The proportion of 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 acts as a bonding agent, flux inhibitor, rheology control agent and suspending agent. It is composed of resin, activator (surfactant, catalyst), thixotropic agent, flux and additives.
The flux used to make solder paste has the same welding function as liquid flux, but it must have other conditions. This flux is the carrier of the solder powder, and its relative density with the solder powder is 1:7.3, which is very 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 paste flux.
A good flux should have a high boiling point to prevent the solder paste from spraying during reflow; high viscosity can prevent the solder paste from settling during storage; low halogen content can prevent corrosion of components after reflow soldering; low The moisture absorption can prevent the solder paste from absorbing water vapor in the air and causing powder oxidation during use.
The rosin or other resin contained in the paste flux can increase the viscosity and prevent the secondary oxidation of the solder during the soldering and film forming process. This component 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 make the solder paste have the characteristics of pseudoplastic fluid, which is also called thixotropy. During the printing process, it is subjected to the shearing action of the squeegee. The viscosity decreases. When passing through the template window, it can quickly drop to the PCB pad. After the external force stops, the viscosity quickly rises. Therefore, it can ensure the high resolution of the graphics after the solder paste is printed, and the high-quality printed graphics can ensure bridges during soldering. The decline of defects. The activator in the flux mainly serves to remove the oxides on the surface of the PCB copper mold pad and the soldering parts of the parts, and at the same time has the effect of reducing the surface tension of tin and lead. The flux in the solder paste is generally composed of a variety of components, which are mixed with different boiling points, polarities and non-polar melting poles, which can not only weld various fluxes, but also enable the solder paste to have a better storage life. Additives are substances with special physical and chemical properties added to adapt to the process and environment. Commonly used are regulators, matting agents, corrosion inhibitors, brighteners and flame retardants.
The content of the flux has a significant effect on the slump, viscosity, and bonding properties of the solder paste; in addition, it can also affect the thickness of the solder paste after soldering, because the low flux content means an increase in the metal content of the solder paste. The content leads to an increase in the thickness of the solder layer after hot melting. For example, when the metal content changes from 90% to 95%, that is, when the flux content changes from 10% to 5%, the thickness of the solder decreases from 4.5mil to 2mil, which is almost doubled. The flux content of each batch of solder paste is very small. Changes will have a great impact on the quality of solder joints. For example, printing the same thickness of solder paste, a 10% change in metal content can turn excess solder joints into solder joints with insufficient solder, resulting in poor soldering strength, especially poor fatigue strength. The solder paste used in the surface mount board should generally contain 88% to 90% of the metal content.
