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What are the Functions of Solder FLux

2022-09-01

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What are the Functions of Solder FLux?

Solder paste is one of the indispensable materials in the packaging industry connecting the chips and pads and forming an electrical interconnection. The various formulations of solder pastes can meet different packaging requirements, but the most basic formula of solder pastes is solder powder and flux.

 

1. What is flux

 

Flux is an important ingredient in making solder pastes. Solder pastes are produced by mixing solder paste alloy powder and flux in proportion. Flux is often composed of rosin mixed with an appropriate amount of activator, surfactant, thixotropic agent, and solvent. The activator consists of organic acid, and the surfactants are usually alkanes. The function of the solvent is to mix the solder paste components uniformly. Rosin is not heat-resistant to high temperatures and will be decomposed at about 300° C. Therefore, when the reflow temperature is too high, the resulting solder joints will have a black color.

 

2. The role of flux

2.1 Removal of base metal oxide layer

Flux plays a crucial role in enhancing the solderability of solder paste. Copper is a very suitable metal for soldering. However, the copper pads are exposed to the air, which makes them susceptible to oxidation. The oxides are difficult to remove with cleaning agents. The oxide layer acts as a barrier, inhibiting the solder paste to wet the pads. Therefore, the oxide layer needs to be removed during soldering. The active organic acids, hydrogen, inorganic salts, organic halides, and other components in the flux can undergo a reduction reaction with the oxides on the pads so that the oxides are decomposed (equations below). The flux also continuously protects the solder joints from oxidation during subsequent reflow soldering processes. However, high activity may lead to more soldering residues, which increases the difficulty of cleaning.

 

Hydrogen: MxOy +yH2 = xM+yH2O (1)

 

Organic acid: CuO+2RCOOH = Cu(RCOO)2+H2O (1)

Cu(RCCO)2+H2+M = 2RCOOH+M—Cu (2)


2.2 Changing the surface tension of the base metals

The size of the surface tension determines the wettability of the solder paste to the pads. If the surface tension is too large, the solder paste will shrink into a spherical shape after curing and cannot completely cover the pads, resulting in uneven solder joint shape, poor reliability, and solder joint lift-off. The surfactant component in the flux has a hydrophilic end and a hydrophobic end structure, which can change the surface tension of the solder paste on the base metals. When the surface tension is reduced, the solder paste can flow smoothly and spread on the base metals. The general solder joint wetting angle is 30-45°.

 

The surfactant should be volatilized in time during the soldering process. No activator residue should be left after soldering. Otherwise, it will continuously corrode the pads.


Fitech is a world-leading provider of microelectronics and semiconductor packaging materials solutions. Fitech’s solder paste, epoxy solder paste, and solder powder have excellent wetting effects, narrow particle size distribution, and outstanding reliability after soldering. Welcome to Fitech’s official website for more information.

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