Analysis of the main components and role of flux

Flux is widely used in the PCB industry, and its quality is directly related to the entire production process and product quality of the electronics industry. With the implementation of RoHS and WEEE directives, lead-free has higher requirements for flux performance, flux has developed from the traditional rosin type to halogen-free, rosin-free, clean-free, and low solid content, and its composition has changed accordingly, and the interaction between various components makes the flux performance better.

<!--[if !supportLists]-->1、   <!--[endif]-->The basic composition of flux

1. Fluxes at home and abroad are usually composed of activators, solvents, surfactants and special components, including corrosion inhibitors, antioxidants, film-forming agents, etc.

The role of each component of the flux There are oxides, dust and other dirt on the surface of the welded metal workpiece, which hinders the diffusion between the metal of the workpiece matrix and the solder in an atomic state, so the oxide must be removed to expose the clean metal matrix, but the atoms on the surface of the cleaned metal matrix will be oxidized immediately in the atmosphere, and the oxidation rate is faster at the welding temperature. Therefore, flux is added during the soldering process to assist in providing an oxide-free metal surface and to maintain these oxide-free surfaces until the solder is soldered to the metal surface. At the same time, with the help of the chemical action of the flux, it is combined with the oxide on the surface of the welded metal to form a liquid compound at the welding temperature, so that the metal atoms on the surface of the welded metal part and the atoms of the molten solder diffuse each other, so as to realize the soldering connection. During the soldering process, the flux also promotes solder flow and diffusion, affecting the balance of solder surface tension in the direction of solder diffusion by reducing surface unevenness.

In addition to chemical activity, the ideal flux should also have good thermal stability, adhesion, expansion force, electrolytic activity, environmental stability, chemical functional groups and their reaction properties, rheological properties, adaptability to general cleaning solutions and equipment, etc. The above effects of flux are achieved through the action of activators, solvents, surfactants and other components.

2. Mechanism of action of activator The main function of activator is to remove oxides on the surface of the pad and solder at the soldering temperature, and form a protective layer to prevent the substrate from oxidizing again, thereby improving the wettability between the solder and the pad. The components of flux activators generally include hydrogen, inorganic salts, acids and amines and their compound compositions.

3. Hydrogen, inorganic salts Hydrogen and inorganic salts (such as stannous chloride, zinc chloride [1], ammonium chloride [2], etc.) use their reducing properties to react with oxides, such as hydrogen in gas fluxes, and the only residue after welding is water; In addition, the reduction of hydrogen can effectively remove oxides from the metal surface and convert oxides into water. MxOy＋yH₂ ＝xM＋yH₂O。 At the same time, hydrogen also provides a shielding gas to the metal surface, preventing re-oxidation of the metal surface before the welding is complete.

4. Organic acids Acid active agents (such as halogenic acids, carboxylic acids, sulfonic acids) are mainly due to the reaction of H⁺ and oxides, such as [3]: the carboxyl group and metal ions of organic acids remove the oxide film of pads and solder in the form of metal soap:

CuO+2RCOOH→Cu(RCOO)₂ +H₂O

Then the organic acid copper decomposes, absorbs hydrogen, and generates organic acid and metallic copper:

Cu(RCOO).₂+H₂+M→2RCOOH+M-Cu Colophony is expressed as C₁₉H₂₉COOH by molecular formula, which has a certain fluxing effect at a certain temperature due to the presence of carboxyl groups; At the same time, rosin is a macromolecular polycyclic compound, which has a certain film-forming property, which transfers heat and plays a covering role in the welding process, which can protect the metal from being re-oxidized after removing the oxide film. Now there are single organic acids as activators, and mixed acids as activators. There is a certain difference in the boiling point and decomposition temperature of these acids, so that the boiling point of the fluxand the decomposition temperature of the activator are distributed in a large range.

5. Organic halides such as carboxylic acid halides, hydrohalides of organic amines. Zhang Yinxue [4] used bromide salicylic acid as the activator, which can thermally decompose hydrogen bromide and salicylic acid to dissolve oxides on the surface of the matrix metal at brazing temperature. In addition, the hydroxyl and carboxyl groups of salicylic acid can react with JH resin to form a polymer resin film during brazing, covering the surface of the solder joint. Hydrohalides of organic amines, such as aniline hydrochloride, react with the copper of the substrate during soldering and produce CuC1₂ and copper complexes. The resulting copper compound mainly reacts with the tin in the molten solder to form metallic copper, which is immediately melted into the solder, and through these reactions and the melting of the copper in the solder, the solder is distributed on the copper plate. The reaction is as follows[5]: Cu+2C₆H₅NH₂·HCl→CuCl₂＋2C₆H₅NH₂＋H₂ CuCl₂＋2C₆H₅NH₂·HCl→Cu[C₆H₅NH₃]₂Cl₄

6. Organic amines and acids are used in the compound use of amino groups - NH₂ contained in organic amines themselves, and the addition of organic amines can improve the welding effect. In order to reduce the corrosive effect of the flux on the copper plate, a certain amount of corrosion inhibitor can be added to the formulated flux, and the corrosion inhibitor is usually organic amine. Organic acids and organic amines will be mixed with a neutralization reaction, the resulting neutralization products are unstable, and will quickly decompose at the welding temperature to regenerate organic acids and organic amines, so as to ensure the original activity of organic acids, and after welding, the remaining organic acids will be neutralized by organic amines, so that the acidity of the residue is reduced and corrosion is reduced. Therefore, the addition of organic amines not only adjusts the acidity of the flux, but also makes the solder joint shiny, minimizing post-solder corrosion without reducing flux activity [6]. At present, it is more appropriate to combine organic amines and organic acids with strong wetting ability. For example, Xue Shuman et al. introduced in the patent [7] with aliphatic dibasic acids, aromatic acids or amino acids as active ingredients of flux. In addition, the addition of a small amount of glycerol to the flux not only contributes to the storage stability of the flux, but also helps the activator to exert its activity. Zhang Mingling added dibromosuccinic acid, dibromobutene glycol, and dibromostyrene to the flux to enhance the activity of the flux [8]. Carboxylic acids (including dicarboxylic acids) are moderately active at low temperatures, and their high temperature activity is significantly improved. The higher activity is the hydrohalide or organic acid of organophosphate, sulfonic acid, organic amine (including hydrazine); The magnitude of the activity of halides and their substituted acids depends on their specific structure.

7. Role of Other Ingredients Flux also contains many other useful ingredients. The main function of the solvent is to dissolve the components in the flux, as a carrier for each component, so that the flux becomes a uniform viscous liquid. Generally, alcohols, esters, alcohol ethers, hydrocarbons, ketones, etc. The alcohol with high boiling point has a good protective effect, but the viscosity is large and it is inconvenient to use; Alcohols with low boiling points have low viscosity but poor protection, so mixed alcohols can be considered [9][10]. Generally, it is a mixture of high boiling and low boiling point alcohols, and some use water-soluble alcohols and water-insoluble ethers as solvents [11]. Li Weihao used a water-soluble polymer with an overbranched structure and an average molecular weight of 2000 as the flux carrier, and the molecular configuration of hyperbranched polymer can not only increase the thermal decomposition temperature of the polymer, but also reduce the viscosity of the polymer and enhance the permeation and wetting properties of the polymer [12]. The main function of surfactant is to reduce the surface tension of the flux and increase the wetting of the flux to the solder powder and pad.

Compared with Sn-Pb (63 - 37), non-lead solders (such as SAC 3O5, etc.) have higher melting points, greater surface tension, long processing time at high temperatures, and large internal stresses generated during rapid cooling, so surfactants play a more prominent role in improving the reliability of non-lead solder solder interconnects. They can be nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants, and fluorinated surfactants.

Corrosion inhibitors are generally pyrroles, such as benzotriazole (BTA), which is a highly effective corrosion inhibitor for copper, and its addition can inhibit the corrosion of copper plates by the active agent in the flux. Benzotriazole is thought to react with copper to form a precipitated film of insoluble polymer. According to chemical analysis and X-ray analysis, Wang Weike believes that the empirical formula of the membrane is BTA₄Cu₃Cl₂·H₂O and (BTA₂Cu)₂CuCl₂·H₂O, and the surface of the polymer and the copper metal are parallel, which is very stable. BTA is easier to form on the Cu₂O layer than on the CuO layer, and the thickness of the film is nearly twice as thick. When the concentration of BTA is greater than l0⁻³mol/L, it can well inhibit the corrosion of copper [9].

The main function of antioxidants is to prevent oxidation of solder, generally phenols (hydroquinone, catechol, 2,6-di-tert-butyl-p-cresol), ascorbic acid and its derivatives, etc. Especially in water-soluble fluxes, antioxidants are a must. F· J. Jasky added polynuclear aromatic compounds to the flux, which released N₂ when heated to form an inert atmosphere and prevent oxidation [13]. The film-forming agent selects hydrocarbons, alcohols and greases, which generally have good electrical properties, play the role of protective film at room temperature and are inactive, and show activity at the welding temperature of 200 °C ~ 300 °C, with the characteristics of no corrosion and moisture-proof.

Surfactants play a significant role in improving the reliability of non-lead solder solder interconnects, and further high-performance surfactants for non-lead solders can be developed in the future. In view of the characteristics of different types of surfactants, it may be possible to develop surfactants with better wettability and lower surface tension through molecular design or composite use

The main function of thixotropic agent is to give the solder paste a certain thixotropic performance, that is, the viscosity of the solder paste becomes smaller in the stressed state to facilitate the printing of the solder paste, and after printing, its viscosity increases in the state of no force to maintain the inherent shape and prevent the solder paste from collapsing. The main function of thickener (also known as tackifier) is to increase the viscosity of flux to give the solder paste a certain viscosity and facilitate the pasting of components to be soldered. Interfacial compound growth inhibitors: Intermetallic compounds (IMCs) are present in the alloy coating layer formed on the copper surface of the pads, and their composition and thickness determine the weldability of the assembly weld. For example, Cu₃Sn and Cu₆Sn₅ are formed in hot air leveling, the former has poor weldability, the latter has better weldability, and the most superficial solder layer is the best welding; In Sn-Pb (63 - 37), the thickness of its IMC is small due to the coverage of the eutectic alloy Sn/Pb; In non-lead solders, other metals (e.g., Ni or Co in SN100CL) are often added to affect the thickness of the IMC layer [14]; In flux, oxalic acid, 2-aminobenzoic acid, quinoline, quinoline-2-carboxylic acid, etc. are often added. These compounds can form an interfacial compound deposition layer at the interface between the solder and the substrate, which inhibits the atomic diffusion between the solder and the substrate, thereby hindering the growth of intermetallic compounds.

In terms of solvents, existing mixed alcohol methods are an effective solution, but as technology evolves, it may be necessary to find a more ideal solvent or combination of solvents. According to the current understanding of the role of solvents, the ideal solvent should ensure that the components are dissolved and the appropriate viscosity is provided, while also having better environmental protection and stability.

2、   Ways to improve

 1. Microencapsulation method In order to reduce the corrosive problem caused by the acidic substance of the active agent, Chen Qiyin et al. [9][10][16][17] used microencapsulation technology. They used polyimide, acrylic resin, cellulose acetate and other membranes to microencapsulate the active agent. The microencapsulated membrane blocks the direct contact between the organic acid and the metal surface to avoid the metal being oxidized, and when the welding reaches a certain temperature, the membrane is destroyed and the organic acid is released to achieve the purpose of welding. In this way, the treated flux has strong flux and non-corrosiveness, so as to truly achieve the purpose of no-cleaning. Wang Weike believes that acids and amines are prone to neutralization reactions, but due to the non-polar protective effect of the filmformer, the coated active material can still remain inert, thereby improving the performance of the flux.

2. Curable flux In common no-clean fluxes, the components generally used are volatile during soldering, but there is usually a little residue, and the volatilization has more effects on the air. Therefore, in order to achieve VOC-free no-cleaning, curable fluxes [18] - [20] have emerged, which are used as fluxes during soldering and as reinforcing materials for soldering parts after heating and curing. Resins containing phenolic hydroxyl groups in curable fluxes, curing agents for curing resins, and curing catalysts.

3. With the continuous development of the electronics industry, the performance requirements for fluxes are also continuously improving. As mentioned earlier, fluxes play an extremely important role in the PCB industry, where their components interact with each other to achieve a variety of functions.

From the perspective of activators, although there are many types of activators and their compounding methods, more new activators can still be explored in the future. For example, in the context of increasingly stringent environmental requirements, the development of activators that can remove oxides efficiently and have a lower environmental impact is one direction. Referring to the mechanism of action of the various activators mentioned above, such as the way organic acids and organic amines are used in combination, can inspire researchers to find more combinations of ingredients with synergistic effects.

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In terms of corrosion inhibitors, although corrosion inhibitors like benzotriazole have a good corrosion inhibitor effect on copper plates, for other metal materials, new corrosion inhibitors may need to be developed. Based on the current principle of corrosion inhibitors, more targeted corrosion inhibitors are designed according to the chemical properties of different metals.

For antioxidants, existing phenols, ascorbic acid and their derivatives play an important role in preventing oxidation of solders. However, with the diversification of welding processes, it may be necessary to develop antioxidants that can effectively prevent oxidation under more extreme conditions (e.g., high temperatures, high humidity, etc.).

The microencapsulation method and curable flux are currently effective means to improve the performance of flux. In terms of microencapsulation technology, the selection of film materials and the process of microencapsulation can be further optimized to improve the fluxing and environmental friendliness of flux. For curable fluxes, researchers can explore more types of curable components to meet the needs of different soldering scenarios. In short, the development of flux needs to comprehensively consider the functional optimization of each component and the improvement of overall performance to meet the evolving needs of the electronics industry.