Types and uses of fluxes

       The types of fluxes are broadly divided into three categories: inorganic, organic and resin.

The fluxing effect of organic flux is between inorganic flux and resin flux, and it mainly contains lactic acid and citric acid-based organic acid components for fluxing. Because the welding residue can remain on the welded object for a period of time without serious corrosion, it can be used for the assembly of electronic equipment.

In the welding of electronic products, the largest proportion of resin fluxes are used, and the main ingredient is rosin. Rosin is inactive when it is solid, but only active when it is in liquid state, insoluble in water, only soluble in organic solvents. Rosin has a melting point of 127°C, and its post-melting activity lasts up to 315°C, covering the optimal temperature for soldering, and its soldering residue is free of corrosion problems, which makes rosin flux widely used in electronic device soldering.

Rosin fluxes are available in liquid, paste, and solid forms, and are used in wave soldering, reflow soldering, and manual soldering processes, respectively.

With the continuous development of science and technology, the research and development of flux is also continuing. Judging from the various fluxes mentioned earlier, while they all play an important role in their respective application scenarios, there are still some limitations. For example, although inorganic fluxes have good flux performance, their high corrosion limits their application range; Although organic flux can be used for the assembly of electronic equipment, the flux effect is at an intermediate level.

Whether liquid, paste, or solid, it's not all about rosin monomer as flux alone. Because rosin is a monomer, the chemical activity is weak, and the wetting of the promoting solder is often insufficient, so it is necessary to add a small amount of active agent and dissolve it in an organic solvent to make a flux to improve the activity. In rosin flux, the one containing only rosin and the solvent that dissolves rosin is inactive rosin flux (R); In addition to rosin and rosin-dissolving solvents, there are four grades of actives with different levels of activity: weakly activated rosin flux (RMA), activated rosin flux (RA) and super-activated rosinflux (RSA).

(1) Inactivated rosin flux (R): It is composed of pure rosin dissolved in ethanol, isopropanol and other solvents, without active agents, and has limited ability to eliminate oxide films, so the weldment is required to have very good weldability, and is usually used in some circuits that are absolutely not allowed to have the risk of corrosion in use, such as pacemakers implanted in the heart.

(2) Weakly activated rosin flux (RMA): The active agent added to this type of flux is lactic acid, citric acid, stearic acid and other organic acid compounds. The addition of these weak activators can promote wetting, and is less corrosive, so it is used in most electronic assembly fields, but it still needs to be professionally cleaned after application in aeronautics, aerospace products or fine-pitch surface mount products.

(3) Activated rosin flux (RA) and super-activated rosin flux (RSA): The strong active agents added to the activated rosin flux include aniline hydrochloride, hydrazine hydrochloride and other organic compounds, the activity of this flux is significantly improved, but the corrosion of chloride ions in the residue after soldering has become a problem that cannot be ignored, so it is rarely used in the assembly of electronic products.

There is also a no-clean flux, which is also a type of rosin flux. By analyzing its composition, it is possible to further understand the components of the flux and the role of these components. However, if you are not an expert in chemistry, ignore the various acids and fats mentioned below and focus on the ingredients and effects.

No-clean flux: the main raw materials are organic solvents, rosin resin and its derivatives or synthetic resin surfactants, organic acid activators, anticorrosive agents, co-solvents, and film-forming agents. To put it simply, all kinds of solids are dissolved in various liquids, and the proportion of each component is different, and the fluxing effect of the flux is also different.

Although resin fluxes are widely used, different grades of fluxes also face different problems, such as activated rosin flux (RA) and super-activated rosin flux (RSA) are rarely used in electronic product assembly because of chloride corrosion in post-soldering residues. Although no-clean flux avoids the cleaning process, the selection and proportion of each component are also quite complex, and a variety of factors need to be considered.

(1) Organic solvents: one or several mixtures of ketones, alcohols and esters, commonly used are ethanol, propanol and butanol; acetone, toluene, isobutyl ketone; Ethyl acetate, butyl acetate, etc. As a liquid component, its main function is to dissolve the solid components in the flux to form a uniform solution, which is convenient for the components to be soldered to evenly coat the appropriate amount of flux components, and at the same time, it can also clean less serious dirt and oil stains on the metal surface.

(2) Natural resin and its derivatives or synthetic resin surfactants: halogen-containing surfactants have strong activity and high fluxing ability, but halogen ions are difficult to clean, the ion residue is high, and halogen elements (mainly chlorides) are strongly corrosive, so they are not suitable for use as raw materials for no-cleanfluxes, and halogen-free surfactants have slightly weak activity but less ion residues.

The surfactant of no-clean flux is mainly a non-ionic surfactant of fatty acid group or aromatic group, and its main function is to reduce the surface tension generated when the solder is in contact with the metal of the lead pin, enhance the surface wetting force, enhance the penetration of (3) organic acid activator, and can also play the role of foaming agent.

(3) Organic acid activator: composed of one or more of organic acid dibasic acids or aromatic acids, such as succinic acid, glutaric acid, itaconic acid, o-hydroxybenzoic acid, azerainic acid, heptanedic acid, malic acid, succinic acid, etc. Its main function is to remove oxides on the lead pins and oxides on the surface of molten solder, and is one of the key components of flux.

(4) Anticorrosive agent: reduce the residue of solid components such as resin and activator after high temperature decomposition.

(5) Co-solvent: Prevent the tendency of solid components such as activators from dissolving, and avoid the poor non-uniform distribution of activators.

(6) Film former: In the process of soldering the lead pin, the coated flux precipitates and crystallizes to form a uniform film, and the residue after high temperature decomposition can be quickly cured, hardened and reduced viscosity due to the presence of film former.

In future research, scientists may be able to overcome these limitations by looking for new substances to replace some of the components in existing fluxes. For example, in order to solve the problem of the corrosivity of inorganic fluxes, some substances with similar chemical effects but very little corrosiveness can be explored. For organic fluxes, try to improve their flux performance while keeping them less corrosive. In the case of resin-based fluxes, especially activated rosin flux grades, it is possible to study how to improve activity while avoiding the residue of corrosive substances such as chloride ions. For no-clean fluxes, how to optimize the ratio of each component to achieve the best flux effect and ensure the minimum side effects is also an important research direction.

In conclusion, there is still a lot of room for flux development, and through an in-depth understanding of its composition and function, it is expected that better performance and more widely used flux products can be developed.