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What is a function of flux in soldering?

A flux (derived from Latin fluxus meaning “flow”) is a chemical cleaning agent, flowing agent or purifying agent. The selection of soldering Flux and cleaning process plays a critical role in the manufacturing yield and product reliability of electronic assemblies.

The function of flux 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 primarily on type of flux used.

The function of Soldering Flux

The all metals except pure gold and platinum, oxidise in air at room temperature. The rate of oxidation can increase with humidity and temperature. Different metals have different affinities for oxygen, hence they differ in their susceptibility to oxidation which is form of corrosion.

Since the metal oxides surfaces are barrier to the formation of such a bond, they must be removed by the use 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 intermetallic bonding can be taken place. The main function of the solder flux’s 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 surface prevent metal from being re oxidized at higher temperatures.
  • Allow molten solder paste to form a solid long lasting electrical and mechanical joints.

Consideration in Selection of Soldering Flux 

The activity in solder flux in generally provides 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.

But due 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 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 issue as a result of high residue of fluxes on board. No clean flux comes to meeting this requirements but it is relatively inactive even at soldering temperature and require clean work environment and change in culture that effect the user and supplier both.

So the selected flux should be 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 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.

The Classification of Soldering Fluxes

The soldering fluxes are basically classified on basis of their activity and constituents which determines its activity. The flux activity is indication of its effectiveness of removing the surface contamination. The fluxes are generally classified as per below:-

Rosin Flux

Rosin Fluxes are normally extracted from the stumps or bark of pine tree 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 compere to the Non-activated Rosin Fluxes (R).These fluxes are ideal for higher containing lead and use full for soldering of general purpose cables, PCB’s and electronic components.

Rosin activated (RA) fluxes are 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 fluxes 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 fluxes but weaker than the inorganic fluxes. Organic acid fluxes are also known as water soluble fluxes as they can dissolve into the water.

The use of Organic acid fluxes 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 need to completely dried before power is applied to it. 

Inorganic Acid Fluxes

Much stronger than organic fluxes and inorganic acid fluxes are perfect choice for soldering surface that are difficult to bond. The can help to remove large amount of oxides from heavily oxidised metallic parts.

These are comprised of inorganic acids and salts like hydrochloric acid, hydrofluoric acid, stannous chlorides, Sodium or potassium fluoride and Zinc chlorides.

The inorganic fluxes are generally used for non-electronic applications such as the brazing of copper pipes. The Major disadvantage is that they leave behind chemically active resides that can cause corrosion and hence serious field failures.

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