During pcb manufacturing process, analysis of the paste flux functions

1,268 Published by FASTPCBA 8月 12,2019

During pcb manufacturing process, analysis of the paste flux functions

During pcb manufacturing process, the proportion of the paste flux in the solder paste is generally 10% to 15%, and the volume percentage is 50% to 60%. As a solder powder carrier, it functions as a binder, a flux inhibitor, a rheology control agent, and a suspending agent. It consists of a resin, an activator (surfactant, catalyst), a thixotropic agent, a flux, and an additive.

pcb manufacturing processpaste flux functions

pcb manufacturing process paste flux functions

The flux used to make the solder paste has the same soldering function as the liquid solder during pcb manufacturing process, but it must have other conditions. This flux is a carrier of solder powder, and its relative density to solder powder is 1:7.3, which is extremely different. In order to ensure good mixing, it should have high viscosity, and its viscosity should be controlled at 50Pa•S. Because it has a certain viscosity, it is also called a paste flux.


Excellent flux should have a high boiling point to prevent the solder paste from ejecting during reflow; high viscosity can prevent the solder paste from settling during storage; low halogen content can prevent corrosion of components after reflow; The low moisture absorption prevents the solder paste from absorbing water vapor in the air during using to cause oxidation of the powder.


The rosin or other resin contained in the paste flux can enhance the viscosity and play a role in preventing secondary oxidation of the solder during welding and film formation. This composition plays an important role in fixing the components. In addition, the flux contains a thixotropic agent, which can adjust the viscosity and printing performance of the solder paste, and the solder paste has a pseudoplastic fluid characteristic, which is also called thixotropic, and is sheared by the scraper during the printing process. The viscosity is lowered, and when it passes through the stencil window, it can be quickly lowered onto the PCB pad during pcb manufacturing process. After the external force is stopped, the viscosity is quickly increased. Therefore, the resolution of the pattern after the solder paste is printed can be ensured, and the high-quality printed pattern can ensure the incline of bridge connection defects in the welding


The activator in the flux mainly functions to remove the surface layer of the PCB copper mold pad and the oxide of the soldering part, and has the effect of reducing the surface tension of tin and lead. The flux in the solder paste is generally composed of muti-kinds of components, which are composed of different boiling points, polarities and non-polar melts, which can weld various fluxes and have a good storage life of the solder paste during pcb manufacturing process. Additives are substances with special physical and chemical properties added to suit the technics and environment. Regulators, matting agents, corrosion inhibitors, brighteners and flame retardants are commonly used.


The flux content has a significant effect on the slump, viscosity, and bonding properties of the solder paste; in addition, it can affect the thickness of the solder after soldering, because the low flux content means that the solder paste metal content increases, high metal content causes an increase in the thickness of the solder layer after hot melt. For example, when the metal content changes from 90% to 95% during pcb manufacturing process, that is, the flux content changes from 10% to 5%, the thickness of the solder decreases from 4.5 mils to 2 mils, almost decrease doubled, and the solder content tiny change of each batch of solder paste will have a large impact on solder joint quality during pcb manufacturing process. For example, printing a solder paste of the same thickness, a 10% change in the metal content can cause the excess solder joint to become a solder joint with insufficient solder joint, resulting in poor soldering strength, especially poor fatigue strength. The solder paste used for the surface mount board should generally contain 88% to 90% of the metal composition.

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