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solder(sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i.e., silver solders) have high melting points and are suited for use with ferrous and high-melting-point nonferrous alloys. Areas to be soldered are cleaned and coated with a flux (such as hydrochloric acid or borax) to prevent oxides from separating the solder from the surface. When brass is used in the solder or when brass surfaces are to be joined, the process is known as brazing, though the name is sometimes applied also to other hard soldering.
a metal or alloy that is introduced into the gap between combinable parts or that is formed between combinable parts as a result of diffusion during soldering. Solder has a lower melting point than soldered materials. The melting point of a solder is one of its most important characteristics and determines the soldering method and equipment to be used and the strength of the joint to be formed. The higher the melting point of a solder, the greater its strength and the strength of the soldered joint over the entire operational temperature range.
Solder must sufficiently wet the materials to be soldered and must flow over these materials in such a way as to cover all solderable gaps. Tight, corrosion-resistant joints are thus formed. Thermal expansion coefficients for both the solder and the soldered material should not differ markedly. In special cases, solders are required to be resistant to heat, acids, and alkalies. They should also be good electrical conductors.
The most common solders contain Sn, Pb, Cu, Ag, Ni, Zn, or Al. Monolithic, powdered, laminated, and self-fluxing solders are differentiated according to performance.
V. P. FROLOV