1. Flow characteristics:
Flux is an indispensable auxiliary material in the SMT welding process.
In wave soldering, flux and solder are used separately whereas in reflow soldering, flux is an important part of the solder paste.
The soldering effect is not only related to the soldering process, components and PCB quality, but also the flux selection is very important.
A well-performing flow should have the following functions:
(1) Remove the oxide on the welding surface, prevent reoxidation of the weld and welding surface during welding, and reduce the surface tension of the weld.
(2) The melting point is lower than that of solder. Before the solder melts, the flux must first melt to fully play the role of a soldering aid.
(3) The diffusion rate of infiltration is faster than that of molten solder, and expansion is generally required by about 90% or more.
(4) The viscosity and specific gravity are lower than those of solder. The viscosity will make infiltration and diffusion difficult. If the specific gravity is large, the weld surface cannot be covered.
(5) No weld bead spatter, toxic gas and strong pungent odor will be generated during welding.
(6) The residue after welding is easy to remove and has the characteristics of non-corrosion, non-hygroscopic and non-conductive.
(7) Non-stick, do not touch your hands after welding, and the solder joint is not easy to sharpen.
(8) Stable storage at room temperature.
2. Chemical composition of the flow:
Traditional flux is generally rosin-based. 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 auxiliary material.
Currently, rosin-based active flux is mainly used in SMT.
As the chemical composition and properties of rosin vary greatly depending on the variety, origin and production process, rosin optimization is key to ensuring flow quality.
The general flux also includes the following components: active agent, film-forming material, additives and solvents.
A. Active agent:
Active flux is an active substance added to flux to improve weldability.
The activity of the active agent refers to its ability to chemically react with the surface oxides of the solder and welded materials 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.
Generally, inorganic active agent has good soldering aid, but it has long action time and high corrosivity, so it is not suitable for use in electronic assembly;
The organic active agent has the advantages of mild action, short time, low corrosiveness and good electrical insulation. It is suitable for electronic assembly.
The active agent content is about 2% – 10%. In the case of chlorinated compounds, the chlorine content must be controlled below 0.2%.
B. Film-forming substances:
By adding film-forming material, a compact organic film can be formed after welding, which protects the solder joint and substrate, and has anti-corrosion and excellent electrical insulation. Commonly used film-forming materials include rosin, phenolic resin, acrylic resin, vinyl chloride resin, polyurethane, etc.
Generally, the addition value is 10% to 20%. Excessive addition will affect the expansion rate and reduce welding aid.
When assembling common household appliances or household appliances with low requirements, film-forming substances are used, and the electrical components after assembly are not cleaned to reduce the cost.
However, in precision electronic assembly, they still need to be cleaned after soldering.
C. Additives:
Additives are substances with special physical and chemical properties added to adapt to the process and environment. Common additives are:
(1) Regulator:
Materials added to adjust stream acidity, such as triethanolamine, can adjust stream acidity;
Adding hydrochloric acid to the inorganic flux can inhibit the formation of zinc oxide.
(2) Matting agent:
It can dull the solder joint and overcome eye fatigue and vision decline during operation and inspection.
Generally, inorganic halides, inorganic salts, organic acids and their metal salts such as zinc chloride, tin chloride, talc, copper stearate, calcium, etc. are added.
(3) Corrosion inhibitor:
The addition of corrosion inhibitor can protect the printed board and lead-free device, has moisture-proof, mildew-proof and corrosion-proof resistance, and maintains excellent solderability.
Most materials used as corrosion inhibitors are organic compounds with nitrides as the main body.
(4) Whitening:
It can make the solder joint shine. Glycerol and triethanolamine may be added. Generally, the addition amount is about 1%.
(5) Flame retardant:
Added materials to ensure safe use and improve flame resistance.
D. Solvent:
Most practical flows are liquids. Therefore, the solid components of the flux must be dissolved in a certain solvent to make it a homogeneous solution.
Isopropanol and ethanol are mainly used as solvents. Various solid components used as flux have good solubility.
(1) It has good solubility for various solid components in flow.
(2) The degree of volatilization is moderate at room temperature and volatilizes quickly at welding temperature.
(3) Low odor and toxicity.
3 . Flow classification:
(1) According to the state, it can be divided into liquid, pasty and solid.
(2) It can be divided into three categories: brushing, spraying and impregnation.
(3) According to the flow activity, it can be divided into non-activated and low-activated.
