Solder is a metal alloy that is used to join the clean and smooth surfaces of metals. It is normally used in a molten state. Normally, solder alloy contains flux (a substance used to promote fusion) and lead. In many modern applications, however, lead is not used for soldering materials. It finds its applications in electronics and in plumbing for joining pipes. Solder alloys are classified according to their composition. The broad classification includes soft solders, hard solders and flux core solders.
Soft Solder Alloy Properties
Soft solder alloys have metal tin in their composition, which is about 5 per cent to 70 per cent in weight. When more tin is added to the solder alloy, it gains more tensile strength and shear strength. Generally, solder alloy of the ratio 60-to-40 of tin and lead is very common, but for electrical work 63-to-37 is the desired ratio. The former variety has higher melting point (370 degree Fahrenheit) than the latter, whose melting point is 361.4 degree Fahrenheit. Soft solders become solid at certain temperatures.
Lead was once used in equal proportions with tin for plumbing since it ensures better leakage prevention. But the use of lead in soft solders is now prohibited because it could cause water poisoning. Other metals such as silver, antimony and copper have replaced lead.
Hard Solder Alloy Properties
When either zinc or silver is used in solder alloy, it is called hard solder alloy, which finds great use in brazing. Hard solder has a high melting point and does not contain lead. Depending upon the hardness, the hard solders are categorised into "enamelling," "hard," "medium" and "easy." Enamelling is used to prevent a joint from coming apart and has a very high melting point. Hard and medium have relatively lower melting points and are used in the process of making an item or when additional soldering is needed. Easy solder has the lowest melting point and is commonly used for repairing things.
Flux Solder Alloy Properties
Flux solder alloy is basically a coiled wire of solder with non acid flux embedded lengthwise. The various metals used in flux solder alloys serve different purposes: silver offers mechanical strength, copper enhances melting point, bismuth reduces the melting point and zinc serves as a low-cost substitute.