What Is Soldering? Differences Between Soft Soldering and Brazing

Another way of joining metal parts is soldering. Unlike welding, in this method the parts to be joined do not melt themselves. Instead a separate metal (the solder), which melts below the melting point of the parts, comes into play and holds the two pieces together.

How Does Soldering Work?

During soldering the metals to be joined keep their solid state, because the melting point of the filler used is lower than that of those metals. The molten filler penetrates between the crystal structure of the parts and forms a sound bond.

Soldering is divided into two according to working temperature:

Soft soldering: carried out below 450°C
Brazing (hard soldering): carried out above 450°C

Soft Soldering

Soft soldering is carried out with filler materials that melt below 450°C. The alloy most used for this purpose is the easily melting lead-tin mixture.

This method is preferred especially for joining thin sheets such as tinplate, zinc and brass, for airtight canning work, and for assembling electronic components.

To make a sound soft-soldered joint, the surface to be soldered must be thoroughly clean and must come into direct contact with the filler. This cleaning is usually achieved with soldering fluid (flux).

What is soldering fluid? It is a cleaning liquid obtained by dissolving zinc chloride in hydrochloric acid.

During the operation, the tip of the heated soldering iron is first cleaned by rubbing it on a block of sal ammoniac. A small amount of filler is then picked up with the iron tip and applied to the surface to be joined. Once the solder cools, the joint is complete.

Soft solders generally consist of a tin-lead alloy and their melting points are always below 450°C. An interesting property of these alloys is that they melt gradually rather than suddenly: first they reach a paste-like consistency, then they turn into a thick liquid, and once they reach a certain temperature they become fully fluid. A special alloy in which tin and lead are mixed in equal proportions (50/50) is the exception to this rule and melts directly at 182°C.

As the tin content rises, the strength of the solder changes as well. A solder containing 8% tin, for example, is used for soldering parts exposed to small forces such as radiators, while a solder containing 60% tin is preferred for tinning electrical cables. Solders with a tin content of 90% are used for tinning vessels, boilers and equipment that comes into contact with foodstuffs.

Brazing (Hard Soldering)

Brazing is carried out with filler materials that melt at temperatures above 450°C, and it is far stronger than soft soldering. This method is preferred for joining metal parts with a high melting point, sheet plates, boilers, pipes and wires. A copper and zinc alloy is generally used as the joining medium.

When brazing, the parts to be joined are first heated and the surfaces are cleaned by applying borax. The filler, in the form of brass wire, is dipped in borax, melted with the torch flame and run onto the surface to be joined. After it cools, the surface is cleaned with 10% sulphuric acid.

The copper-to-zinc ratio of the brass alloys used in brazing, and the working temperature, vary according to which material is being joined. A brass filler containing 42% copper, for example, works at 845°C, while one containing 54% copper works at 900°C and is used for brazing steel and bronze alloys.

Differences Between Soft Soldering and Brazing

The main differences between the two methods can be summed up as follows:

The difference in temperature is the deciding factor: soft soldering is done below 450°C and brazing above it.

The filler wires used in brazing are very varied. Materials such as silver, aluminium, copper, gold, magnesium and nickel can be used for this purpose. In soft soldering the wires used are fixed, and consist of a lead-tin alloy.

Soldering Compared with Welding

Although soldering and welding resemble each other, there are important differences between them:

In soldering, the materials to be joined do not have to be of the same kind. In welding, materials of the same kind, or with similar properties, are generally joined.

In welding the material itself melts, while in soldering only the filler melts. Soldering is preferred in situations where excessive heat could damage the material.

Soldering is applied where joining by welding would not be practical or economical. Where strength is required, welding is preferred.

Frequently Asked Questions

In welding the joined metal itself melts, while in soldering only the filler melts and the parts to be joined keep their solid state.

450°C. Soldering carried out below this temperature is called soft soldering, and above it brazing.

It cleans the surface to be soldered so that the filler can make direct and sound contact with the metal.

Brass wires of copper-zinc alloy are generally used. Materials such as silver, aluminium, gold, magnesium and nickel can also be chosen for brazing.

Soldering is preferred where the material could be damaged by excessive heat, where metals of different kinds have to be joined, or in work where welding would not be practical or economical.