Page 42 - Curriculum Visions Dynamic Book

Solder
Soldering is a process of joining two
pieces of metal together with a filler metal. Soldering metals have to be chosen to alloy with the metals they are joining and so provide a strong joint.
The composition of the solder is also designed to have a low melting point, so that it can be applied easily without damaging the metals being soldered.
Common solders are mixtures, or alloys, of lead and tin. Both lead and tin have low melting points, but a mixture of the two is stronger and has a lower melting point than either metal. Some materials, such as wires, are tin plated to make soldering easier.
The most common use of lead-tin soldering is to join copper wires or pipes.
To make a soldered joint
The use of lead solder relies on the way that a liquid
will pull itself into a small space, called capillary action. The metal
surfaces will only join if they are clean,
so in general the tarnished surface is scraped away using steel wool, and the metal left bright and shiny. A material called a flux is then spread over the metals to be joined to prevent them from tarnishing again as they are heated.
For joining wires a soldering iron is used, and the solder has small rods of flux embedded in it. For soldering larger items, such as pipes, the heat has to be applied by a source such as a blowtorch. It is important that the heat is applied evenly so that the solder can run quickly and smoothly.
 The most commonly used solder consists of 50% tin and 50% lead. Solders containing a higher proportion of tin
(say two-thirds tin, one-third lead) can be applied at a lower temperature but tend to be more brittle.
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 Copper pipework is joined
by solder. A flux is applied to the surfaces to be joined to prevent them from oxidising in the heat before the solder melts.
These ridges contain solder reservoirs. When the joint
is heated with a blowtorch, the solder melts. A close fit ensures that capillary action causes the solder to flow evenly throughout the joint.