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Flux: Various Types & How to Choose

Attempting to divide flux materials into corrosive and non-corrosive categories is a misleading and inaccurate method of classification. Every material used as a fluxing agent is corrosive to some degree. It is this corrosiveness that chemically cleans a metal's tarnished surface creating an environment where solder can flow and bond. A more accurate method of classification is to first classify the available fluxes as rosin based or water-soluble and then either organic or inorganic and then determine the various sub-groups or categories of each of these classifications.

I - Rosin Based fluxes (Organic)

These fluxes are made from rosin, (the purified product is known as water-white rosin) which is extracted from pinesap. A wide variety of compounds may be added in order to increase the flux's cleaning and deoxidizing abilities. Therefore this classification can be subdivided into three separate groups, as follows:

  1. R (rosin only) - This type of flux is the least active and is generally recommended for use on surfaces that are all ready very clean. It is intended for this type of flux to leave virtually no residue behind.
  2. RMA (rosin mildly activated) - This type of flux contains activators that have been added in order to enhance its cleaning and deoxidizing abilities. It will leave a minimal amount of inert residue behind. That residue should be non-corrosive, tack free and be substantially free from ionic contamination after cleaning.
  3. RA (rosin activated) - This type of flux also contains activators that have been added and is the most aggressive of the rosin-based fluxes. Although it leaves the most residues behind, these residues can be easily removed by using the appropriate type of flux cleaners.

These flux groups are the only ones specified for mil spec work (ANSI/IPC-SF-818 Class 3 or Mil-F-14256E) in electronic applications.

II - Water Soluble Fluxes

(The flux itself is not water soluble, but the residue that remains after soldering usually is.)

Organic Materials:

These Non-Rosin based organic fluxes are more active than Rosin Activated fluxes and can be divided into three groups.

  1. Organic Acids - These, being organic materials, are temperature-sensitive. They are slow acting, with only a marginal ability to remove tarnishes. They remain corrosive after use and any condensed fumes must be removed. Not all are water-soluble and generally organic solvents are used for clean up. Included in this group of acids are: oleic, stearic, citric, lactic, and others.
  2. Organic Halogens - These, being organic materials, are temperature-sensitive. They are fast-acting with good tarnish removing abilities and are used because of the easily available halogen ion. They are more corrosive by comparison than other organic fluxes. Their condensed fumes must be removed. Cleaning should take place immediately after soldering is completed.
  3. Amines & Amides - These, being organic materials, are also very temperature-sensitive. Amines are organic derivatives of ammonia, while combining a carboxylic acid and a nitrogen compound (like ammonia) forms amides. This is a group of additives that are used because they do not contain halogens. Derivatives of amines and amides (like aniline phosphate) are also used as fluxing materials.

Inorganic materials:

This type of flux is the most active, aggressive fluxing material of all and can be divided into three groups.

  1. Inorganic Acids - These acids, not often used by them selves, are a vital part of inorganic solder-flux combinations. They are fast cleaning materials and will remove all common types of oxidation. They are stable and active at soldering temperatures and are very corrosive before, during and after the soldering process is complete. Their condensed fumes must be removed (generally by using aqueous solutions) or be neutralized. Cleaning should take place immediately after soldering is completed.
  2. Inorganic Salts - These salts are less dangerous than acids in fluxes. They are fast cleaning materials that become very active when molten and are stable at soldering temperatures. They are not as corrosive when they are in salt form except in humid atmospheres. Their condensed fumes must be removed (most are water-soluble) or neutralized. It could be necessary to soak them in a slightly acidic solution to form soluble complex salts and then continue with normal aqueous rinsing procedures.
  3. Inorganic gases - These gases become chemically active at elevated temperatures. Clean surfaces, free of foreign materials, are required for this type of flux to perform adequately. In addition, special equipment will be required, because of the hazardous nature of this group. This group includes materials like dry hydrogen and hydrogen chloride.

Factors to consider in choosing your Fluxing Agent

Understanding the various types of fluxes available is important. However there are some very specific operating parameters that are also required of flux materials in order to maintain soldering as an economical method of joining metal surfaces. As you evaluate these operating parameters, please remember that they are "intentionally" not being listed in any specific order of priority, because their level of importance may change dramatically from one application to the next.

We hope you found this information helpful and educational. If you require more detailed information please feel free to contact us.

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