Introduction of Main Components of Flux
Date：2019-10-18 Auto：
Flux is widely used in PCB industry. Its quality directly affects the whole production process and product quality of electronic industry. With the implementation of RoHS and WEEE directives, lead-free has put forward higher requirements for the performance of flux. Flux has developed from traditional rosin type to halogen-free, rosin-free, cleaning-free, and low solid content. Its composition has also undergone corresponding changes. The interaction of various components makes the performance of flux more excellent.
Flux is usually a mixture with rosin as the main component and is an auxiliary material to ensure the smooth progress of the welding process. Welding is the main technological process in electronic assembly. Flux is the auxiliary material used in welding. The main function of flux is to remove the oxide on the surface of solder and the base metal to be welded so that the metal surface can reach the necessary cleanliness. It prevents surface reoxidation during welding, reduces surface tension of solder and improves welding performance. The performance of flux directly affects the quality of electronic products.
Flux at home and abroad is generally composed of activator, solvent, surfactant and special components. Special components include corrosion inhibitor, anti-oxidant and film forming agent.
In recent decades, rosin resin flux, which is mainly composed of rosin, resin, halide-containing active agent, additive and organic solvent, is commonly used in the soldering process of electronic products. Although this kind of flux has good weldability and low cost, it has high residue after welding. The residue contains halogen ions, which will gradually cause problems such as electrical insulation performance degradation and short circuit. To solve this problem, rosin resin flux residue on electronic printed boards must be cleaned. This will not only increase the production cost, but also the residual cleaning agent for rosin resin flux is mainly fluorine-chlorine compound. This compound is a substance that depletes the ozone layer in the atmosphere and is banned and eliminated. There are still many companies that use the above-mentioned process of using rosin resin finger scale flux solder and then cleaning with cleaning agent, which has low efficiency and high cost.
The main raw materials of the washless flux are organic solvent, rosin resin and its derivatives, synthetic resin surfactant, organic acid activator, anticorrosive agent, cosolvent and film forming agent. Simply speaking, various solid components dissolve in various liquids to form uniform and transparent mixed solutions, in which the proportions of various components are different and the roles they play are different.
Organic solvent: one or a mixture of ketones, alcohols and esters, commonly used are ethanol, propanol and butanol; Acetone, toluene isobutyl ketone; Ethyl acetate, butyl acetate, etc. As a liquid component, its main function is to dissolve the solid components in the flux to form a uniform solution, which is convenient for the components to be welded to be evenly coated with an appropriate amount of flux components. At the same time, it can also clean light dirt and oil stains on the metal surface.
Natural resins and their derivatives or synthetic resins
Surfactant: Halogen-containing surfactant has strong activity and high welding aid capability. However, due to the difficulty in cleaning halogen ions, high ion residue and strong corrosiveness of halogen elements (mainly chlorides), it is not suitable for use as raw material for wash-free flux. Halogen-free surfactant has slightly weak activity but little ion residue. Surfactants are mainly non-ionic surfactants of fatty acid group or aromatic group. Their main functions are to reduce the surface tension generated when the solder contacts the lead pin metal, enhance the surface wetting force, enhance the penetration force of organic acid activator, and also act as foaming agent.
Organic acid activator: it is composed of one or more of organic acid diacid or aromatic acid, such as succinic acid, glutaric acid, itaconic acid, o-hydroxybenzoic acid, sebacic acid, pimelic acid, malic acid, succinic acid, etc. Its main function is to remove oxides on the lead pins and oxides on the surface of molten solder. It is one of the key components of flux
Anticorrosive agent: substance that reduces the residue of solid components such as resin and activator after pyrolysis
Cosolvent: Prevent the tendency of solid components such as activator from desolventizing from the solution, and avoid the uneven distribution of activator.
Film Forming Agent: During the soldering process of lead pins, the coated flux precipitates and crystallizes to form a uniform film. The residue after pyrolysis can be quickly solidified, hardened and reduced in viscosity due to the presence of film forming agent.
Dirt such as oxide, dust and the like exist on the surface of the metal workpiece to be welded, which hinders the mutual diffusion between the base metal of the workpiece and the solder in an atomic state. Therefore, oxide and the like must be removed to clean the surface and expose the metal substrate. However, atoms on the surface of the metal substrate to be cleaned are immediately oxidized in the atmosphere, and the oxidation speed is faster at the welding temperature. Therefore, flux is added during the welding process to assist in providing metal surfaces without oxide layer and to maintain the oxide-free state of these surfaces until the solder and metal surfaces complete the welding process. At the same time, depending on the chemical action of the flux, it combines with the oxide on the surface of the metal to be welded to form a liquid table at the welding temperature, so that the metal atoms on the surface of the metal to be welded and the atoms of the molten solder diffuse mutually to achieve the purpose of soldering connection. In the welding process, flux can also promote the flow and diffusion of solder, and affect the balance of solder surface tension in the direction of solder diffusion by reducing surface roughness.
In addition to chemical activity, ideal flux should also have good thermal stability, adhesion, expansion, electrolytic activity, environmental stability, chemical functional groups and their reaction characteristics, rheological characteristics, adaptability to general cleaning solutions and equipment, etc.
The above-mentioned function of flux is realized through the action of activator, solvent, surfactant and other components.
The main function of the activator is to remove oxides from the solder pad and the solder surface at the soldering temperature and form a protective layer to prevent reoxidation of the substrate, thus improving the wettability between the solder and the solder pad.
The components of flux activator are generally hydrogen, inorganic salts, acids and amines, and their compound compositions.
Hydrogen and inorganic salts such as stannous chloride, zinc chloride and ammonium chloride react with oxides by their reducibility, for example, hydrogen in gas flux, water is the only residue after welding; Moreover, the reduction of hydrogen can effectively remove the oxide on the metal surface and convert the oxide into water.
MxOy＋yH2＝xM＋yH2O
At the same time, hydrogen also provides protective gas for the metal surface to prevent reoxidation of the metal surface before welding is completed.
organic halide
Such as carboxylic acid halides, hydrohalides of organic amines.
Zhang Yinxue uses salicylic acid bromide as an activator, which can thermally decompose hydrogen bromide and salicylic acid to dissolve oxides on the surface of matrix metal at brazing temperature. In addition, hydroxyl and carboxyl of salicylic acid can react with JH resin to cross-link into a polymer resin film to cover the surface of the solder joint during soldering.
During soldering, the molten flux reacts with the copper of the substrate to generate CuC12 and copper complex. As a result, the generated copper compound mainly reacts with tin in the molten solder to generate metallic copper, and the copper is immediately melted into the solder. Through these reactions and the melting of copper in the solder, the solder is spread on the copper plate. The reaction was as follows:
Cu＋2C6H5NH2•HCl→CuCl2＋2 C6H5NH2＋H2CuCl2＋2C6H5NH2•HCl→Cu［C6H5NH3］2Cl
The organic amine itself contains amino-NH: which is active. The addition of organic amine can promote the welding effect.
In order to reduce the corrosion effect of flux on copper plate, a certain amount of corrosion inhibitor can be added to the prepared flux, and the corrosion inhibitor usually selects organic amine. Mixing organic acid and organic amine will produce neutralization reaction to produce neutralization product. This neutralization product is unstable and will decompose rapidly at the welding temperature to regenerate organic acid and organic amine, thus ensuring the original activity of the organic acid. After welding, the remaining organic acid will be neutralized by the organic amine again, reducing the acidity of the residue and reducing corrosion. Therefore, after the organic amine is added, the acidity of the flux can be adjusted, the welding spot can be bright, and the corrosion after welding can be reduced to the minimum without reducing the activity of the flux
At present, it is most suitable to combine organic amine and organic acid with strong wetting ability. For example, Xue Shuman et al. introduced in their patent a flux compounded with aliphatic diacid, aromatic acid or amino acid as active ingredients.
In addition, adding a small amount of glycerin to the flux is not only helpful for the storage stability of the flux, but also helpful for the activation of the activator. Zhang Mingling added dibromosuccinic acid, dibromobutylene glycol and dibromostyrene to the flux to enhance its activity.
Carboxylic acids (including dicarboxylic acids) have moderate activity at low temperatures, and their high temperature activity is significantly improved. Higher activity is organic phosphate, sulfonic acid, halogen acid salt or organic acid salt of organic amine (including hydrazine); The activity of halogens and their substituted acids depends on their specific structures.
The flux also contains many other useful components.
The main function of the solvent is to dissolve the components contained in the flux and serve as a carrier for each component to form a uniform viscous liquid. Generally, they are alcohols, esters, alcohol ethers, hydrocarbons, ketones, etc.
Alcohols with high boiling point have better protection effect, but they have high viscosity and are inconvenient to use. Alcohol with low boiling point has low viscosity but poor protection, so the method of selecting mixed alcohol [9] [10] can be considered. Generally, it is a mixture of high-boiling and low-boiling alcohols, and some use water-soluble alcohols and water-insoluble ethers as solvents [11]. Li Weihao uses hyperbranched structure and water-soluble polymer with average molecular weight of 2000 as flux carrier. The hyperbranched molecular configuration can not only increase the thermal decomposition temperature of polymer, but also reduce the viscosity of polymer and enhance the permeability and wetting property of polymer.
The main function of the surfactant is to reduce the surface tension of the solder and increase the wettability of the solder powder and pad. Compared with Sn-Pb(63-37), non-lead solders (e.g. SAC3O5, etc.) have higher melting point, higher surface tension, longer processing time at high temperature and greater internal stress generated during rapid cooling. therefore, surfactant plays a more prominent role in improving the reliability of non-lead solders' welding interconnection. They can be nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants and fluorine-containing surfactants.
Corrosion inhibitors are generally pyrroles, such as benzotriazole (BTA), which is a high-efficiency corrosion inhibitor for copper. The addition of the inhibitor can inhibit the corrosion of copper plates caused by active agents in flux. It is generally believed that benzotriazole reacts with copper to form a precipitation film of insoluble polymer. According to chemical analysis and X-ray analysis, Wang Weike believes that the empirical formulas of the film are BTA4Cu3Cl2•H2O and (BTA2Cu)2CuCl2•H2O, and the surfaces of the polymer and the metal copper are parallel and very stable. BTA film formation on Cu2O layer is easier than that on CuO layer, and the thickness of the film is nearly twice as thick. When BTA concentration is greater than l0-3 mol/l, copper corrosion can be well inhibited.
The main function of anti-oxidant is to prevent solder oxidation, which is generally phenol (hydroquinone, catechol, 2,6-di-tert-butyl-p-methylphenol), ascorbic acid and its derivatives, etc. Especially in water-soluble flux, there must be antioxidant. F•J• Jasky adds polynuclear aromatic compounds to the flux, releasing N2 to form an inert atmosphere during heating to prevent oxidation.
The film-forming agent is selected from hydrocarbons, alcohols and fats, which generally have good electrical properties, do not show activity as a protective film at normal temperature, show activity at a welding temperature of 200-300 deg c, and have the characteristics of no corrosion, moisture resistance and the like
The main function of thixotropic agent is to endow solder paste with certain thixotropic properties, that is, the viscosity of solder paste becomes smaller under stress, so as to facilitate solder paste printing. After printing, its viscosity increases in an unstressed state to maintain its inherent shape and prevent solder paste from collapsing.
Thickener (also known as tackifier) is mainly used to increase the viscosity of the solder paste to give it a certain viscosity and facilitate the adhesion of components to be soldered.
Interface compound growth inhibitor: The alloy coating layer formed on the copper surface of the pad contains intermetallic compound (IMC), whose composition and thickness determine the weldability during assembly and welding. For example, Cu3Sn and Cu6Sn5 are formed in hot air leveling. The former has poor solderability while the latter has better solderability, while the solder layer on the outermost layer is the best one to weld. In Sn-Pb(63-37), the thickness of IMC is small due to the coverage of eutectic alloy Sn/Pb. In non-lead solder, other metals (such as Ni or Co in SN100CL) are often added to affect the thickness of IMC layer [14]; In flux, oxalic acid, 2- aminobenzoic acid, quinoline, quinoline -2- carboxylic acid, etc. are often added. Such compounds can form a layer of deposited layer of interfacial compound at the solder and the interface, which can inhibit the atomic diffusion between the solder and the substrate, thus hindering the growth of intermetallic compounds
