Analysis on the function of each component of the

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Abstract: according to the research status of flux, this paper introduces the functions of the main components of flux, puts the driven pointer back to zero, mainly expounds the functions and mechanisms of the active components, and summarizes and prospects the methods and directions of improving the performance of flux

flux is widely used in PCB industry, and its quality directly affects the whole production process and product quality of electronic industry. With the implementation of RoHS and WEEE directives, lead-free puts forward higher requirements for the performance of flux. Flux has developed from traditional rosin type to halogen-free, rosin free, clean free and low solid content, and its composition has also changed accordingly. The interaction of each component makes the performance of flux more excellent

1 basic composition of flux

fluxes at home and abroad are generally composed of activators, solvents, surfactants and special components. Special ingredients include corrosion inhibitor, antioxidant, film-forming agent, etc

2 the role of various components of the flux

there are oxides, dust and other dirt on the surface of the welded metal workpiece, which hinder the mutual diffusion between the base metal of the workpiece and the solder in the atomic state. Therefore, the oxides must be removed to make the surface clean and expose the metal substrate, but the atoms on the surface of the cleaned metal substrate are oxidized immediately in the atmosphere, and the oxidation speed is faster at the welding temperature. Therefore, during the welding process, add flux to help provide metal surfaces without oxide layer, and maintain these surfaces in an oxide free state until the welding process is completed between the solder and the metal surface. 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 platform 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 can diffuse each other, so as to achieve the purpose of tin welding 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, the ideal flux should also have good thermal stability, adhesion, expansion, electrolytic activity, environmental stability, chemical functional groups and their reaction characteristics, rheological properties, adaptability to general cleaning solutions and equipment, etc. The above functions of flux are realized through the action of activator, solvent, surfactant and other components

2.1 action mechanism of activator

the main function of activator is to remove the oxides on the surface of solder pad and solder at the welding temperature, and form a protective layer to prevent the reoxidation of the substrate, so as to improve the wettability between solder and solder pad. The components of the flux activator are generally hydrogen, inorganic salts, acids and amines, as well as their composite compositions

2.1.1 hydrogen, inorganic salts

hydrogen and inorganic salts such as stannous chloride, zinc chloride [1], ammonium chloride [2] react with oxides by their reducibility, such as hydrogen in gas flux, and water is the only residue after welding; Moreover, the reduction of hydrogen can effectively remove the oxides on the metal surface and convert the oxides into water. MxOy + YH 2 = XM + YH 2 o at the same time, hydrogen also provides protective gas for the metal surface to prevent reoxidation of the metal surface before welding is completed

2.1.2 organic acids

acid activators (such as halogen acid, carboxylic acid, sulfonic acid) are mainly due to the reaction between h+ and oxides, such as [3]: the carboxyl group of organic acids and metal ions remove the oxide film of pads and solders in the form of metal soap:

cuo + 2rcooh → Cu (rcoo) 2 + H 2o

then the organic acid copper decomposes and absorbs hydrogen, And produce organic acid and metal copper:

Cu (rcoo) 2 + H2 + m → 2rcooh + m-cu

rosin (colophony) is expressed as C 19 h29cooh by molecular formula. Because it contains carboxyl group, it has a certain welding aid effect at a certain temperature; At the same time, rosin is a kind of macromolecular polycyclic compound, so it has a certain film-forming property. It transfers heat and plays a covering role in the welding process, and can protect the metal after removing the oxide film from being oxidized again

now there are single organic acids as activators and mixed acids as activators. There is a certain difference between the boiling point and decomposition temperature of these acids. Such a combination can make the boiling point of flux and the decomposition temperature of activator present a large interval distribution

2.1.3 organic halides

such as carboxylic acid halides and hydrogen halides of organic amines. Zhang Yinxue [4] uses brominated salicylic acid as activator, which can thermally decompose the oxides on the surface of the base metal dissolved by hydrogen bromide and salicylic acid at the brazing temperature; And the hydroxyl and carboxyl groups of salicylic acid can react with JH resin to crosslink into polymer resin film during brazing, which covers the surface of the solder joint. When welding, the molten flux reacts with the copper of the substrate to produce CuC12 and copper complex. As a result, the generated copper compound mainly reacts with tin in the molten solder to produce metallic copper, which 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 is as follows [5]:

cu + 2C 6 h5nh2? HCl→CuCl2+2 C6H5NH2+H2

CuCl2+2C6H5NH2? HCl → Cu [c6h5nh3] 2cl4

2.1.4 organic amine is used in combination with acid

organic amine itself contains amino NH: active, and the addition of organic amine can promote the welding effect. In order to reduce the corrosion effect of the flux on the copper plate, a certain amount of corrosion inhibitor can be added to the prepared flux, and the organic amine is usually selected as the corrosion inhibitor. The mixture of organic acid and organic amine will produce neutralization reaction and produce neutralization products. This neutralization product is unstable. It will decompose rapidly at the welding temperature and regenerate organic acids and amines, which can ensure the original activity of organic acids. After welding, the remaining organic acids will be neutralized by organic amines, reducing the acidity of residues and corrosion. Therefore, the addition of organic amines can not only adjust the acidity of the flux, but also make the solder joint bright. Without reducing the activity of the flux, the corrosion after welding is minimized [6]

at present, it is most suitable to combine organic amines with strong wetting ability with organic acids. For example, Xue Shuman and others introduced the flux with aliphatic dibasic acid, aromatic acid or amino acid as the active ingredient in patent [7]. In addition, adding a small amount of glycerol to the flux will not only contribute to the storage stability of the flux, but also contribute to the activity of the activator. Zhangmingling added dibromo succinic acid, dibromo butene glycol, dibromo styrene, etc. to the flux to enhance the activity of the flux [8]. At low temperature, carboxylic acids (including dicarboxylic acids) have moderate activity, and their high-temperature activity is significantly improved; The higher activity is the hydrogen halite or organic acid salt of organic phosphate, sulfonic acid and organic amine (including hydrazine); The activity of halogenated compounds and their substituted acids depends on their specific structures

2.2 role of other ingredients

flux also contains many other useful ingredients. The main function of solvent is to dissolve the components contained in the flux, as the carrier of each component, making it a uniform viscous liquid. Generally, they are alcohols, esters, alcohol ethers, hydrocarbons, ketones, etc. The alcohol with high boiling point has good protective effect, but it has high viscosity and is inconvenient to use; Alcohols with low boiling points have low viscosity but poor protection, so the method of mixing alcohols can be considered [9][10]. Generally, it is a mixture of high boiling point and low boiling point alcohols, and some use water-soluble alcohols and water-insoluble ethers as solvents [11]. Li Weihao uses the hyperbranched structure and water-soluble polymer with an average molecular weight of 2000 as the flux carrier. The hyperbranched molecular structure can be divided into about 1000 subclasses, which can not only improve the thermal decomposition temperature of the polymer, but also reduce the viscosity of the polymer and enhance the permeability and wettability of the polymer [12]. The main function of surfactant is to reduce the surface tension of flux and increase the wettability of flux to solder powder and pad

compared with Sn Pb (), non lead solders (such as sac 3o5, etc.) have higher melting point, higher surface tension, longer treatment time at high temperature, and higher internal stress during rapid cooling. Therefore, surfactants play a more prominent role in improving the reliability of non lead solder welding interconnection. They can be nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants and fluorinated surfactants

corrosion inhibitors are generally pyrrole, such as benzotriazole (BTA), which is a high-efficiency inhibitor of copper. Its addition can inhibit the corrosion of copper plate caused by the active agent in the 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 formula of the membrane is BTA 4 cu3cl2? H2O and (bta2cu) 2cucl2? H2O, and the polymer is parallel to the surface of metal copper, which is very stable. The film formation of BTA on Cu 2O layer is easier than that on CuO layer, and the thickness of the film is nearly twice as thick. When the concentration of BTA is greater than 1 mol/L, the corrosion of copper can be well inhibited [9]

the main function of antioxidant is to prevent solder oxidation, which is generally phenols (hydroquinone, catechol, 2,6-di-tert-butyl-p-cresol), ascorbic acid and its derivatives. Especially in water-soluble flux, there must be antioxidant. F? J? Continuously improve the method and logic of oscillation test. Jasky adds polynuclear aromatic compounds to the flux, releasing N2 during heating to form an inert atmosphere to prevent oxidation [13]. Hydrocarbon, alcohol and grease are selected as film-forming agents. These substances generally have good electrical properties. They play a protective film role at room temperature and are not active. They show activity at the welding temperature of 200 ℃ ~300 ℃, and have the characteristics of no corrosion and moisture resistance

thixotropic agent is mainly used to give the solder paste a certain thixotropic property, that is, the viscosity of the solder paste becomes smaller under stress, so as to facilitate the printing of solder paste. After printing, its viscosity will increase in a stress free state to maintain its inherent shape and prevent the solder paste from collapsing. The main function of Thickener (also known as tackifier) is to increase the viscosity of the flux, so as to give the solder paste a certain viscosity, which is convenient for pasting the components to be welded. Interface compound growth inhibitor: the alloy coating formed on the copper surface of the pad contains intermetallic compounds (IMC), whose composition and thickness determine the weldability during assembly and welding

for example, when Cu 3Sn and Cu 6Sn5 are formed in hot air leveling, the former has poor weldability, the latter has good weldability, and the solder layer on the top layer is the best welding; In Sn Pb (), the IMC thickness is small due to the coverage of low eutectic alloy sn/pb; In non lead solders, other metals (such as Ni or CO in sn100cl) are often added to affect the thickness of IMC layer [14]; In the flux, oxalic acid, 2-aminobenzoic acid, quinoline, quinoline-2-carboxylic acid and so on are often added. Such compounds can form an interface compound deposition layer at the solder and interface, which can inhibit the atomic diffusion between solder and substrate, thus hindering the growth of intermetallic compounds [15]

3 improved method

3.1 microcapsule method

in order to reduce the corrosive problem caused by the acidic substances of the active agent, Chen Qiyin et al. [9][10][16][17] adopted microcapsule technology. they

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