Basic knowledge Correct soldering - soldering methods and differences to welding

Author / Editor: Frauke Finus / Theresa Knell

Soldering is a thermal process that is used for the conclusive joining of workpieces. Soldering is an electrical connection method.

In contrast to welding, a laser does not melt the base material during soldering. Instead, it is merely wetted by the filling material. The two joining partners are connect at the micro level. This is an important characteristic, because a fusion of both materials would lead to an increased susceptibility to corrosion.
In contrast to welding, a laser does not melt the base material during soldering. Instead, it is merely wetted by the filling material. The two joining partners are connect at the micro level. This is an important characteristic, because a fusion of both materials would lead to an increased susceptibility to corrosion.
(Source: Trumpf)

Soldering is a method for joining two workpieces. This process produces a liquid phase, which is achieved either by fusion soldering (melting of a solder) or by diffusion soldering (diffusion at the boundary layers). The deeper layers of the workpiece are not melted and only a surface alloy is produced. This process does not reach the liquidus temperature of the raw materials. After the solder has solidified, a material-locking connection similar to welding is formed.

What is the difference between soldering and welding?

During welding, the liquidus temperature of the parts that are to be joined is exceeded by a lot, whereas in the case of metal soldering the bond may be chemically identical, but the liquidus temperature is hardly or not at all exceeded. Depending on the materials used for metal soldering or welding (welding wire, solder paste, solder wire), the resulting connection differs in its chemical composition.

What is the difference between brazing and soft soldering?

The following processes are used for industrial metal brazing:

  • Brazing
  • Soft soldering
  • High-temperature brazing

The difference between brazing and soft soldering is in the liquidus temperature of the solder. In soft soldering the temperature is below, in brazing above 450°C. High-temperature brazing, on the other hand, is used for joining highly stressed components and is carried out in special vacuum furnaces. This flux-free brazing process takes place at temperatures of above 900°C; cobalt, nickel and copper brazing alloys are used as materials.

History of soldering

Eutect offers solutions for copper enameled wire soldering.
Eutect offers solutions for copper enameled wire soldering.
(Source: Eutect)

Soldering is a very old joining technique that was verifiably used as early as 5000 BC. The then known metals gold, silver and copper were processed into jewelry or cult objects, with soldering serving as the joining technique. The copper salts formed in the CO atmosphere of the charcoal fire were reduced by so-called reaction or diffusion soldering. By chemically reacting with silver and gold, the copper contained in the material forms a solderable alloy. The resulting product has a lower melting point than the pure metals gold, silver or copper. It was not until centuries later that an existing alloy was used as a soldering material. The Egyptian gold mask of King Tutankhamun is one example for these already highly developed soldering processes.

What is needed to solder metal?

  • Soldering materials
  • Soldering flux
  • Instrument for heat generation (soldering iron or torch)

1. Soldering materials

So-called brazing alloys are used to produce a brazed joint. They are usually available as solder wire or solder paste. Solder materials often already contain a flux.In the case of soft solders, the flux is incorporated into the materials hollows , whereas in the case of brazing it is available as a colored coating. The flux often remains as a residue on the solder joint. If soldering is to be carried out without flux (e.g. vacuum soldering), the surface of the workpieces to be joined must be kept clean; soldering itself must be carried out under inert gas or in a vacuum. Glass solders are used for soldering glass and ceramics. They consist of the glass powder with a low melting point and are applied as a paste.

2. Soldering flux

The parts to be soldered must be tidy and free of oxidizing spots in order to allow for the diffusion process to occur during metal soldering. Since soldering is almost always carried out under the influence of air and the oxygen in the air leads to oxidation of the surfaces to be soldered, a flux is applied to the areas to be soldered. The flux deoxidizes the surface and prevents further oxidation. Furthermore, the inclusion of foreign bodies, which can happen during metal soldering, is made impossible and the surface tension of the liquid solder is reduced. Many fluxes must be completely removed after soldering, otherwise corrosion may occur.

3. Heat generation

The heat required for soldering is generated by means of a soldering iron or torch. In some cases, hot steam, laser or an arc (arc soldering) is used. A soldering iron is used for metal soldering and melts the metal to be soldered and then connects it by means of soft soldering. During the soldering process, the metal parts are heated by means of a soldering iron and wetted with solder, which melts in the heat. This creates a durable and electrically conductive connection between the two metal parts. A distinction is made between externally heated soldering irons - used when a large amount of heat is dissipated through the soldering material - electric soldering irons - fitted with replaceable soldering tips - and soldering irons, which are characterized by a short preheating time prior to soldering.

In this video Kuka demonstrates how to automate laser welding, laser cutting, laser soldering and laser hybrid welding with the laser.

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What should be considered in soldering?

Prior to soldering, the workpieces should always be free of dirt, lacquers, oxides and grease. Metallic surfaces should always be tidy for soldering metal in order to allow good wetting by the flux. This makes a significant contribution to the soldering process. In the field of electronics, it can be useful to tin the wires to be soldered first. This reduces the soldering time and the flux dissolves the oxide layers that are constantly forming on the wires during the soldering process. This protects the workpieces from renewed oxidation and increases the flowability of the solder. However, during soldering, the temperature of the workpieces must correspond to the temperature of the liquid solder. Workpieces that are very sensitive to heat, such as electronic components, maximum soldering times must always be observed. Since the strength of soft solders is usually below that of the workpieces to be soldered, a very thin solder layer should be used when high mechanical forces are exerted.

The process of laser soldering.
The process of laser soldering.
(Source: Trumpf)

Which materials can be soldered?

Provided that the correct solder and flux are used, almost all metals are suitable for soldering. Copper and nickel as well as their alloys, iron, steels and precious metals can be processed with commercially available universal solders and universal fluxes. For the brazing of aluminum and aluminum alloys, hard metals, chromium, molybdenum and tungsten as well as for materials which are similar to those of the soft soldering material, the use of special brazing alloys and fluxes is necessary. For titanium, zirconium and beryllium as well as ceramics made of metal oxide, special soldering alloys and the use of customized brazing processes are required.

Types of soldering materials

Soldering alloys are divided into brazing and soft soldering alloys.

1. Brazing materials

Brazing alloys are alloys based on brass or nickel silver which have a high silver content and a softening temperature of more than 450°C. Brazing alloys are available in bar or rod form, as wire or foil or as pastes. Commercially available brazing pastes already contain a flux, making the additional use of a flux superfluous. In contrast to soft solder, brazing alloys are suitable for joints that are subject to high mechanical or thermal stress. A brazed joint is usually characterized by a higher strength than the workpieces to be brazed. When brazing metal, it should be noted that stainless steels with a chromium, molybdenum or tungsten content are usually difficult to braze. In case of doubt, soldering tests are necessary. If these tests are unsatisfactory, these materials should not be soldered, but welded. Gas burners and industrial lasers are used as heat sources for brazing alloys.

Common brazing alloys are:

  • Silver solders
  • Alloys of silver, copper, cadmium and zinc with low nickel/manganese content.
  • Brass solders
  • Alloys of copper and zinc to which silver, silicon and manganese have been added.
  • Phosphorus solders
  • Phosphorus and copper alloys.
  • Brazing alloys for aluminum

2. Soft soldering

Transformer copper profiles in transformer construction: The components are soldered using an Eldec MICO-L 50 MF generator and an auxiliary device for holding the workpiece.
Transformer copper profiles in transformer construction: The components are soldered using an Eldec MICO-L 50 MF generator and an auxiliary device for holding the workpiece.
(Source: Emag)

In contrast to brazing joints, soft solders only exhibit average strength after brazing. This technology is therefore mainly used in electronics, household installations and art (lead glass technology). Today, electric soldering irons with exchangeable soldering tips are used for soft soldering. These irons reach a temperature of about 60°C to 420 °C during soldering and can be set digitally and analogously. Due to the risk of solder and flux splashing, safety glasses should always be worn during soldering. It is also recommended that work be carried out below an air extraction device due to the harmful vapors emitted by the flux.

Frequently used soft solders are tin solders: These solders are alloys of tin and lead and contain small amounts of iron, antimony, copper and nickel. The melting point of tin solders is below 330°C for metal soldering.

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This article was first published by belchnet.