they can, only with limited success due to the very different melting points of the materials. Joining aluminum and steel is one such example. For these and similar joining tasks, soldering is the perfect alternative.