How to choose the right soldering flux

A General Overview
Flux is a key contributor to most soldering applications. It is a compound that is used to lift tarnish films from a metals surface, keep the surface clean during the soldering process, and aid in the wetting and spreading action of the solder. There are many different types and brands of flux available on the market; check with the manufacturer or reseller of your flux to ensure that it is appropriate for your application, taking into consideration both the solder being used and the two metals involved in the process. Although there are many types of flux available, each will include two basic parts, chemicals and solvents.
The chemical part includes the active portion, while the solvent is the carrying agent. The flux does not become a part of the soldered joint, but retains the captured oxides and lies inert on the joints finished surface until properly removed. It is usually the solvent that determines the cleaning method required to remove the remaining residue after the soldering is completed. It should be noted that while flux is used to remove the tarnish film from a metals surface, it will not (and should not be expected to) remove paint, grease, varnish, dirt or other types of inert matter. A thorough cleaning of the metals surface is necessary to remove these types of contaminates. This will greatly improve the fluxing efficiency and also aid in the soldering methods and techniques being used.
Detailed Examination
All common untreated metals and metal alloys (including solders) are subject to an environmental attack in which their bare surfaces become covered with a non-metallic film, commonly referred to as tarnish. This tarnish layer consists of oxides, sulfides, carbonates, or other corrosion products and is an effective insulating barrier that will prevent any direct contact with the clean metal surface which lies beneath. When metal parts are joined together by soldering, a metallic continuity is established as a result of the interface between the solder and the surfaces of the two metals. As long as the tarnish layer remains, the solder and metal interface cannot take place, because without being able to make direct contact it is impossible to effectively wet the metals surface with solder.

The surface tarnishes that form on metal are generally not soluble in (and cannot be removed by) most conventional cleaning solvents. They must, therefore be reacted upon chemically in order to be removed. This required chemical reaction is most often accomplished by the use of soldering fluxes. These soldering fluxes will displace the atmospheric gas layer on the metals surface and upon heating will chemically react to remove the tarnish layer from the fluxed metals and maintain the clean metal surface throughout the soldering process.

The chemical reaction that is required will usually be one of two basic types. It can be a reaction where the tarnish and flux combine forming a third compound that is soluble in either the flux or its carrier. An example of this type of reaction takes place between water-white rosin and copper oxides. Water-white rosin, when used as a flux is usually in an isopropyl alcohol carrier and consists mainly of abietic acid and other isomeric diterpene acids that are soluble in several organic solvents. When applied to an oxidized copper surface and heated, the copper oxides will combine with the abietic acid forming a copper abiet (which mixes easily with the unreacted rosin) leaving a clean metallic surface for solder wetting. The hot molten solder displaces the rosin flux and the copper abiet, which can then be removed by conventional cleaning methods.

Another type of reaction is one that causes the tarnish film, or oxidized layer to return to its original metallic state restoring the metals clean surface. An example of this type of reaction takes place when soldering under a blanket of heated hydrogen. At elevated temperatures (the temperature that is required for the intended reaction to take place is unique to each type of base metal) the hydrogen removes the oxides from the surface, forming water and restoring the metallic surface, which the solder will then wet. There are several other variations and combinations that are based on these two types of reactions.

Once the desired chemical reaction has taken place (lifting or dissolving the tarnish layer) the fluxing agent must provide a protective coating on the cleaned metal surface until it is displaced by the molten solder. This is due to the elevated temperatures required for soldering causing the increased likelihood that the metal’s surface may rapidly re-oxidize if not properly coated. Any compound that can be used to create one of the required types of chemical reactions, under the operating conditions necessary for soldering, might be considered for use as a fluxing material. However most organic and inorganic compounds will not hold up under the high temperature conditions that are required for proper soldering. That is why one of the more important considerations is a compounds thermal stability, or its ability to withstand the high temperatures that are required for soldering without burning, breaking down, or evaporating.

When evaluating all of the requirements necessary for a compound to be considered as a fluxing agent, it is important to consider the various soldering methods, techniques and processes available and the wide range of materials and temperatures they may require. A certain flux may perform well on a specific surface using one method of soldering and yet not be at all suitable for that same surface using a different soldering method. When in doubt it never hurts to check with the flux, or solder manufacturer for recommendations.

2 Responses to How to choose the right soldering flux

  • solderingguru says:

    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:

    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.

  • solderingguru says:

    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.

    Safety – When the operating parameters concerning safety are being considered, it is extremely important to remember both personal safety and overall plant safety. Be sure that all individuals involved with the use, or handling of fluxes have been properly educated and instructed to follow all safe handling and operating guidelines as determined by the appropriate governing agencies. It is very important to obtain, read and fully understand; any directions that have been supplied by the manufacturer; the MSDS covering the materials that are in use, and any other literature or documentation that may be available.

    Personal – Be sure that all of the necessary safety equipment and materials are provided for and they are being properly used. All work should be done in a safe and properly ventilated area. All operating personnel should be properly trained in whatever method of soldering is being performed. They should also understand and be familiar with the proper handling of all of the materials that are being used during the soldering application and the possible hazards that may be encountered.

    Plant – In order to minimize many of the potential risks that may be encountered the flux material you intend to use should be inert to the various materials it might normally be expected to come into contact with during use and should have a high flash point with a sufficient margin for safety. It should also have a slow decomposition rate to prevent the possibility of explosions in the event of overheating.

    Economics – When evaluating your total operating costs, you should be considering not only the initial price of the flux, but also the time required for soldering, the cleaning materials that are needed, and any other expenses, that may often outweigh the flux materials cost. Consider also that the more expensive flux may in fact save you more money over time with increased reliability and fewer instances of rework.

    Time & Temperature – These two factores often go hand in hand and the requirements of each should be determined and evaluated both separately and together. There may be specific instances where a higher level of heat may be utilized to decrease the amount of time needed to perform the desired soldering application. The faster the intended soldering process takes place, the less time there will be for any excessive heat to travel into unwanted areas. This will help to eliminate the possibility of thermal damage to any temperature sensitive components.

    Time – The shortest soldering times possible, without jeopardizing safety, quality or efficiency, are preferred in all applications. This is especially true where there is the possibility of exposing heat sensitive components to longer soldering times, which can be detrimental. The flux that is selected for the application should be one that will rapidly affect the solder system, allowing for the quickest and most efficient method of soldering to take place.

    Temperature – It is important to always match the thermal characteristics of the flux to the overall temperature range required for the entire soldering process. The temperature range required is determined by the solder alloy that is being used and also by the method and equipment that are being used, to perform the soldering application. If there are unique considerations that cause a specific flux to be required for the soldering process it may be possible to use the flux outside of its recommended range, provided the flux reaches a temperature high enough to activate its cleaning and deoxidizing function, but not high enough to cause it to breakdown, or deteriorate.

    Corrosion – As discussed earlier, every material used as a fluxing agent, is corrosive to some degree. This corrosion must be controlled or the solder joint may be weakened and eventually fail. In electrical connections, the problem may be magnified because of changes that affect the electrical characteristics, like increased resistance caused by a decrease in the diameter of a conductor. Therefore if the corrosiveness of a flux cannot be controlled or avoided, it becomes necessary to ensure that its products, or byproducts can be easily and adequately removed after the soldering process is completed.