What is Soldering?

What is Soldering?

Soldering is a process of joining two metal surfaces together using a filler metal known as solder, which has a lower melting point than the base metal.

The solder is melted and applied to the joint area, where it cools and solidifies, creating a bond between the two metal surfaces.

Unlike welding, soldering does not involve heating the metal surfaces to their melting point, and the joint is typically weaker than a welded joint.

Soldering is commonly used in electronics, plumbing, and jewelry making, among other applications.

Base metals that can be Soldered?

Unlike fusion welding, soldering does not require heating the parts to be joined to their melting point.

The filler metal, also known as solder, has a different composition than the base metal.

The joint is formed either by filling a braze gap through the capillary effect or by filling a soldering groove.


Many metals can be soldered, including copper, brass, aluminum, gold, silver, and nickel. However, not all metals can be easily soldered due to factors such as their melting point, reactivity with the solder, or other properties.

For example, stainless steel can be difficult to solder because it has a high melting point and can form a chromium oxide layer that interferes with the soldering process.

It is important to choose the appropriate solder and flux for the specific base metal being used in order to achieve a strong, durable bond.

Related reading: How Strong is Silver Solder?


Soldering is a suitable method for joining almost all types of steel, especially when it is important to maintain the specific properties of the material.

The temperature used for soldering can be carefully controlled to allow for the integration of heat treatment processes, such as tempering, annealing, and precipitation hardening.

Related reading: Solder not stick to soldering iron: Solution is here.

This allows for precise control of the properties of the joint and ensures that the material maintains its desired characteristics even after the soldering process.

In contrast to welding, which can cause distortion and damage to the base material, soldering is a gentler process that can be carefully controlled to achieve the desired results.


Currently, it is common to use brazing and soft soldering techniques to join aluminum and its alloys.

These methods make it possible to create mixed joints, allowing aluminum to be combined with other materials such as high-alloy steel, which is highly resistant to corrosion.

In fact, it is quite common to see these materials used together in various applications. The ability to create strong, reliable joints between dissimilar materials like these is a valuable advantage of brazing and soldering techniques.


Copper and its alloys, both low and high-alloyed, can be joined through brazing and soft soldering techniques.

Combining copper with steel is also possible and presents no difficulties. However, copper brazing can be limited due to the potential softening of cold-formed or precipitation-hardened copper, which can impact the desired properties of the material.

The oxygen content in the copper plays an important role in determining its suitability for soldering, and in some cases, whether it can be soldered at all.

As a result, it is essential to carefully consider the specific properties of the materials being joined in order to achieve a successful, strong bond.

Types of Solders

There are several types of solders available, each with its own unique properties and applications. Here are some of the most common types of solders:

  1. Soft solder: This type of solder is typically made of tin and lead, or tin and antimony. Soft solder has a lower melting point than other types of solder, making it ideal for joining metals with low melting points, such as copper and brass.
  2. Hard solder: Also known as silver solder, hard solder is made of a silver alloy and has a higher melting point than soft solder. Hard solder is ideal for joining metals with higher melting points, such as steel and stainless steel.
  3. Flux-cored solder: This type of solder contains a flux core, which helps to remove oxidation and other impurities from the joint area, ensuring a strong, clean bond.
  4. Lead-free solder: Due to concerns about the environmental and health impacts of lead, lead-free solders have become increasingly popular. These solders typically use a combination of tin, silver, and copper, among other metals.
  5. Aluminum solder: Specially designed for joining aluminum and its alloys, aluminum solder typically contains a flux core to help remove oxidation from the joint area.
  6. Copper solder: Copper solder is designed for joining copper and its alloys and typically contains a flux core to help remove oxidation from the joint area.

Related Reading: Soldering Techniques.

Soldering Flux

Fluxes are non-metallic substances, often made from materials such as silicate, carbonate, borate, chloride, or fluoride.

Related reading: Types of Soldering Flux

They are used during soldering to perform several essential tasks and possess specific properties, including:

  • Having a melting point approximately 50°C lower than the melting point of the solder being used.
  • Forming a dense coating that remains intact even at elevated temperatures.
  • Being highly viscous, allowing them to adhere to vertical surfaces without dripping or running.
  • Eliminating oxides and preventing their formation during heating, ensuring a clean, strong bond.
  • Protecting the joint area from air, which can interfere with the soldering process.
  • The soldering temperature must fall within the effective range of the flux.

One notable exception to the use of flux is copper-phosphorus brazing solders, also known as “CP solders.” When soldering copper, flux is typically not required.

Related Reading: Difference Between Solder Paste And Flux


What is the difference between soldering and welding?

Soldering and welding are both techniques used to join two pieces of metal together.
However, welding involves melting the metal at the joint and fusing it together, while soldering involves melting a filler material (solder) and using it to join the pieces together. Soldering does not require the pieces being joined to reach their melting points, unlike welding.

What materials can be soldered?

Many metals can be soldered, including copper, brass, steel, aluminum, and silver. The choice of solder will depend on the specific materials being joined and the desired properties of the joint.

What types of solders are available?

There are several types of solders available, including soft solder, hard solder, flux-cored solder, lead-free solder, aluminum solder, and copper solder.
The choice of solder will depend on the specific materials being joined and the desired properties of the joint.

Do I need to use flux when soldering?

Flux is typically used during soldering to remove any oxides from the surface of the metal and to protect the joint area from air.
However, some solders, such as copper-phosphorus brazing solders, do not require the use of flux.

What equipment do I need for soldering?

The basic equipment needed for soldering includes a soldering iron or torch, solder, flux, and a heat-resistant surface to work on. Other tools, such as pliers and wire cutters, may also be helpful.

Is soldering difficult to learn?

Soldering can take some practice to master, but with the right equipment and technique, it can be a relatively straightforward process.
There are many resources available, including instructional videos and tutorials, to help you learn how to solder.

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