In the welding process of smt chip processing, the substance that can purify the welding metal and the welding surface and help welding is called flux, or flux for short. Flux is an indispensable process material in the soldering process. Liquid fluxes are used in both wave and hand soldering processes. Flux and solder are used separately. Flux is an important part of solder paste during reflow soldering. The quality of solder is very fast, which is not only related to the quality of solder alloys, components, PCBs, and soldering processes, but also has a very important relationship with the performance of flux and the selection of flux. What are the requirements for the chemical properties of the flux?

The physical properties of the flux mainly refer to the melting point, surface tension, viscosity, mixing degree, etc. related to the welding performance.

It is required that the flux has a certain chemical activity, good thermal stability and good wettability, which can promote the expansion of the solder. The flux residue left on the substrate is non-corrosive to the substrate and has good cleaning performance. The chlorine content is within the specified range.

The general requirements are as follows:

1. The appearance of the flux should be uniform, transparent, free of precipitation, delamination, and no foreign matter. Flux should not emit toxic, noxious or strongly irritating odor gases and fumes to help protect the environment. During the valid shelf life, its color should not change.

Second, the viscosity and density are smaller than those of molten solder, and it is easy to replace. The density of the flux can be diluted with solvent, and it should be 0.80-0.95g/cm3 at 23 degrees. No-clean flux should be within (100±1.5)% of its nominal density.

3. The surface tension is smaller than that of solder, the wetting and expansion rate is faster than that of molten solder, and the expansion rate is greater than 85%

Fourth, the melting point is lower than that of solder, and the flux can fully play the role of the flux before the solder melts.

5. The non-volatile content is not more than 15%, there is no welding beads splashing during welding, and there is no harmful and strong irritating odor.

6. The surface of the residue after welding should not be sticky or sticky, and the chalk powder on the surface should be easy to remove.

7. No-clean flux requires solid content <2.0%, no halide, less residue after welding, no moisture absorption, no corrosion, good insulation performance, and insulation resistance >1*10 square ohm.

8. Washing, semi-washing and solvent cleaning fluxes are required to be easy to clean after soldering.

Nine, stable storage at room temperature.