Role of flux

1、 Characteristics of flux:

Flux is an indispensable auxiliary material in SMT welding process. In wave soldering, flux and solder are used separately, while in reflow soldering soldering, flux is an important part of solder paste. In addition to the quality of welding process, components and PCB, the selection of flux is very important. The flux with good performance shall have the following functions:

(1) Remove oxides from the welding surface to prevent re oxidation of the solder and welding surface during welding, and reduce the surface tension of the solder.

(2) The melting point is lower than that of the solder. Before the solder melts, the flux must melt first to give full play to its role.

(3) The diffusion rate of infiltration is faster than that of molten solder, and usually requires expansion to around 90% or more.

(4) The viscosity and specific gravity are smaller than those of solder, which makes it difficult to infiltrate and diffuse. If the specific gravity is large, it cannot cover the surface of solder.

(5) During welding, there is no spatter of welding beads, no toxic gas, and no strong irritating odor.

(6) The residue after welding is easy to remove and has characteristics such as non corrosion, non hygroscopicity, and non conductivity.

(7) Non stick, do not touch hands after welding, and solder joints are not easy to tip.

(8) Stable storage at room temperature.

2、 Chemical composition of flux:

The traditional flux is usually based on rosin. Rosin has weak acidity and hot melt flowability, as well as good insulation, moisture resistance, corrosion-free, non-toxic, and long-term stability, making it a rare welding material. At present, active flux based on rosin is mostly used in SMT. Because the chemical composition and performance of rosin vary greatly with the variety, place of origin and production process, the optimization of rosin is the key to ensure the quality of flux. The general flux also includes the following components: active agent, film-forming material, additive, solvent, etc.

A. Active agent:

Active flux is an active substance added to the flux to improve its soldering ability. The activity of an activator refers to its ability to react chemically with the surface oxides of the solder and the soldered material, in order to clean the metal surface and promote wetting. Active agents are divided into inorganic active agents, such as zinc chloride, ammonium chloride, etc; Organic active agents, such as organic acids and organic halides. Inorganic activators usually have good soldering properties, but they have a long action time and high corrosiveness, making them unsuitable for use in electronic assemblies; Organic surfactants have a gentle effect, short duration, low corrosiveness, and good electrical insulation, making them suitable for use in electronic assemblies. The active agent content is about 2% -10%. If it is a chloride containing compound, its chlorine content should be controlled below 0.2%.

B. Film forming substance:

Adding film-forming substances can form a tight organic film after welding, protecting the solder joints and substrate, with anti-corrosion and excellent electrical insulation. Commonly used film-forming substances include rosin, phenolic resin, acrylic resin, vinyl chloride resin, polyurethane, etc. The general addition amount is 10% -20%, and excessive addition will affect the expansion rate and reduce the soldering effect. In ordinary household appliances or electrical assemblies with low requirements, film-forming substances are used, and the assembled electrical components are not cleaned to reduce costs. However, in precision electronic assemblies, cleaning is still necessary after welding.

C. Additives:

Additives are substances with special physical and chemical properties added to adapt to the process and environment. Commonly used additives include:

(1) Regulator: material added to adjust the acidity of flux, such as triethanolamine, which can adjust the acidity of flux; Adding hydrochloric acid to inorganic flux can inhibit the formation of zinc oxide.

(2) Matting agent: can make solder joints dull and overcome eye fatigue and visual impairment during operation and inspection. Generally, inorganic halides, inorganic salts, organic acids and their metal salts are added, such as zinc chloride, tin chloride, talc, copper stearate, calcium, etc.

(3) Corrosion inhibitor: The addition of corrosion inhibitor can protect printed boards and device free leads, with moisture, mold, and corrosion resistance, while maintaining excellent solderability. Most of the substances used as corrosion inhibitors are organic compounds with nitrogen compounds as the main body.

(4) Brightener: can make solder joints glow, and can be added with glycerol, triethanolamine, etc., with a general addition amount of about 1%.

(5) Flame retardant: A material added to ensure safe use and improve flame resistance.

D. Solvent:

Most practical flux is liquid. Therefore, the solid component of flux must be dissolved in a certain solvent to make it a homogeneous solution. Most solvents are isopropanol and ethanol. As a flux, various solid components have good solubility.

(1) It has good solubility for various solid components in flux.

(2) The degree of volatilization is moderate at room temperature and rapidly evaporates at welding temperature.

(3) Low odor and toxicity.

3、 Classification of welding flux:

(1) According to the state, there are three types: liquid, paste, and solid.

(2) There are three categories according to their purpose: brushing, spraying, and impregnation.

(3) According to the activity of flux, it can be divided into inactive and low active.