What do welding additives do?

1. Characteristics of flux: Flux is an indispensable excipient in the SMT (surface mount technology) soldering process. In wave soldering, flux is used separately from solder; Whereas, in reflow soldering, flux is an important part of solder paste. In addition to the quality of the soldering process, components and PCB (printed circuit board), the choice of flux is extremely important. The flux with excellent performance should have the following functions: (1) Remove the oxide on the soldering surface to prevent the solder and the soldering surface from oxidizing again during soldering, and reduce the surface tension of the solder. Solders (e.g., tin-lead alloys or lead-free solders) have a high surface tension in the liquid state. The high surface tension makes it difficult for the solder to spread across the metal surface. Flux reduces the surface tension of the solder, making it easier for the solder to flow and spread across the surface of the metal being soldered. This is just as a drop of dish soap on the surface of the water reduces the surface tension of the water, making it easier for objects to float or spread on the surface. In this way, the flux ensures that the solder is evenly covered over the solder area, creating a good solder joint. (2) The melting point is lower than that of the solder, and the flux should be melted before the solder melts, so that the effect of the flux can be fully exerted. (3) The wetting and diffusion rate is faster than that of molten solder, and the expansion rate is usually required to be about 90% or more than 90%. (4) The viscosity and specific gravity are smaller than that of solder, and too large viscosity will lead to difficulty in wetting and diffusion, and the specific gravity will not be able to cover the surface of the solder. (5) There will be no welding bead splash during welding, and there will be no toxic gas and strong irritating odor. (6) The residue after welding is easy to remove, and has the characteristics of non-corrosion, non-hygroscopic and non-conductive. (7) Non-sticking, no sticking after welding, and the solder joint is not easy to pull the tip. (8) Stable storage at room temperature.

2. Chemical composition of flux: Traditional flux is usually based on rosin. Rosin has weak acidity and hot-melt fluidity, and has good insulation, moisture resistance, non-corrosive, non-toxic and long-term stability, which is a rare flux material. At present, most of the active fluxes used in SMT are rosin-based. Because there are great differences in the chemical composition and properties of rosin due to different varieties, origins and production processes, the optimization of rosin is the key to ensure the quality of flux. General purpose fluxes also contain the following components: active agents, film-forming substances, additives, and solvents. In the SMT process, the role of flux is more complex and critical. In the production line of printed circuit boards, the flux is precisely applied to the parts of the PCB that need to be soldered by a printing machine. Due to the very high precision requirements of SMT soldering, the flux must be able to effectively remove oxides and reduce surface tension between the tiny pads and component pins. During the reflow soldering process, the flux retains its activity at high temperatures, ensuring that the solder can accurately flow to each solder joint when multiple components are soldered at the same time, resulting in a high-quality solder joint.

A. Activator: Activator is an active substance added to flux to improve the ability of flux. The activity of an active agent refers to its ability to chemically react with solder and oxides on the surface of the soldered material to clean the metal surface and promote wetting. Active agents are divided into inorganic active agents, such as zinc chloride, ammonium chloride, etc.; Organic active agents, such as organic acids and organic halides. Generally, the inorganic active agent has good flux resistance, but the action time is long and corrosive, so it is not suitable for use in electronic assembly; The organic active agent has mild action, short time, low corrosiveness and good electrical insulation, and is suitable for use in electronic assembly. The active agent content is about 2% - 10%, and if it is a chlorine-containing compound, the chlorine content should be controlled below 0.2%.

B. Film-forming substance: After adding film-forming substance, it can form a tight organic film after welding, protecting solder joints and substrates, and has anti-corrosion and excellent electrical insulation. Commonly used film-forming substances include rosin, phenolic resin, acrylic resin, vinyl chloride resin, polyurethane, etc. Generally, the amount of addition is 10%-20%, and adding too much will affect the expansion rate and reduce the effect of fluxing. In ordinary household appliances or electrical assembly with low requirements, film-forming substances are used, and the electrical parts after assembly are not cleaned to reduce costs, but they still need to be cleaned after welding in precision electronic assembly.

C. Additives: Additives are substances with special physical and chemical properties added to adapt to the process and environment. Commonly used additives are:

(1) Regulator: the material added to adjust the acidity of the flux, such as triethanolamine to adjust the acidity of the flux; The addition of hydrochloric acid to the inorganic flux inhibits the formation of zinc oxide.

(2) Matting agent: It can make the solder joint dull, and overcome eye fatigue and vision deterioration during operation and inspection. Generally, inorganic halides, inorganic salts, organic acids and their metal salts are added, such as zinc chloride, tin chloride, talc, copper stearate, calcium, etc.

(3) Corrosion inhibitor: The addition of corrosion inhibitor can protect the printed board and the deviceless leads, with moisture-proof, mildew-proof and anti-corrosion performance, and maintain excellent solderability. Most of the substances used for corrosion inhibitors are organic compounds with nitrides as the main body. The protective film left by the flux after soldering not only prevents the solder joint from being oxidized again, but also prevents the solder joint from being attacked by corrosive substances in the surrounding environment. In harsh operating environments, such as high humidity or the presence of chemical contaminants, flux protective films can extend the life of the solder joints and protect the integrity of the entire electronic circuit.

(4) Brightener: It can make the solder joint glow, and glycerin, triethanolamine, etc. can be added, and the general amount is about 1%.

(5) Flame retardant: a material added to ensure safety and improve flame resistance.

D. Solvent: Most of the practical fluxes are in liquid form. For this purpose, the solid components of the flux must be dissolved in a certain solvent to make it a homogeneous solution. Most of them use isopropanol and ethanol as solvents, and have good solubility to various solid components in fluxes.

(1) It has good solubility to various solid components in flux.

(2) The degree of volatilization is moderate at room temperature, and it volatilizes rapidly at welding temperature.

(3) Low odor and low toxicity.

3. Classification of flux:

(1) According to the state, it can be divided into three categories: liquid, paste and solid.

(2) According to the use, it can be divided into three categories: brushing, spraying and impregnation.

(3) According to the active size of the flux, it is divided into unactivated and low activated.

The role of flux in the soldering process is multifaceted and irreplaceable. It ensures solder quality by removing oxides, reducing surface tension, and has a profound impact on the reliability of solder joints and the overall performance of electronic devices, both in manual soldering and in automated SMT processes. With the continuous development of electronic technology, the performance requirements for flux are also constantly improving, and flux will continue to play an important role in the field of electronic manufacturing in the future.