Flux of SMT Process

1. Definition of flux for SMT processing:

In the process of SMT and welding, the material which can purify the welding metal and welding surface auxiliary help the soldering of components and solder paste in the SMT is called flux, it is very important that cannot be ignored in the SMT process.


2. The main composition of flux for SMT processing:

Flux is composed of activator, tackifier, solvent, additives etc.

1). Activator

Activators mainly include rosin, glyceryl stearate, hydrochloric acid, hydrazine, triglycolic acid etc.

Rosin as a natural flux is currently recognized as the most suitable material for flux. Because rosin is mainly composed of rosin acid, rosin acid starts to soften at 74°C and activates around 170-175°C. The activation reaction becomes violent as the temperature rises, and it can play a very key auxiliary role in patch processing.

Function: remove oxides in the welding part of pad and components.

2). Tackifier

Tackifier is mainly include rosin, rosin resin, polybutene and so on.

Its function: Purify the metal surface and keep the sticky with SMD.


Tackifier is mainly include glycerol, ethylene glycol

Its function:dissolve solid or liquid components in the solvent, adjust density, viscosity, fluidity, thermal stability and protection, and prevent premature solidification.


Additives is mainly include paraffin (wax emulsion).Ointment base.

Its function: anti-dispersion, collapse and other poor welding.

3. Classification of fluxes for SMT processing:

The current flux is divided into three kind: inorganic flux, organic flux, and resin series.  

1). Inorganic series flux:

It has strong chemical action and very good flux performance, but it has great corrosive action and can easily damage metals and solder joints,generally cannot be used in welding electronic products.It is an acidic flux and dissolves in water which also called water-soluble flux, and includes 2 types of inorganic acid and inorganic salt.

2). Organic series flux:

The effect of flux is between the inorganic flux and the resin flux,it is also an acidic, water-soluble flux. The water-soluble flux containing organic acid is based on lactic acid and citric acid. Because its welding residue can remain on the welded object for a period of time without serious corrosion, it can be used in the assembly of electronic products, but it is generally not used in the solder paste of SMT, because it does not have the viscosity of rosin flux (to prevent the movement of the chip components).

3). Resin series flux:

In the soldering of electronic products, the resin-based flux is the largest. Because it can only be dissolved in organic solvents, it is also called organic solvent flux, and its main component is rosin. The characteristic solder residue of rosin does not have corrosion problems make rosin a non-corrosive flux and is widely used in the soldering of electronic devices. For different application needs, rosin flux has three forms: liquid, paste and solid. The solid flux is suitable for soldering iron, and the liquid and paste flux are suitable for wave soldering respectively.

4. The role of flux for SMT processing:

1). Remove the oxide on the surface of the welded metal;

2). Prevent high temperature re-oxidation of the metal surface during welding;

3). Maintain the surface strength of the soldering material, enhance the thermal stability of the solder, enhance wettability, and improve solderability;

4). Promote heat transfer to the welding area.

5.The performance characteristics of the flux:

1). It should have good thermal stability. Generally, the thermal stability temperature is not less than 100°C.

2). The flux should start to play its due role before the solder melts, and within a reasonable range, it can better remove the oxide film during the soldering process, maintain the surface strength of the soldering material, and enhance the thermal stability of the solder. But it should not be too different.

3). Residues after welding should not be corrosive and easy to clean; no toxic and harmful gases should be precipitated; water-soluble resistance and insulation resistance that meet the requirements of the electronics industry; no moisture absorption, no mold; stable chemical properties, easy to store.

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