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The primary purpose of flux is to reduce/remove metal oxides from the surfaces of printed circuit boards to be soldered, and to act as a mechanism for removing other dirt/debris.

The electronics manufacturing industry utilises several flux chemistries. The type of flux used very much depends on the final application of the printed circuit board assembly. Certain applications require all residues to be cleaned off to prevent long term reliability problems caused by phenomena such as a dendritic growth whilst others allow certain residues to be left on.

Fluxes are identified by the activity level of the flux and its residue. The activity level describes the flux's ability to clean the surfaces to be soldered. The activity of the flux residue id often represented is by the letters: L=low, M=moderate and H=high.

Types of Flux Chemistries

The three common types of cleanable flux chemistries used today are Water Soluble, RMA only, and RMA/Low Solids (Hybrid).

Water Soluble - This is the most aggressive of all flux types as it contains mostly organic acids. It is primarily used to improve solder wetting especially on printed circuit boards with oxidised pads or lands. These fluxes contain polar ions that are easily removed by a polar solvent such as water. Because of their solubility in water, these fluxes are environmentally desirable, although the use of a no-clean flux may be environmentally more desirable. These water-soluble fluxes need to be cleaned using either a saponifier or deionised (DI) water.

RMA Only - Rosin Mildly Activated fluxes are less active than water-soluble fluxes. As a result fewer residues are left behind after soldering.

Rosin or colophony is a naturally occurring product that is extracted from pine trees. The composition of rosin varies, but a typical formula is C19H29COOH.

Rosins contain a small amount of unsaponifiable hydrocarbons; for rosin flux removal, saponifiers (a form of alkaline chemical used to make the water soapy) must be added. Deionised water is recommended for final rinsing.

RMA/Low Solids (Hybrid) - This flux leaves behind low activity benign residues that do not need to be washed off. However, it is active enough to solder like an RMA with the option to be cleaned. This too can be cleaned using a heated saponifier.

"No-clean" fluxes are also known as "low solid" or "low residue" fluxes. There is a fourth type of flux, very rarely used in electronics manufacturing, inorganic flux.

Inorganic Fluxes

Inorganic fluxes are highly corrosive, and are comprised of inorganic acids and salts. These fluxes are capable of removing oxides of ferrous and nonferrous metals such as stainless steel, which cannot be soldered with weaker fluxes.

Inorganic fluxes are generally used in non-electronic applications such as the brazing of copper pipes. They are, however, sometimes used for lead-tinning applications in the electronics industry.

Inorganic fluxes should not be considered for electronics assemblies (PTH or surface mount) because of potential reliability problems. They leave chemically active residues than can cause corrosion.

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Guide Written by Paul Wilson

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