A Beginner's Guide to What is Solder Flux and How to Use It

Working of soldering flux
Soldering flux is a very useful material that assists in obtaining a perfect soldering bond. Click here to learn more about the function and usage of soldering flux.
Did You Know?
Nowadays, soldering wires are available in the market which have a tubular structure. The hollow region inside is filled with flux, which in the case of small soldering jobs, eliminates the need for applying flux externally.
Be it the complex motherboard of a computer or a simple electronics hobby project, there is no bypassing the art of soldering. It is one of the most basic activities that must be performed while building most electronics or electrical circuits.

We call soldering an art, because even though it may appear simple enough, it actually requires a significant amount of practice to get it right. Most importantly, one needs to learn the correct technique and use the right tools to obtain the perfect bond. Many beginners and non-professionals simply tend to heat up the gun and start soldering right away. This may work most of the time, but the end result simply cannot be called professional, and is rarely reliable in the long run.

Therefore, to achieve the perfect, reliable solder, one should make use of a material known as a soldering flux. In the following lines, we shall find out what soldering flux is, and how it works. We will also examine the basic process of using it. But before that, let's understand some of the problems that are commonly faced while soldering without using soldering flux.
Soldering and Oxidation
Repair of electronic
When a solder melts to form a joint between two metals, it doesn't just act as an external glue. In fact, solder, when heated by the gun, reacts chemically with the metal surfaces that need to be joined, and forms a metallurgical bond.
For effective soldering to take place, there are two prime requirements:

1) The solder must be metallurgically compatible with the metals that need to be bonded.

2) The surfaces of the metals to be bonded must be as clean as possible, free from dust, oxides, and grime, which can hamper effective bonding.
Prevention of accumulation of dust and grime can be achieved by optimum and clean storage, and the use of effective alcohol-based cleaning solutions can help in their removal. However, the removal of oxidation from contacts and metal surfaces requires something more.

Oxides can form on almost any metal surface when oxygen reacts with it. The most commonly known oxidation is that which happens to iron - rust. However, oxidation also occurs on tin, aluminum, silver, copper, etc. Since metals are used as the basic conductors of electricity, it follows that almost all electric/electronic components are vulnerable to oxidation.

Oxidation forms oxides, which degrade the surface of metals, preventing the solder from bonding properly with the surface. As such, soldering may become difficult or even impossible, unless oxides are completely removed from them.
Purpose of Soldering Flux
Soldering iron composition
Thus, clearly, a reliable bond can only be formed if the surfaces are clean and free from oxides. Oxidation takes place at a very rapid rate, especially when the metals are heated, leading to the formation of an oxide film on their surfaces.

While cleaning with appropriate solvents does help in removing the oxidation film, it tends to reoccur almost instantaneously. For this reason, a specially designed material, known as a soldering flux, is used to get rid of the oxidation film. It removes oxidation and prevents it from reoccurring again until the soldering bond is formed.

In its unheated state, the flux is non-corrosive and non-conductive. Therefore, it can be safely applied to electrical contacts on the circuit. At solder temperatures, the flux liquefies into a highly corrosive liquid, which scrapes the oxides from the surfaces to be soldered. Flux also prevents the reformation of new oxides.

The flux is designed to melt at below-solder temperatures. This allow it to spread over the surface and perform the fluxing action―removing oxides and getting rid of them―prior to the actual soldering process.

If the tip of the soldering gun is allowed to remain on the surface where the flux is applied, the flux first liquefies and then quickly evaporates. It is therefore important that the soldering should take place immediately after the flux melts, and before it can evaporate.
How to Use Solder Flux
Soldering paste
There are many different types of solder fluxes. The commonly used ones include Rosin (No Clean), Rosin (Mildly Activated), and Water Soluble. Soldering flux is available as paste contained in small jars/tin cans, and also as liquid contained in jars/bottles or flux pens.

The type of flux also varies depending upon their particular applications. For soft soldering, organic flux is typically used. Inorganic halogenides/acid based flux may be used in non-electrical applications. Flux used for welding, brazing, and soldering (electronics) is typically inorganic in nature, but may include organic compounds as well. These tend to be activated at higher temperatures.

The following is the general procedure for the use of soldering flux:

1) Before using the soldering flux, a good solvent should be used to clean the metal contacts to get rid of the grime, dust, and excessive oxidation that may be present.

2) Then, an even coat of the flux must be applied to those surfaces where the actual soldering is slated to take place. Note that, at this stage, heat must not be applied.

3) When the tip of the soldering gun is hot and ready, it should be placed on the metal contacts covered with flux. The flux should be allowed to melt and spread, covering the metal surface. This will remove the oxide layer and keep it from forming again, until the flux is present.

4) As soon as the flux starts to vaporize, the soldering wire should be introduced and melted by the gun. This will ensure that the solder bonds before oxidation reoccurs, allowing for effective soldering to take place.
Thus, solder flux is a very useful material that removes oxides which form on electrical contacts, and helps the metals being soldered in bonding together more effectively.