Function of flux

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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, 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. Fluxes with good properties should have the following functions:

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

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

(3) The infiltration diffusion rate is faster than that of melted solder, which usually requires expansion of about 90% or more.

(4) Viscosity and specific gravity are smaller than solder. Viscosity makes it difficult to infiltrate and diffuse.

(5) There is no spatter of beads, no poison gas and strong irritant odor during welding.

(6) The residue after welding is easy to remove, and has the characteristics of non-corrosion, non-moisture absorption and non-conductivity.

(7) Non-sticking, not touching hands after welding, solder joint is not easy to sharpen.

(8) Storage is stable at room temperature.

Chemical composition of flux:

Traditional flux is usually based on rosin. Rosin has weak acidity and hot melt fluidity, and has good insulation, moisture resistance, non-corrosive, non-toxic and long-term stability. It is a rare welding aid material. At present, most of the active flux used in SMT is based on rosin. Because the chemical composition and properties of rosin vary greatly with the variety, 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: activator, film forming material, additives and solvents.

A. Activators:

Activator is a kind of active substance added in flux to improve the soldering ability. The activity of the activator means that it reacts chemically with the oxide on the surface of solder and soldered material in order to clean the metal surface and promote wettability. Activators can be divided into inorganic active agents, such as zinc chloride, ammonium chloride, and organic active agents, such as organic acids and organic halides. Usually inorganic activator has good soldering aid, but it has long action time and high corrosiveness, so it is not suitable for electronic assembly. Organic activator has soft action, short time, low corrosivity and good electrical insulation, so it is suitable for electronic assembly. The content of active agent is about 2%-10%. If it is a chlorinated compound, its chlorine content should be controlled below 0.2%.

B. Film-forming substances:

Adding film-forming material can form a tight organic film after welding, which protects solder joints and substrates, and has corrosion resistance and excellent electrical insulation. The commonly used film-forming materials are rosin, phenolic resin, acrylic resin, vinyl chloride resin, polyurethane, etc. Generally, the addition amount is between 10% and 20%. Excessive addition will affect the expansion rate and reduce the effect of welding aids. In ordinary household appliances or electrical appliances with low requirements, film-forming materials are used, and the electrical components after assembly are not washed to reduce costs. However, after welding in precision electronic assembly, cleaning is still necessary.

C. Additives:

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

(1) Regulators: materials added to adjust the acidity of flux, such as triethanolamine, can adjust the acidity of flux, and hydrochloric acid can inhibit the formation of zinc oxide in inorganic flux.

(2) Extinction agent: can make solder joint extinction, in operation and inspection to overcome eye fatigue and visual decline. Inorganic halides, inorganic salts, organic acids and metal salts such as zinc chloride, tin chloride, talc, copper stearate, calcium and so on are generally added.

(3) Corrosion inhibitor: adding corrosion inhibitor can protect PCB and device-free lead, with moisture-proof, mildew-proof, corrosion-proof performance, and maintain excellent solderability. Most of the materials used as corrosion inhibitors are organic compounds containing nitrides as the main body.

(4) Brightener: It can make the solder joint glow. It can add glycerol, triethanolamine and so on. Generally, the amount of the brightener is about 1%.

(5) Flame retardants: materials added to ensure safe use and improve fire resistance.

D. Solvents:

Practical flux is mostly liquid. Therefore, the solids of flux must be dissolved in a certain solvent to make it a homogeneous solution. Isopropanol and ethanol are mostly used as solvents. The solids used as flux have good solubility.

(1) It has good solubility for all kinds of solids in flux.

(2) The volatilization degree is moderate at room temperature, and volatilizes rapidly at welding temperature.

(3) Low odor and toxicity.

Classification of flux:

(1) According to the state, it can be divided into three categories: liquid, pasty and solid.

(2) There are three types of coatings, spraying and impregnation according to their uses.

(3) According to the activity of flux, it can be divided into inactivated and low-activated.