Composition and Requirements of Solder Paste Flux


1. The basic composition of flux in solder paste.

The basic composition of flux in solder paste is shown in Table 1.


Table 1 Basic composition of solder paste flux.




Base material

Necessary tackiness for fixing
components; cleaning the soldering surface

Rosin Resin


Cleans the metal surface 

Amine, Amine hydrochloride, etc


Preventing the separation of solder  powder and flux and adjusting the
solder paste thixotropic.

Hydrogenate castor oil, carnauba wax, etc


Adjusting solder paste viscosity

Ethylene glycol, Glycol; Diethylene glycol,
Diethylene glycol

2. Basic requirements for solder paste flux.

The solder paste flux is the carrier of the solder powder. A good solder paste flux should meet the following conditions.


First, the solder paste flux should have a high boiling point to prevent the paste solder from splashing during reflow;


Secondly, the solder paste flux must have sufficient viscosity to prevent the solder alloy from settling during storage;


Third, the halogen content of the solder paste flux should be as low as possible to prevent corrosion of components after reflow soldering;


Fourth, the solder paste flux should have low hygroscopicity, preventing the solder alloy from absorbing water vapor in the air and oxidation.


The expansion ability of the solder paste flux during the reflow process is related to the rosin mixture, thickener, and activator. Rheology, slump, viscosity curve, reflow temperature curve, etc. can generally be used to describe the quality of the prepared solder paste.


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