Fluxing Agents: Types and Applications

Introduction: What Are Fluxing Agents?

Fluxing agents - The word flux derives from the Latin word “fluxus” which is equivalent to flow. So, what has our soldering flux got to do with flow? Electrical engineers know flux in reference to electric and magnetic flux lines and microwave engineers know there is something called power flux. Specifically, the reader who is in the field of solar energy is familiar with the word solar flux. We also use the term “influx” quite commonly, as in “a sudden influx of immigrants….” So, why is the soldering flux called so? The reason lies in history and metallurgy. Many mineral ores are mixtures of many compounds. Smelting is a process used for the extraction of some metals eg, iron from their ores. Limestone and other materials when added to the contents of a smelting furnace help the slag (A thick pasty composite containing the impurities and the fluxes themselves) become more fluid. Because of this, these materials are fluxing agents. However, it is common practice to call them simply as fluxes. Fluxing agents, or fluxes are also used in joining metals, and that is how we know it in the photovoltaic domain. Soldering and brazing are used for joining metals.  

Fluxing Agents in Soldering

In soldering,  fluxing agents, or more popularly, a flux is a chemical that helps us make good solder contacts. The function of the fluxing agents (or flux) is somewhat like that of the primer in a painting job. Persons experienced in electric circuit soldering must have noted that the common solder (tin-lead) does not attach well (rather it tends to from beads) on copper surfaces that have remained exposed to the atmosphere for some time. But once treated with fluxing agents, solder flows on these surfaces and attaches to them readily.  

What Do Fluxing Agents Do?

How does the flux (fluxing agents) perform this miracle? Actually, it does three things when we apply fluxing agents to a wire or a metal surface.

• It works chemically as a reducing agent to remove any oxidation from the surface and also helps clean the surface of other impurities or deposits. Oxidized surfaces of metals do not allow good solder contacts. An oxide-free surface of metals like copper can be easily wetted by molten solder and the solder adheres to it on cooling.
• The fluxing agents also prevent subsequent access of oxygen to the surface covered by it. And this way it prevents re-oxidation of the heated surface which could otherwise quickly get oxidized.
• In certain situations, molten fluxing agents also facilitate heating of the joint by conducting heat from the molten solder or soldering tool.

Types of Fluxing Agents

Fluxing agents are normally divided into three classes: rosin flux, acid flux (organic), and acid flux (inorganic).

• Rosin flux- Out of these three, rosin fluxing agents are definitely the oldest. Yet it is still counted among the most common fluxing agents used for soldering electrical components. It has the property that it is only active when heated and does not react with electrical circuits in the unheated state. Unwanted rosin flux can be cleaned after a soldering operation using a solution of isopropyl alcohol.
• Organic acid flux is another common type of fluxing agent used in soldering electrical circuits. It is more efficient in cleaning oxides from electrical leads and does it more quickly. It leaves a residue after soldering which is conductive. That means larger residues can cause electrical short circuits. Hence, traces of fluxing agents residue must be cleaned thoroughly after soldering. Luckily, it is water-soluble and therefore, can be cleaned with water easily.
• Inorganic fluxes- Are meant for higher temperature soldering/brazing and are normally used only on stronger metals such as stainless steel, iron, zinc. However, halogen-based fluxing agents like ammonium chloride may be used on copper for electric soft-soldering work.

Organic Fluxing Agents -Composition

Organic fluxing agents generally consist of four major ingredients. These are:

• Activators– These are chemicals that decompose or dissolve the metal oxides thus revealing an un-oxidized metal surface that is better wetted by the solder. (Chemical action of the activator is further discussed later in the article). An activator may also help to solder by an exchange reaction with the base metal.
  • Very active fluxes may contain chemicals like metal halides (normally ammonium chloride or zinc chloride), hydrochloric acid, citric acid, phosphoric acid, and hydrobromic acid. Fluxing agents for soldering and brazing aluminum often contain fluorides as activators. Mineral salts with amines are also aggressive activators. Some of these are corrosive at ordinary temperatures. Hence, fluxing agents containing such aggressive activators must be removed carefully after soldering operation. Further, they should not be used for finer electronic soldering work.
  • Milder activators include carboxylic acids, di carbolic acids, and fatty acids like oleic acid and stearic acid, and sometimes, amino acids. Milder fluxing agents also have halides or organohalides.
• Vehicles– These are non-volatile solids or liquids which can stand high temperatures without decomposing. Solids should be able to readily melt at the soldering temperature. Solid vehicles are natural or synthetic resins often based on natural or modified rosin (which could be pimaric acid, abietic acid, and possibility other resin acids). Water-soluble organic fluxes generally contain vehicles based on glycols, and higher polyglycols, surfactants based on polyglycol, and glycerol. They function to:
  • Dissolve and Carry away products of activator reaction with oxides.
  • Act as a barrier between the exposed hot metal surface and atmospheric oxygen and thus prevent re-oxidation.
  • Facilitate heat transfer.
• Solvents– They facilitate processing and deposition to the metal surfaces. They generally get dried out during preheating before the actual soldering occurs. If not completely removed they may cause boiling off and a spattering of solder particles molten solder or paste.
• Additives– These are primarily used to “tune” the fluxing agents properties. Additives may be corrosion inhibitors, surfactants (especially nonionic), stabilizers or antioxidants, tackifiers, thickeners, and other rheological (consistency and flow) modifiers for solder pastes, plasticizers for flux-cored solders, and possibly, dyes.

Inorganic Fluxing Agents

Inorganic fluxing agents also contain ingredients doing similar functions, but at higher temperatures for brazing and other similar applications at elevated temperatures, where organic fluxes are not stable. Commonly used materials are fluorides and chlorides, borates, borax, and fluoroborates. Halogenides are also used but are highly corrosive. They are used in the brazing of magnesium and aluminum alloys.

Properties of Fluxing Agents

Several properties of fluxing agents are important:

• Activity– It is the ability of the flux to dissolve the oxides on the metal surface and to facilitate wetting the surface with solder. Very active fluxes will tend to be corrosive also.
• Corrosivity – Acidic or otherwise reactive nature of the flux may cause corrosion by its residues. Very active fluxes may be corrosive even at room temperatures.  Fluxes containing halides (compounds of chlorine, fluorine, etc.) and some mineral acids are extremely corrosive.  Some of the water-soluble flux residues may be hygroscopic. Their residues on the metal may cause corrosion and a reduction in electrical insulation.  Their residues must be carefully removed after the soldering operation. That is why cleaning the contacts after soldering is standard good practice. Some fluxes, especially those based on borax form hard glass-like coatings that are hard to remove. As far as possible, less corrosive solders must be used.
• Cleanability– As cleaning away the flux and its residues is important, we look at the cleanability property. Fluxing agents which possess higher content of solids will often leave larger residues. Decomposition under the heat of some vehicles may also release products that form hard-to-clean, even charred deposits. You may have noted such a happening during hand-soldering. Some flux residues can be cleaned with water, others with organic solvents and some with both. No-clean fluxes are sufficiently volatile and leave no deposits. There are some fluxing agents which leave harmless residues and need no cleaning. As a general rule, flux residues may impair the adhesion of conformal coatings, or cause undesired insulation.
• Volatility– Volatile materials will leave little or no residues. But if too volatile, they will go away before they can contribute their function.
• Residue tack– In case residue is not removed and is tacky, it can attract and accumulate dust. This can cause problems with insulation.
• Viscosity- Solder pastes, must be easy to apply and at the same time thick enough not to spread to undesired locations when applied. Solder paste can also work as a temporary adhesive for holding electronic parts in place before and during the process of soldering. Fluxes to be applied by foam demand a low viscosity.
• Flammability– We are dealing with chemicals in the presence of electric voltage and heat. We must be cognizant of the possibility of a flash. This is especially relevant to glycol-based vehicles and also for organic solvents.
• Percentage of Solids– when the percentage of solid material in the fluxing agents is low, the chances of leaving residues are less.  Fluxing agents with solid content low, even 1-2%, are termed low solids fluxing agents, and hence, low-residue fluxing agents or no-clean fluxing agents. They will normally be composed of weak organic acids, with some amount of rosin or other resins added in.
• Conductivity- some fluxes may leave conductive residues, leading to random faults on circuits working at high impedances. Different types of fluxes cause different conductivity levels in their residues.

Activator Action

As mentioned earlier, the role of the activator is primarily to disrupt the chemical bond and to remove the oxide layer, and prevent reoxidation both on the metal surface and the molten solder. The reaction of oxide decomposition is: Metal oxide + Acid → Salt + Water Salts have an ionic in nature and can product failure through metallic leaching or dendrite growth. Leaching is the absorption of metal by the salt body. This will reduce insulation. Dendrites are very thin thread-like structures of metallic molecules that form inside the salt body and may cause a short circuit. That is why flux residues must be removed when using highly active fluxes. The products of the reaction are usually water-soluble and can be cleaned easily. Hydrochloric acid (HCl) molecules can react with copper oxide at elevated temperatures to give water-soluble and mechanically weak copper chloride. It reacts with rosin to produce salts of copper and abietic acid which itself is soluble in molten rosin. Some activators have metal ions which can have an exchange reaction with the underlying metal.  Solders containing such fluxes facilitate soldering by chemically forming a thin layer of solderable metal on the exposed base metal. Examples are the fluxes containing zinc, tin, or cadmium compounds. These compounds may be chlorides and sometimes fluorides or fluoroborates. Common highly active activators are mineral acids, along with amines, halides, water, and/or alcohols: For example:

• Hydrochloric acid which is most common
• Phosphoric acid whose use is limited because it polymerizes at elevated temperatures.

Inorganic acids attack metals even at room temperature and more so at elevated temperatures. That demands care in handling, application, and storage. As soldering causes high temperatures, we prefer to use compounds that produce acids as products. Examples are:

• zinc chloride reacts with moisture at high temperatures forming hydrochloric acid and oxychloride.
• ammonium chloride thermally decomposes to hydrochloric acid and ammonia.
• amine hydrochlorides decompose to hydrochloric acid and the relevant amine.

Rosin and Resin?

The term Rosin and Resin are quite often confused. Rosin is a solid form of natural resin obtained from the gums of pines and other coniferous plants. Resin, on the other hand, is a solid or highly viscous, generally polymerizable substance of plant or synthetic origin. Fluxes are normally termed as Rosin flux if they are based on natural resin, but the distinction is often violated.  

Rosin 

Rosin is also called colophony. The term derives from colophonia resina, meaning "resin from Colophon". (Colophon is an ancient Greek city). Rosin is produced by heating the liquid resin to vaporize and remove the volatile components.  

Rosin Properties

It is a semi-transparent solid with colors varying from yellow to black. Ingredients are organic resin acids, mostly abietic acid, but the presence of pimaric acid and isopimaric acid. Rosin is popular because of its excellent flux properties. Rosin is virtually nonreactive and noncorrosive at normal temperatures. In a molten state, it is mildly reactive to metallic oxides. Varieties of rosin soften between 60 and 70 °C and is fully fluid at about 120 °C. In the molten state rosin is mildly acidic and can dissolve thinner layers of oxides from the copper surface without any further additives. However, for removing thicker surface contamination or speedier action, additional activators can be added.  

Three Types of Rosin

There are three types. The composition and the quality of each type differ with the source tree and location. It may vary from year to year.

• Gum rosin(from pine tree).  Gum rosin has a mild odor and a weak tendency to crystallize from solutions. Hence it is preferred for flux preparations.
• Wood rosin(extracted from tree stumps), has a stronger odor, and a higher tendency to crystallize.
• Tall oil rosin. Tall oil is a byproduct of the Kraft process used to prepare the pulp of coniferous trees. Tall oil is thermally more stable and hence forms less decomposition deposits during soldering. Thus, tall oil rosin is popular in fluxes.

Rosin Fluxing Agents

Natural rosin can be used unmodified, or it can be chemically improved through polymerization, esterification, or hydrogenation. The purpose is to improve thermal stability, cleanability, solution viscosity, and harder residue (or alternately, softer residue). By forming of an ethoxylated rosin amine, rosin can be also made into a water-soluble rosin flux. Early fluxes consisted of mixture of rosin and vaseline in equal amounts. Fluxes can be prepared from synthetic resins also. These are usually based on polyol esters and fatty acids. These resins have better fume odors and a less tacky residues. However, their activity as a flux and their solubilities tend to be lower than those of natural resins.  

Rosin Fluxing Agents Categories

Rosin fluxing agents categorization is based on level of activity: L for low, M for moderate, and H for high. Here are some type letters with their meaning

• R (Rosin) - Pure rosin, with no activators, mildest and with low activity.
• WW (Water-White) Purest grade, no activators, hence, low activity, (sometimes treated synonymous with R type)
• RMA (Rosin Mildly Activated) - containing mild activators, but no halides.
• RA (Rosin Activated) - With strong activators, contains halides and has high activity.
• OA (Organic Acid) - Activated rosin with organic acids, possesses high activity, is highly corrosive, needs cleaning with water.
• SA (Synthetically Activated) - Rosin added strong synthetic activators, possesses high activity; readily soluble in organic solvents like alcohols and chlorofluorocarbons for cleaning.
• WS (Water-Soluble) - Normally based on halides (organic or inorganic); leaves highly corrosive residues.
• SRA (Super-activated rosin) - Having very strong activators hence, possessing very high activity.
• IA (Inorganic Acid) – this is rosin activated with inorganic acids like phosphoric acid or hydrochloric acid. Possesses the highest activity level and is highly corrosive.

Grades R, WW, and RMA are used, the joint cannot be cleaned easily or where corrosion risk is too high. More active grades of rosin flux will require thorough cleaning of the residues.  

Important Note

Improper cleaning may actually aggravate the corrosion by releasing any trapped activators from the fluxing agents residues.  

Activator Types for Rosin Fluxing Agents

There are several groups of chemicals which are used as activator for rosin fluxes: Halide activators are organic halides, e.g. diethylammonium chloride and dimethylammonium chloride. Organic acids like acetic acid, formic acid, propionic acid, sebacic acid oxalic acid, malonic acid, etc.  

Solder Fluxing Agents Specifications

Fluxes are specified according to several standards.

• DIN EN 29454-1 / ISO 9454-1 is the most common in Europe. A four-character code specifies fluxing agents type, base, activator, and type (physical form, eg solid or liquid) in the same order. The first three characters are numerals 1 to 3; the second and third are also numerals again 1 to 3, and the fourth character is a letter, A, B, or C.  For example, 3.2.1C (The form code is often omitted) indicates inorganic flux, acid, with phosphoric acid activator, in paste form. For rosin fluxing agents with halides, it will be 1.1.2.
• J-STD-004(United States)-Two letters indicate the base followed by one letter for Activity, and a number to indicate whether halides are included in the activators.

The base composition codes are:

• IN: inorganic 
• OR: organic 
• RE: resin 
• RO: rosin

Activity codes are:

• H: high 
• L: low 
• M: moderate

Halide Content Code Number:

• 0 for halide free (less than 0.005% by weight)
• 1 for halide content depending on activity:
  • Low activity-less than 0.5%
  • Moderate activity-0.5 % to 2 %
  • High activity- greater than 2.0

Code ROL0 would mean rosin (RO) low activity (third letter L) halide free (4^th character 0).  

Special Fluxing Agents

Special fluxes are needed for certain materials which are difficult to solder.

• Aluminum metal and its alloys are an example. They develop a passivating layer of aluminum oxide which are difficult to disrupt by ordinary fluxes. Salts or complexes of salts must be chosen which have the ability to penetrate into cracks in the oxide. A redox action undoes the oxide and a metallic layer formed which can be wetted with the solder.
• Magnesium alloys. Molten acetamide has shown good results as a flux for tin-indium solder on magnesium at low temperatures. Acetamide is able to dissolve surface oxides magnesium as well as aluminum.
• Stainless steel is difficult to solder due to its low thermal conductivity and stable, self-healing oxide layer on its surface. Zinc chloride in dissolved hydrochloric acid is commonly used as a flux for stainless steels. However, any flux remnants must be removed carefully after soldering else it will cause corrosion. Phosphoric acid is another highly effective flux for steel. However, application is limited due to its tendency to polymerize at elevated temperatures.

A comprehensive list of various fluxing agents in alphabetic order is available in Wikipedia.