Popular Science: Understanding of PCB soldering Flux Correctly

In the welding process, the material that can clean the welding metal and the welding surface and help the welding is called flux. Flux is an indispensable craft material in the soldering process. Liquid flux is used in wave soldering and manual soldering processes. Flux and solder are used separately. In the reflow process, the flux is an important part of the solder paste. The quality of the solder is not only related to the solder alloy, components, PCB quality, soldering process, but also has a very important relationship with the performance of the flux and the flux choosing .

First, the requirements of the physical / chemical properties of the flux

The physical properties of the flux mainly refer to the melting point, surface tension, viscosity, and mixing properties associated with the soldering properties. The flux is required to have a certain chemical activity, good thermal stability, good wettability, and promote the expansion of the solder. The flux residue remaining on the substrate is non-corrosive to the substrate, has good cleaning property, and the chlorine content is Within the prescribed range. The basic requirements are as follows.

(1) The appearance of the flux should be uniform, transparent, free from sedimentation or delamination, and free from abnormal object. The flux should not emit toxic, harmful or strong irritating gases and thicker fumes to protect the environment.

(2) The viscosity and density are smaller than that of the molten solder and are easily replaced. The density of the flux can be diluted with a solvent and should be 0.80 to 0.95 g/cm3 at 23 °C.

(3) The surface tension is smaller than that of the solder, and the expansion speed is faster than that of the molten solder, and the expansion ratio is >85%.

(4) The melting point is lower than that of the solder. Before the solder melts, the flux can fully exert the fluxing effect.The non-volatile content should be no more than 15%, no welding bead splashing during welding, no toxic gas and strong irritating odor.

(5) The surface of the residue after welding should be non-sticky and not on hand. The surface chalk powder should be easily removed.

(6) Water cleaning, semi-aqueous cleaning and solvent cleaning fluxes require easy cleaning after welding.

(7) Storage is stable at normal temperature.

Second, the composition of the flux

The flux is usually a mixture of rosin or non-rosin-type synthetic resin as a main component, and is mainly composed of a rosin or non-rosin-type synthetic resin, an active agent, a film former, an additive, and a solvent.

1) Rosin type or non-rosin type synthetic resin.

A rosin-type or non-rosin-type synthetic resin is a main component of the flux. Rosin is a kind of natural resin obtained by distilling the viscous liquid secreted by pine. It is a transparent and brittle solid substance. Its color is from yellowish to light brown. The surface is slightly shiny. It has a rosin smell and is soluble in alcohol. Acetone, glycerin, benzene and other organic solvents, insoluble in water.

Synthetic resins have a higher melting point (decomposition) temperature, so synthetic resin fluxes can be used at higher temperatures than rosin fluxes.

(2) Activating agent

The activator is a strong reducing agent. The main function of the activator is to purify the solder and the surface of the welded part .

(3) Film former

The film former can form a tight organic film after soldering to protect the solder joints and the substrate from corrosion resistance and excellent electrical insulation.

(4) Additives

The additives mainly include corrosion inhibitors, surfactants, thixotropic agents, matting agents, etc. The main function of the additives is to obtain some special physical and chemical properties of the flux to meet the needs of different requirements of products and process occasions.

5) Solvent

Solvents mainly include ethanol, isopropanol, ethylene glycol, propylene glycol, glycerin, etc., all of which are organic alcohol solvents.

Third, the function of flux

Whether it is manual welding, dip soldering, wave soldering or reflow soldering, the surface of the soldering (welded metal or masterbatch) shoud be cleaned, heated, wetted, diffused and dissolved, cooled and solidified during the forming of solder joints. The flux can play the following roles during the soldering process.

(1) Removing oxides from the surface of the soldered metal

Except pure gold and platinum in nature, all metals placed in the air oxidize at room temperature, forming an oxide layer on the surface, which hinders the occurrence of welding.

(2) Preventing reoxidation of metal surfaces during soldering

The flux is applied to the metal surface during soldering to isolate the metal surface from the air, which can effectively prevent the metal surface from being oxidized again at high temperature during soldering.

(3) Reduce the surface tension of the solder, enhance the wettability, and improve the solderability

The activation reaction of the rosin acid and the active agent in the flux is decomposition, reduction and displacement. In the chemical reaction, heat and activation energy are emitted, which helps to reduce the surface tension and viscosity of the molten solder, and at the same time, the activation energy of the metal surface can promote the flow of the liquid solder on the surface of the metal to be welded, increase the surface activity, thereby improving the liquid solder invasion.

(4) Promote heat transfer to the weld zone

Since the flux reduces the surface tension and viscosity of the molten solder, the fluidity of the liquid solder is increased, thereby facilitating the rapid and efficient transfer of heat to the weld zone and accelerating the expansion speed.

The above is the wisdom knowledge of FASTPCBA Electronics, I hope to help you! For more information, please visit http://www.jbpcba.com Thank you for your attention and comments! ! !