What is Soldering? (2 Types of Soldering You Should Know!)

What is soldering

I’m sure you might have seen this type of circuit board used in electronics. The connections on this circuit board are made by using a soldering process.

Let’s dive deeper into the Soldering process, types of Soldering and many more…

What is Soldering?
Definition: Soldering is defined as a process of joining two metal pieces by adding an extra material or alloy called “solder”.

The extra material used for soldering is in the form of a filler rod.

In soldering process, the parent metal is not melted but the bond between two metals occurs after the solidification  of molten solder.

Few important things about Soldering

  • The commonly used solder (or filler rod) for the soldering process is made up of tin and lead.
  • The melting point of solder is lower (around 150 °C to 350 °C).
  • The solder (or filler rod) is dipped in the flux before the soldering process, in order to prevent contamination.
  • The solder used for soldering process must have a lower melting point than the melting point of the metals to be joined. And the fusion point of solder and base metal should be near to each other so that proper soldering takes place.

Types of Soldering

There are basically 2 different types of Soldering as mentioned below.

#1) Hard soldering

hard soldering of silver ring
Hard soldering of silver ring

In hard soldering process, the melting point of the solder is above 450 °C.

Also in the hard soldering process, the heating is usually done with the Oxy-acetylene gas torch.

Hard soldering is used for joining precious and semi precious metals like gold, silver, copper, brass, etc.

You can see in the above image that hard soldering is used for the joining process in a silver ring.

#2) Soft soldering

soft soldering

In soft soldering process, the melting temperature of the solder is below 450 °C.

The heating of the solder is usually done by using a solder iron (see above image).

The joints obtained by soft soldering process are not so hard as compared to the joints obtained by hard soldering process.

Soft solder mainly consists of lead and tin alloy. But other metals are also added to the solder for lowering the melting point.

In soft soldering process, it is necessary to use flux to reduce the contamination during the soldering operation.

Soldering flux is either in the liquid state or it is like a semi liquid paste.

There are different types of flux used for the soldering process.

  • Zinc chloride
  • Commercial hydrochloric acid
  • Alcohol, etc.

Most common solders (or filler material) used in soft soldering process are alloys of tin and lead in different proportions.

Lead is toxic to the environment, so lead free solders are developed nowadays and they are widely used.

Solder composition percentage

Various solders and their percentage composition are mentioned below.

  • Soft solder: lead (37%), tin (63%)
  • Plumber’s solder: lead (70%), tin (30%)
  • Electrician’s solder: lead (58%), tin (42%)

Having discussed the two different types of soldering, now let me tell you how soldering works? 

Soldering process (6 Steps)

The soldering process steps and mentioned below.

Step 1: Clean the surface

Step 2: Turn on the soldering iron

Step 3: Apply flux to the solder

Step 4: Heat the solder and apply it to the joint

Step 5: Leave the joint to cool down

Step 6: Clean the soldering iron after soldering is completed.

Detailed soldering procedure

First things first, solder does not stick to the metal surface that is rusted or dirty.

So the first step is to clean the workpiece surface so that the dirt and oil or grease can be removed.

The surface cleaning can be done by filing or brushing by using emery cloth.

After the cleaning of surface, the electric soldering iron is plugged into the socket and it is allowed to reach a particular temperature.

Meanwhile a flux is added to the solder wire. Commonly used flux is ZnCl2, which protects the surface from contamination during the soldering process.

The heated tip of the soldering iron is contacted with the solder which melts and spreads around the joint.

The tip used in soldering iron is made up of copper which can absorb and give up the heat during soldering operation.

The melting point of solder in soft soldering process is below 450 °C and for the hard soldering process, the melting point of solder is above 450 °C.

How does soldering work?

Soldering takes place in 3 stages.

  1. Wetting: When the solder is contacted with the soldering iron tip, it melts and then it wets the surface of the workpiece.
    This molten solder and the solid base material comes in contact and forms a layer of alloy whose depth is that of a few crystal diameters.
  1. Flowing: The heat is liberated during the alloying of the solder with the workpiece surface. And this allows the solder to flow and penetrate easily over the soldering gap.
    The molten solder penetrates due to capillary effect in the solder gap.
  1. Bounding: In this process, both the materials (molten solder and solid base material) get diffused with each other.
    It involves the migration of individual atoms in a mixed phase.

Applications of soldering

The applications of soldering process are mentioned below.

  • Soldering is generally used for obtaining a clean leak proof joint which is not very strong.
  • Soldering is applicable for joining low resistance electrical circuits and wires.
  • Soldering is used for manufacturing printed circuit boards (PCBs).
  • Soldering is used to seal the leakage in automobile radiators.
  • Hard soldering is used for joining precious and semi precious metals like gold, silver, copper, brass, etc.
  • Hard soldering is also used for plumbing, and other metal working processes.

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