PCB manufacturing smt processing flux function


In the PCB manufacturing SMT processing, the specific 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

pcb manufacturing

The flux used to make solder paste for PCBs has the same soldering function as liquid solder, 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 that the materials and pads are well mixed in the PCB manufacturing, they should have high viscosity, and the 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 being sprayed during reflow in PCB manufacturing; high viscosity can prevent the solder paste from settling during storage; low halogen content can prevent components corrosion after reflow soldering; low moisture absorption prevents the solder paste from absorbing water vapor in the air during usage to cause powder oxidation


The rosin or other resin contained in the paste flux can enhance the adhesion in the PCB manufacturing process, and plays a role in preventing secondary oxidation of the solder during the soldering and film formation process, which 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 template window, it can be quickly lowered onto the PCB pad. 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 bridge decline in the welding.


The activator in the PCB manufacturing 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 a plurality 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. Additives are substances with special physical and chemical properties added to adapt to the process 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 stacking 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%, that is, the flux content changes from 10% to 5%, the thickness of the solder decreases from 4.5 mils to 2 mils, almost decreased doubled, and the solder content of each batch of solder paste small changes will have a large impact on solder joint quality. 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, 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 component. PCB manufacturing SMT processing plants need to follow up the minimum spacing of customer materials when selecting solder paste, and choose solder paste reasonably.


The above is the PCB manufacturing industry consultation provided by FASTPCBA, I hope to help you!

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