What is PCBA Flux

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The content of activator in PCBA flux is about 1%--5%. Organic chemical amines and ammonia compounds, citric acid and salts and organic chemical halides are usually used as activators.

Soldermask is an important part of today's printed circuit board technology and a measure of the strength of a PCBA company. The reason why PCBA solder mask has become so common is because the solder acts as a protective cover for the printed circuit board. In this article, let's follow FS Technology to explore PCBA flux

In addition to some home build circuits today, even many prototyping boards have solder resist, so its use in commercial production of printed circuit boards is arguably common.

What is PCBA Flux

In order to promote the soldering process and prevent the metal from oxidizing reaction, generally regular turnkey PCBA companies will add flux during manual soldering.

Flux applied to PCB board

A common flux consists of a base material and an activator. The activator acts as a wetting agent in the flux, it can make the solder more wet by removing oxides from the metal, and is a very common chemical in the PCBA industry. In addition, the flux also contains other solvents and additives, which can inhibit corrosion and reduce the failure rate of soldering PCBA. Common fluxes include solid, semi-solid and liquid forms. Assembly companies need to consider their own needs when choosing, that is, welding methods, types of welding materials, etc. If you are hand soldering in small batches, FS Technology recommends that you apply it with a flux pen, or use a solder wire core. If you use the automatic soldering process in the printed circuit board manufacturing process, FS Technology will not introduce it in detail because there are too many types.

The role of PCBA flux

Remove surface oxides.

As we all know, the direct contact of metal material with oxygen will form a layer of oxide whose thickness is about 2 × 10-9 ~ 2 × 10-8m. Before soldering the PCBA board, we need to remove the oxide on the surface of the board, otherwise the oxide film will inevitably prevent the wetting of the solder to the base material, and the soldering will not proceed normally. PCBA flux can easily realize redox reaction, so as to achieve the purpose of eliminating oxide film.

The process of using PCBA flux

Reduce the surface tension of the material.

The surface tension of the sheet will affect the soldering quality, and another function of the flux paste is to reduce the tension of the material. The surface tension of the molten solder prevents it from flowing to the surface of the base metal, preventing normal wetting. When the flux covers the surface of the molten solder, the surface tension of the liquid solder can be reduced, so that the wetting performance is significantly improved.

In addition, the role of flux also includes: promoting heat transfer to the welding area, preventing secondary oxidation of metal surface due to high temperature during welding, improving the heat resistance of welding materials, and improving solderability. Due to the length of the article, it is not explained in detail. If you are interested in the content of this part, you can check it in FS Technology blog .

Characteristics and composition of PCBA flux


Good thermal stability, the general thermal stability temperature is not less than 100 ℃

The flux should work before the solder material melts



Additives include corrosion inhibitors, surfactants, thixotropic agents and matting agents
Pine resin: PCB assembly companies with SMT technology generally use pine resin as a pure natural flux, which is the most suitable raw material for flux found by humans.


The function of the solvent is to dissolve the liquid or liquid components in the organic solvent. We can maintain the PCBA board by adjusting the relative density, viscosity, fluidity and heat resistance. Commonly used solvents are ethanol and isopropanol.


The activator can also be called a strong reducing agent, and its main function is to purify the surface of the solder and the welded parts, reduce the oxidized metal plate or protect the metal plate from being oxidized. The content of activator in PCBA flux is about 1%--5%. Organic chemical amines and ammonia compounds, citric acid and salts and organic chemical halides are usually used as activators.

Clean excess flux

Film former

Demulsifier is the most widely used film-forming agent in SMT patch processing. In order to facilitate the distinction, FS Technology divides this substance into two categories, one is natural resin, and the other refers to synthetic organic compounds. The key to demulsifiers is to maintain spot welds and base steel plates with corrosion resistance and dielectric strength

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