A Guide to Soldering and Brazing

3rd March 2020

A guide to soldering and brazing


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		Soldering is the process in which two or more pieces or tubes of metal are joined together. The bonding is achieved by melting a filler metal – called solder – to a liquid state with a soldering gun or blow torch and depositing it onto the join. The melted solder expands into the pores of the base metals, opened by the higher temperature, and cools to form a solid bond.



		There are two forms of soldering: Soft soldering Hard soldering The difference between soft and hard soldering lies in the temperature of the heating process to melt the solder and achieve the solvent action.

		Soft soldering
		Temperatures required to melt soft solder range from 90 to 450 degrees C. Solder must also have a lower melting point than the base metals (the metals being bonded. In the case of soft solder, this would be lower than 450 degrees C.

		Soft soldering is used for joining small, intricate parts which may damage when soldering has to be carried out at higher temperatures than 450 degrees C. For example, fixing electronic components or wiring to circuit boards…

		…or joining copper plumbing pipe and the repair of any joints which are not affected by high operating temperatures of certain applications such as a central heating system. As the operating temperature approaches the melting point of the solder, the joint would simply weaken or snap.

		Hard soldering
		Brazing and silver soldering are forms of hard soldering. Both use higher temperatures for the melting of hard solder and the subsequent bonding which takes place.

		In other words, where soft soldering occurs at temperatures less than 450 degrees C, hard soldering occurs at temperatures over 450 degrees C.

		Soft solder is usually a tin-based alloy (a combination of two or more metals) and melting is usually achieved with a solder gun although some soft solders may require a gas torch.

		Brazed joints are used to achieve higher joint strength and fatigue resistance. Hard solder for brazing therefore requires stronger filler metals than tin, such as a brass based alloy. This means it needs a higher temperature heat source such as a gas torch or furnace to melt the solder.

		Brazing Like soft soldering, brazing connects two pieces of base metal by melting filler metal and distributing it in its liquid state between the pieces, fusing them together. This time, however, soldering occurs at temperatures over 450 degrees C.

		Where the primary benefit of soft solder is the low temperature used to avoid heat damage to electronic components, for example, a brazed joint is known for its tensile strength, particularly when it functions at high operating temperatures. In this respect, brazing is especially useful for central heating, ventilation and air conditioning appliances.

		As brazed solders are created primarily for strength, other applications include repairing heavy cast-iron objects or wrought iron furniture.

		Silver soldering This is another form of hard soldering as it uses high temperatures (over 600 degrees C) to make bonded components. Silver soldering is used to assemble silver jewellery and other precious and semi-precious metals.

		Silver solder is an alloy of varying proportions of silver, which melts at different temperatures depending on the application and requires a blow torch, for example, as its heat source.