Solders are special alloys used in brazing. Soldering is carried out or with the aim of creating mechanically strong (sometimes sealed) seam, or with the aim of obtaining a constant (not burst or moving) electrical contact with small transfer resistance. When soldering the joints and solder are heated. Since the solder has a melting point much lower than the metals to be joined, then it melts, while the solder metals remain solid. On the border between the molten solder and the solid metal there are complex physical and chemical processes. The solder wets the metal, spreads it and fills the gaps between joined parts. The solder diffuses into the base metal, the base metal dissolves in the solder, thereby forming an intermediate layer that after hardening of the pieces into one.
Solders can be divided into two groups: soft and solid.
To include soft solders with a melting point up to 400 °C and to a solid - solders with a melting point above 500°C. Besides temperature melting solders are significantly different in mechanical properties. Soft solders have a limit of tensile strength not higher than 50-70 MPa, and solids up to 500 MPa.
Solder type choose, consistent with the originally soldered metals or alloys, the required mechanical strength, corrosion resistance, and cost - soldering of current-carrying parts with a specific electrical conductivity of the solder.
Soft solders are mainly alloys of tin-lead (mark, STL) tin content of 18% (STL-18) to 90% (STL-90). The conductivity of these solders is 9-13% of the conductivity of standard copper, and the temperature coefficient of linear expansion αl = (26-27)⋅10∧-6/K. There are soft solders with additives of aluminium, silver. Still more fusible solder, composed of bismuth and cadmium. They are used with low temperature brazing; mechanical strength thereof is very low. Bismuth solders have a high fragility.
The most common hard solders copper-zinc (SCZ) and silver (SSr).
Generalized characteristics of some solders are given in table 1.
Not related to the actual solders special types of metal materials used in vacuum technology for bushings, in-mould into the glass and operating at relatively low temperatures, so that the use are particularly refractory, but precious metals (tungsten, molybdenum, platinum, etc.) is not required. For these materials of particular importance is the temperature coefficient of linear expansion αl that for obtaining vacuum-tight input should be consistent αl glass. Note Kovar (brand 29NK), used for solder in solid glasses; an alloy of the approximate composition: Ni 29%, Co 18%, Fe rest; it is ρ equal to 0,49 μOhm⋅m, αl is (4 - 5)⋅10∧-6 /K.
The platinum is a bimetallic wire with a core of Nickel steel N42 (containing Ni 42 to 44% by mass) and an outer layer of copper. The content of copper in the platinum - 25 to 30 % of the total mass of the wire. The name "platinum" is because the coefficient of thermal expansion platinum wire close to the coefficient of thermal expansion of platinum.