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

Flux in soldering ? and what is flux in soldering needs more for joint formations. Such types of questions are asked by engineers and technicians or even company purchase departments  especially if you work with wires and electronics.

Soldering fluxes plays a very crucial role during the soldering process without causing poorly made joints with bridged pins or other related defects.The cleaning process plays a critical role in the manufacturing yield and product reliability of electronic assemblies.

flux in soldering used for soldering is to remove and other metallic impurities from the soldering surface and prepare a clean surface solid jointing. After soldering process completion cleaning may be required to remove the flux residues that determine the primary on the type of flux used.

What is flux in soldering and soldering fluxes do in soldering

All metals except pure gold and platinum, oxidized in air at room temperature. The rate of oxidation can increase with humidity and temperature.

Since the metal oxides surfaces are barrier to the formation of such a bond, they must be removed by the use solder flux, in the joining process of welding or brazing.In the soldering the function of flux is to chemically react with oxides and quickly produce a fresh tarnish free surface at soldering temperature.

So that solid inter-metallic bonding can take place. The main function of the solder flux can be broken down in the below parts.

  • Cleaning the dirt and oxidize layer on the metallic surface to be soldered.
  • Reducing the surface tension and viscosity of molten solder to improve the wettability.
  • When contact and spread on metal surfaces prevent metal from being re oxidized at higher temperatures.
  • Allow molten solder paste to form a solid long lasting electrical and mechanical joints

How to select best types of soldering fluxes electronics

The activity in solder flux is generally provided due to the presence of halides (chlorides, bromides) in flux. Higher the activity or the halide content in flux, more the performance and effectiveness of removing the oxides form the soldering surface and better the strength of soldering joints.

  • While use of flux in soldering in soldering to high activeness of Flux, undesirable corrosive by product will left behind the board as a result of higher reaction which will cause the reliability problem in the field.
  • There are mainly two point’s needs to consider in selecting a suitable flux for the electronic assembly of product, the flux should be inactive at room temperature before and after the soldering but active at soldering temperature to promote easy removal of tarnish and oxides.
  • Practically the soldering iron flux should be active slightly below the soldering temperature to ensure that the soldering surface is ready for soldering temperature.The active flux is generally not recommended due to unwanted product reliability issues as a result of high residue of fluxes on board.
  • No clean flux comes to meet these requirements but it is relatively inactive even at soldering temperature and requires a clean work environment and change in culture that affect the user and supplier both.

So the selected soldering flux should provide the balance between activity and clean ability. The low residue or no clean fluxes which are halide free and use organic acids to boot flux with solder activity.They require boards and components of good solderability and also required pertaining and the use of just in time inventory system to ensure the good solderability. 

In addition an inert environment such as Nitrogen to prevent oxidation of soldering surface during soldering process may be necessary.

How to use flux when soldering-Step by Step Guide

Working with electronics, flux in soldering how to use it is very important to make solid and shiny joints. Before starting activity, you need to prep your soldering iron at required tep and the iron tip must be cleaned properly and iron placed on a stand.

flux in soldering

When the iron’s tip hot apply small amount of solder and clean it with a sponge that is called tinning the tip with solder.Tinning helps the tip to reduce wear, Protect from oxidation and also improve heat transfer.

flux in soldering

Step 1- It is always good to scrub the two pieces you wish to solder by a steel wool or scotch brite that help to remove oxide layers on the surface.

Step 2- Apply the flux with solder to both pieces of the terminal that needs to be joined with help of brush.

Step 3-Without burning away the flux, heat the joint by solder iron with help of adjustable temp control set iron the right temperature (400′ C/ 752′ F).

Flux is dangerous so always take precautions and avoid the skin for direct contact with skin and avoid inhaling the fumes.

What type of flux is used in Electronics soldering

The soldering flux in soldering is basically classified on basis of their activity and constituents which determines its activity. to know what is flux used for and soldering flux how to use The flux activity is indication of its effectiveness of removing the surface contamination.Types of Solder Flux per below:-

Rosin Flux

Rosin Fluxes are normally extracted from the stumps or bark of pine trees thus it is a natural product. Composition of rosin varies from batch to batch but general formula is C19H19COOH.It is further classified into Non-activated Rosin Flux (R),Rosin mildly Activated (RMA) and Rosin activated (RA).

Non-activated Rosin Fluxes (R) are not activated so best suited for the uses of soldering surfaces that are already clean or having minimal amount of oxidizes are present.(R) types rosin fluxes are used for soldering of copper wire, Printed circuit boards semiconductors and other hand soldering jobs.

Rosin mildly Activated (RMA) are mild activated with greater capacity of cleaning compared to the Non-activated Rosin Fluxes (R).These flux in soldering are ideal for higher containing lead and useful for soldering of general purpose cables, PCBs and electronic components.

Rosin activated (RA) fluxes are the highest activated fluxes and provides the strongest cleaning ability among the rosin Fluxes. And suitable for hard to clean soldering surfaces and best suited for soldering for electrical components.

Low Residue or No- clean Flux and Solder Paste

In current technology using no clean or low residue flux in soldering is eliminating the need of cleaning of printed circuit board assembly.Many companies especially in Europe have considered the rosin (R and RMA) fluxes as no clean and have not generally clean them due to not creating any reliability issue when not cleaned (especially if they were halide free ).Since the ban on CFCs, No-Clean fluxes become more common worldwide in compare to the fluxes required cleaning.

The driving force of using no clean fluxes and solder paste is that they save not only cleaning cost but also the capital expenditure investment and floor space.

Organic Acid Fluxes

The Organic Acid fluxes are stronger than the rosin flux in soldering but weaker than the inorganic fluxes. Organic acid fluxes are also known as water soluble fluxes as they can dissolve into the water.The Organic acid fluxes use of flux in soldering is justified for mixed assemblies (type II and III) for both military and commercial applications.

Since they are water soluble, regular water can be used to remove the flux residues taking care that the entire assembly needs to completely dried before power is applied to it.

Inorganic Acid Fluxes

Much stronger than organic fluxes and inorganic acid fluxes are perfect choices for soldering surfaces that are difficult to bond. They can help to remove large amounts of oxides from heavily oxidized metallic parts. These are inorganic acids and salts like hydrochloride acid, hydrofluoric acid, stannous chlorides, Sodium or potassium fluoride and Zinc chlorides.

The inorganic use of What is flux in soldering used in soldering for non-electronic applications such as the brazing of copper pipes. The Major disadvantage is that they leave behind chemically active residues that can cause corrosion and hence serious field failures.what is flux used for.

Tips for using soldering with liquid flux

  • 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 very important to keep clean when working with electronics.
  • Always keep your soldering iron in a stand while it is on for safety and reliability of equipment.
  • Use all precaution wearings during soldering for safety purposes.

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