Traditional solder that used to be used for soldering in the electronics industry used to be a mixture of tin and lead. Different types of solder are used for different purposes. Solder is of course used in plumbing. The type of solder used for electronics usually contains 60% tin and 40% lead. This is often known as "60/40" solder. It is what is termed a "Eutectic Mixture". Although this is a chemical term, basically this type of mixture melts at a temperature well below that which might normally be expected.
In additional to the metal constituents flux is also required to ensure that good joints are made. Flux is an acidic mixture that helps remove oxides from the area of the joint and therefore helps the solder flow easily over the joint and form a good bond. The flux can be seen as a brown liquid as a solder is heated, and it sometimes gives off a pungent smoke that can act as a irritant. This is one reason why soldering should always be carried out in a well ventilated area.
For plumbers solder, a separate flux is used, but for electronics solder, it is contained within the solder itself. The solder used for electronics is normally in the form of a wire, and if it is carefully cut small cores of flux can be seen running through the solder. The amount contained is correct for making electronics joints and separate flux should never be used.
