The main active ingredient in the flux is rosin, which will be decomposed by tin at about 260 degrees Celsius, so the temperature of the tin bath should not be too high.
Flux is a chemical substance that promotes welding. In solder, it is an indispensable auxiliary material and its role is extremely important.
Dissolve the solder parent oxide film
In the atmosphere, the surface of the soldered parent material is always covered with an oxide film, and its thickness is about 2×10-9～2×10-8m. During welding, the oxide film will inevitably prevent the solder from wetting the parent material, and the welding cannot proceed normally. Therefore, the flux must be applied to the surface of the parent material to reduce the oxide on the surface of the parent material, so as to achieve the purpose of eliminating the oxide film.
Reoxidation of the soldered parent material
The parent material needs to be heated during the welding process. At high temperatures, the metal surface will accelerate oxidation, so the liquid flux covers the surface of the parent material and the solder to prevent them from oxidation.
Tension of molten solder
The surface of molten solder has a certain tension, just like rain falling on a lotus leaf, which will immediately condense into round droplets due to the surface tension of the liquid. The surface tension of molten solder will prevent it from flowing to the surface of the base material, affecting the normal wetting. When the flux covers the surface of the molten solder, it can reduce the surface tension of the liquid solder and significantly improve the wetting performance.
Protect the welding base material
The original surface protection layer of the material to be welded has been destroyed during the welding process. Good flux can quickly restore the role of protecting the welding material after welding. It can speed up the transfer of heat from the soldering iron tip to the solder and the surface of the object to be welded; suitable flux can also make the solder joints beautiful
Possessing performance
⑴ The flux should have an appropriate active temperature range. It starts to work before the solder melts, and plays a better role in removing the oxide film and reducing the surface tension of the liquid solder during the soldering process. The melting point of the flux should be lower than the melting point of the solder, but it should not be too different.
⑵ The flux should have good thermal stability, and the general thermal stability temperature should not be less than 100℃.
⑶ The density of the flux should be less than the density of the liquid solder, so that the flux can be evenly spread on the surface of the metal to be welded, covering the solder and the surface of the metal to be welded in a thin film, effectively isolating the air and promoting the wetting of the solder to the parent material.
⑷ The residue of the flux should not be corrosive and easy to clean; it should not precipitate toxic and harmful gases; it should have water-soluble resistance and insulation resistance that meet the requirements of the electronics industry; it should not absorb moisture and produce mold; it should have stable chemical properties and be easy to store. [2]
Types
Flux can be classified into hand dip soldering flux, wave soldering flux and stainless steel flux according to its function. The first two are familiar to most users. Here we explain stainless steel flux, which is a chemical agent specially designed for stainless steel welding. General welding can only complete the welding of copper or tin surfaces, but stainless steel flux can complete the welding of copper, iron, galvanized sheet, nickel plating, various types of stainless steel, etc.
There are many types of flux, which can be roughly divided into three series: organic, inorganic and resin.
Resin flux is usually extracted from the secretions of trees. It is a natural product and has no corrosiveness. Rosin is the representative of this type of flux, so it is also called rosin flux.
Since flux is usually used in combination with solder, it can be divided into soft flux and hard flux corresponding to solder.
Soft fluxes such as rosin, rosin mixed flux, solder paste and hydrochloric acid are commonly used in the assembly and maintenance of electronic products. In different occasions, they should be selected according to different welding workpieces.
There are many types of flux, which can generally be divided into inorganic series, organic series and resin series. Inorganic series flux
The inorganic series flux has strong chemical action and very good flux performance, but it has great corrosive effect and belongs to acidic flux. Because it dissolves in water, it is also called water-soluble flux, which includes two types: inorganic acid and inorganic salt.
The main components of flux containing inorganic acid are hydrochloric acid, hydrofluoric acid, etc., and the main components of flux containing inorganic salt are zinc chloride, ammonium chloride, etc. They must be cleaned very strictly immediately after use, because any halide remaining on the welded parts will cause serious corrosion. This type of flux is usually only used for welding non-electronic products. It is strictly forbidden to use this type of inorganic series flux in the assembly of electronic equipment.
Organic
The fluxing effect of organic series flux is between that of inorganic series flux and resin series flux. It also belongs to acidic and water-soluble flux. Water-soluble flux containing organic acid is based on lactic acid and citric acid. Since its soldering residue can remain on the soldered object for a period of time without serious corrosion, it can be used in the assembly of electronic equipment, but it is generally not used in SMT solder paste because it does not have the viscosity of rosin flux (which prevents the movement of patch components).
Resin series
The resin type flux is used in the largest proportion in the welding of electronic products. Since it can only be dissolved in organic solvents, it is also called organic solvent flux, and its main component is rosin. Rosin is inactive in solid state and only active in liquid state. Its melting point is 127℃ and its activity can last up to 315℃. The optimal temperature for soldering is 240-250℃, so it is within the active temperature range of rosin, and its soldering residue does not have corrosion problems. These characteristics make rosin a non-corrosive flux and widely used in the welding of electronic equipment.
For different application needs, rosin flux has three forms: liquid, paste and solid. Solid flux is suitable for soldering iron, while liquid and paste flux are suitable for wave soldering.
In actual use, it is found that when rosin is a monomer, its chemical activity is weak and it is often not sufficient to promote the wetting of solder. Therefore, a small amount of activator needs to be added to improve its activity. Rosin series fluxes are divided into four types: inactivated rosin, weakly activated rosin, activated rosin and super-activated rosin according to the presence or absence of activators and the strength of chemical activity. They are called R, RMA, RA and RSA in the US MIL standard, and the Japanese JIS standard is divided into three grades according to the chlorine content of the flux: AA (less than 0.1wt%), A (0.1~0.5wt%) and B (0.5~1.0wt%).
① Inactivated rosin (R): It is composed of pure rosin dissolved in a suitable solvent (such as isopropyl alcohol, ethanol, etc.). There is no activator in it, and the ability to eliminate the oxide film is limited, so the welded parts are required to have very good solderability. It is usually used in some circuits where corrosion risk is absolutely not allowed during use, such as implanted cardiac pacemakers.
② Weakly activated rosin (RMA): The activators added to this type of flux include organic acids such as lactic acid, citric acid, stearic acid, and basic organic compounds. After adding these weak activators, wetting can be promoted, but the residue on the parent material is still not corrosive. In addition to high-reliability aviation and aerospace products or fine-pitch surface-mounted products that need to be cleaned, general civilian consumer products (such as recorders, TVs, etc.) do not need to set up a cleaning process. When using weakly activated rosin, there are also strict requirements for the solderability of the welded parts.
③ Activated rosin (RA) and super-activated rosin (RSA): In the activated rosin flux, the strong activators added include basic organic compounds such as aniline hydrochloride and hydrazine hydrochloride. The activity of this flux is significantly improved, but the corrosion of chloride ions in the residue after welding becomes a problem that cannot be ignored. Therefore, it is generally rarely used in the assembly of electronic products. With the improvement of activators, activators that can decompose residues into non-corrosive substances at welding temperatures have been developed, most of which are derivatives of organic compounds.