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Soldering/brazing – introduction

Soldering and brazing are similar processes used to join metal parts. The soldering or brazing material simply fills the gap between the two parts, ie , it is the filler. The key feature is that the material being used, the filler, melts but neither of the parts being joined melt. This is contrast to welding where the parts being welded will melt though only in the locality of the weld.

It follows from this that in soldering/brazing the filler melts at a lower temperature than either of the parts. When welding the filler melts at a temperature very close to the melting point of the two parts being welded. Usually the filler is made of the same material or very similar material as the parts being joined.

The difference between soldering and brazing is simply the temperature at which the filler melts. For many people soldering is when the filler melts below 400ºC. If the filler only melts above this temperature then the process is called brazing.

Sometimes soldering below 400C is referred to as being soft soldering whilst soldering above this is hard soldering.

When the filler is a silver alloy then the process will be brazing and the silver rod will be a silver brazing alloy. However in model engineering circles silver brazing rods are commonly called silver solder and the process is called  silver soldering.

In general all fillers are quite runny when molten. They will not fill gaps. On the other hand because of their runniness they are drawn into the smallest of gaps. The size of gaps varies from 0.05mm  to 0.2mm.

The need for fluxes

In all soldering process the filler must not just fill the gap between the two parts it must also fuse with the surface on each part. Most metals, even if they have just been cleaned will have a thin layer of oxide on them. This means that it is always necessary to use a flux that will remove any oxide layer. When this happens it can be seen that the filler “wets” the surface of the parts. It is quite impossible for the filler to physically hold two parts together without this happening.

The range of soldering processes

Soldering can be used wherever the filler melts at a lower temperature than either of the two parts being joined and the filler can be made to wet both surfaces. Whether this can be done depends upon a suitable flux being available.

This leaves a range from about 200ºC to 1000ºc. Over this range there will be many possible fillers. At the bottom end will be lead/tin alloys that are used for soldering electronic components. One with 60% tin and 40% lead melts at 183ºC.As the amount of lead increases the melting point goes up. In the middle might be silver solders. These work over the range from 600ºc to 800ºc  At the top might be fillers made of brass or bronze.

In general the higher the melting point of the material the stronger the join all else being equal.

Joint design

metals that can be silver soldered

choice of filler

cleaning of parts

flux

heating

cleaning off flux

All fluxes, by their very nature, are corrosive

Hazards

Most soft solders used to contain lead. Lead is poisonous. Lead is now banned from many types of soft solder. This is certainly true of solder used in electronics and solders used in plumbing.

Silver solders used to contain cadmium. This is now banned. Cadmium is very poisonous. It was possible to overheat the solder and cause it to produce fumes containing cadmium oxide.

Fluxes for hard solders often contain fluorine compounds. These can cause skin irritation.

lead

cadmium

fluorine compounds