What is a flux used for | How to Use Flux in Soldering

How to Use Flux in Soldering: How does it work? The flux used in soldering is more critical for joint formation. Engineers and technicians often ask these questions in the electronics and wires industry. Even company purchase departments may request them.

Soldering is a common technique used in electronics manufacturing. In this article, we’ll explain What is flux used for and how to Use Flux in Soldering.

Soldering involves metal wire or other conductive material to form electrical connections. The process requires a particular type of solder called flux. This chemical compound cleans away oxidation from the metals before soldering. Without proper cleaning, the solder won’t stick properly.

The purpose of flux in soldering is to remove other metallic impurities from the soldering surface and prepare a clean, solid joint. After the soldering process is completed, cleaning is required depending on the type of flux used.

What is flux in soldering?

Flux in soldering is used to clean up the surface of the solder joint before soldering. Flux acts as a cleaning agent and removes oxides and other impurities from the metal surfaces. Solders are usually fluxed by dipping into a container of molten solder containing flux.

Flux also prevents corrosion between the two surfaces from being joined. Flux is a liquid mixture of rosin (a naturally occurring resin) and other chemicals. When heated, the rosin melts and flows into the joint, where it reacts with the metal oxide layer to form a eutectic alloy which bonds the solder to the base metal.

How to Use Flux in Soldering-Step by Step Guide
  • Metal oxidation occurs when metals come into contact with oxygen. Contact between metal and air causes an oxide layer to form, preventing further oxidation.
  • Flux removes oxides and other impurities from the surface of metals.
  • They reduce the surface tension and viscosity of molten solder to improve wettability.
  • Flux is usually mixed with solder powder and applied to the joint area where the wire will be connected.

What causes oxidation during soldering?

Oxidation occurs when metals react with oxygen from the air. The metal reacts with oxygen to form oxides, which are less conductive than the original metal. This means that the solder joint becomes weaker and may eventually break. To prevent oxidation, keep the work area clean and dry. Use flux to remove any oxide buildup on the surface of the metal parts.

Soldering surface oxidation is caused when the solder melts and then cools down. The oxide layer forms on top of the metal and protects the metal from further oxidation. However, if the solder is too thick, the oxide layer may be too thin and allow oxygen to penetrate the metal. This causes corrosion which leads to failure of the joint.

  • Oxygen is present in solder.
  •  Oxide layers form on the surface of the metals.
  • These oxide layers protect the metals from further oxidation.
  • But if the oxide layer is too thin, it cannot provide adequate protection against further oxidation.
  • As a result, the metal becomes more susceptible to oxidation.
  • This can lead to the failure of the solder joint.
  • To prevent this, the oxide layer must be removed so that the solder can flow smoothly.
  • This is done using flux.
  • Flux is a paste that contains chemicals that help remove the oxide layer.
  • Flux should be applied to the joints before the solder.

So that solid inter-metallic bonding can take place. The primary function of the solder flux can be broken down into the below parts. Solder flux is used to remove the metal oxide surfaces that, in the joining process, prohibit a bond between the metals. At soldering temperatures, flux chemically reacts with oxides to produce new, tarnish-free surfaces.

How to choose the right soldering flux?

Soldering flux is used to remove oxides from metals. The best solder fluxes are made of rosin, derived from pine trees. There are two types of rosin fluxes: organic and non-organic. Organic fluxes contain rosin, while non-organic fluxes do not. When choosing a soldering flux, look for one with a high percentage of rosin.

Rosin fluxes contain rosin, which acts as a solvent for oxide layers on the metal surface. Rosin fluxes tend to work well on tin/lead alloys, while rosinless fluxes work better on copper and silver. Rosin flux is generally better for electronics work because it doesn’t leave any residue behind. Water-soluble fluxes dissolve quickly when heated, making them ideal for quick repairs.

No-clean flux is a type of flux that contains no cleaning agents such as sodium hydroxide. This type of flux has been used to remove solder from printed circuit boards for decades. The advantage of using no-clean flux is that it removes all traces of flux residue from the board after soldering.

A suitable flux should leave no residue after being cleaned off with soap and water.

Two things must be considered when assembling an electronic device. Before and after soldering, the flux must remain inactive; however, it must become activated during soldering. Solder flux should be used at a lower than normal temperature so that when the solder melts onto the tip of the soldering iron, there won’t be any unwanted residue left behind after melting. Flux residues may cause unexpected failures during production runs. Therefore, we recommend using inactive flux for PCB fabrication.

How to use Flux when Soldering -Step by Step Guide

The best way to use solder flux is to wet the iron tip with the flux first. Then wipe off excess flux with a paper towel. Next, place some solder onto the iron tip and touch the solder to the area where you want to solder.

The solder should start melting immediately. Once the solder starts melting, apply pressure to the solder while holding the iron against the soldered item. After applying enough pressure, the solder should begin to cool and solidify.

Step 1- It is best when soldering two pieces to use steel wool or Scotch Brite to clean off any oxides before doing so.

Step 2- Using a brush, apply flux with solder to both pieces of the terminal that need to be connected.

Step 3- To heat the joint without burning the flux, use adjustable temperature control to set the iron to the right temperature (400°C/752°F). Inhaling fumes or direct skin contact with a flush is dangerous, so always take precautions.

What Type of flux is used in Electronics Soldering?

Rosin Flux

Natural Rosin Fluxes are obtained from the stumps of pine trees. Therefore, these are natural products that are extracted from pine trees. Generally, rosin is composed of C19H19COOH, but the formula varies from batch to batch. The rosin flux is further divided into non-activated (N), mildly activated (RMA), and started (RA).

Non-activated Rosin Fluxes (R) are not activated and are therefore best suited to use on clean or minimally oxidized soldering surfaces. For other hand soldering jobs, rosin fluxes of type (R) are used for soldering copper wire, printed circuit boards, and semiconductors. The Rosin Mildly Activated (RMA) Fluxes are strong cleaners that are more powerful than their Non-Activated (R) counterparts. These fluxes in soldering are ideal for handling higher-containment leads for general-use cables, PCBs, and electronic components.

Rosen-activated (RA) fluxes provide the best cleaning performance among all rosin-activated fluxes. This product’s soldering for electrical components is the best choice for soldering hard-to-clean surfaces.

Low Residue or No- clean Flux and Solder Paste:

A printed circuit board assembly can be soldered without being cleaned in current technology. It is common practice in Europe for companies not to clean rosin fluxes (R and RMA) since they don’t cause any reliability issues if not cleaned (especially if they are halide-free). Globally, No-Clean fluxes are becoming more prevalent since the ban on CFCs.

Older pastes and fluxes don’t require cleaning after use, resulting in significant savings on cleaning costs and capital expenditure.

Organic Acid Fluxes

These fluxes are stronger in soldering than rosin fluxes, but they are less efficient than inorganic fluxes. In addition to organic acids, water-soluble fluxes are also known as organic acid fluxes. Organic acid fluxes are justified with mixed assemblies (II and III).

Once the assembly has been thoroughly dried, the flux residues can be removed using normal water since they are water-soluble.

Inorganic Acid Fluxes

Soldering surfaces that are difficult to bond with inorganic acid fluxes are perfect. These fluxes have a much higher melting point than organic fluxes. Frequently oxidized, these methods can remove metallic parts. Hydrochlorides, hydrofluoric acids, stannous chlorides, sodium fluorides, and zinc chlorides fall into this category.

Solder flux in non-electronic applications is used for brazing copper pipes, an inorganic flux in soldering. Chemically active residues can cause serious field failures because they leave behind chemically active residues that can cause corrosion.

How to use liquid flux when soldering?

  • For flux for electrical soldering, use rosin-based flux to make solid joints.
  • For plumbing, joints recommended acid flux when soldering pipes because acids are more prone to remove oxide layers.
  • Solder top tining is essential to keep clean when working with electronics.
  • Always keep your soldering iron in a stand while it is on for the safety and reliability of the equipment.
  • Use all precautionary wearings during soldering for safety purposes.

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Hello, this is Manoj, A Soldering and DIY Tool expert. Writing is one of my hobbies. With more than 20 years of broad experience. I love researching, discovering, and sharing new products with others, I hope you’ll enjoy the greatest featured products to make life easier, more fun, and more productive.

1 thought on “What is a flux used for | How to Use Flux in Soldering”

  1. This is Ray come from China, appreciate your sharing. May I get answer from your site, the question is I know the useful of flux in SMT reflow process, but how does the flux working? Normally after reflow the residual flux is clean and transparent, why few found flux were opaque and yellow without gloss.


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