Surface insulation impedance test
In the experiment, a certain amount of flux was evenly coated on a comb electrode or a ring electrode made of a specific material and dried at a temperature of about 85℃ for 30 minutes as a test piece. First, measure the insulation resistance of the specimen under normal conditions, then put the specimen in a constant temperature and humidity box with a temperature of (40 2)℃ and a humidity of about 90%, take it out after 96 h, and put it in a box with a temperature of (20 2)℃. Only when the surface insulation resistance is greater than108 Ω can the reliability requirements be met.
Foreign countries have higher requirements for the surface insulation resistance of non-cleaning flux, and generally require bias voltage and long-term hot flashes. Observe the aging effect of flux residue after welding on surface insulation resistance, so as to measure the reliability of no-clean flux.
Corrosion test of bronze mirror
Drop the non-cleaning flux to be tested on the copper plate (40.0mm×40.0 mm×0.2 mm) to make it flow naturally, then put it in an oven at 80℃ for 2 h, take it out and cool it, and then put it in a wet box (temperature 40℃, humidity 93%) for 72 h to check the color change of the copper plate. If the color turns dark green, it will corrode, if the color remains unchanged or remains.
Non-adhesion test
Sprinkle chalk powder on the surface of solder coated with no-clean flux, and then wipe it off without adhesion; When tested by gauze method, there is no flux residue on the gauze, and there is no obvious gauze trace on the test board. The results show that the no-cleaning flux has excellent non-adhesion performance.
solderability
Put HLSnPb50(D8 mm×4 mm) solder in the center of a clean copper plate (50 mm×50mm× 1 mm) coated with non-cleaning flux, drop two drops of flux on the solder respectively, then put it in a thermostat at 275℃ 1 min, take it out and measure its overflow area. According to the overflow area, the strength of soldering aid can be judged.
Composition and function of non-cleaning flux
The components of no-clean flux include solvents, activators and other additives. Other additives include surfactants, corrosion inhibitors, film-forming agents and antioxidants. Users can choose the appropriate flux according to the type, composition and welding process conditions of solder, so the formula of flux is flexible and varied.
3.3. 1 solvent:
It is to dissolve the components contained in the flux as the carrier of each component to make it a uniform and viscous liquid. At present, the commonly used solvents are mainly alcohols, such as ethanol and isopropanol. Although the cost of methanol is low, it has a strong toxic effect on human body, so methanol is rarely used by regular flux manufacturers at present. One of ketone, alcohol and ester or their mixture. Commonly used liquid components are ethanol, propanol, butanol, acetone, toluene isobutyl methyl, ethyl acetate and butyl acetate. Its main function is to dissolve the solid components in the flux and form a uniform solution, which is convenient for the parts to be welded to be evenly coated with the appropriate flux components. At the same time, it can also clean light dirt and oil stains on the metal surface. Generally, it is a mixture of high-boiling and low-boiling alcohols, and some use water-soluble alcohols and water-insoluble ethers as solvents.
Activator:
Organic acids or organic acid salts are mainly used, and inorganic acids or inorganic acid salts are basically not used for electronic assembly fluxes, and sometimes used for other special fluxes. 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 the oxide on the lead pin and melt the oxide on the solder surface, and it is one of the key components of flux.
Action mechanism of activator a
The main function of activator is to remove the oxide on the surface of solder pad and solder at welding temperature, and form a protective layer to prevent the substrate from being oxidized again, thus improving the wettability of solder and solder pad. Flux activators are generally composed of hydrogen, inorganic salts, acids and amines, and their compounds. Spreading power, electrolytic activity, environmental stability, chemical functional groups and their reaction characteristics, rheological characteristics, adaptability to general cleaning solutions and equipment, etc. The above-mentioned functions of flux are realized through the actions of activators, solvents, surfactants and other components.
1) hydrogen and inorganic salts
Hydrogen and inorganic salts, such as stannous chloride, zinc chloride, ammonium chloride, etc. Through its reducibility, it reacts with oxides, such as hydrogen in gas flux, and water is its 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 the metal surface from being oxidized again before welding is completed.
2) Organic acids
Acidic activators (such as halogen acid, carboxylic acid and sulfonic acid) are mainly due to the reaction of H+ with oxides, such as carboxyl groups of organic acids and metal ions removing oxide films of pads and solders in the form of metal soaps;
CuO+2RCOOH - Cu(RCOO)2+H2O
Subsequently, the organic acid copper decomposes and absorbs hydrogen to produce organic acid and metallic copper:
Copper (RCOO) 2+H2+M-2RCOOH+M-Cu
Rosin is represented by the molecular formula C9H29COOH. Because it contains carboxyl group, it has a certain welding aid effect at a certain temperature. At the same time, rosin is a polymer polycyclic compound, so it has certain film-forming property, can transfer heat and cover during welding, and can protect the metal from being oxidized again after the oxide film is removed.
At present, there are single organic acids and mixed acids as activators. The boiling point and decomposition temperature of these acids are different to some extent, so that the boiling point of flux and decomposition temperature of activator can be distributed in a larger range.
3) organic halide
Such as carboxylic acid halides and organic amine hydrohalides. Zhang Yinxue uses salicylic acid bromide as an activator. At brazing temperature, hydrogen bromide and salicylic acid can be thermally decomposed to dissolve oxides on the surface of base metal. In addition, the hydroxyl and carboxyl groups of salicylic acid can react with ⅲ resin to cross-link into a polymer resin film during brazing, covering the surface of solder joints.
Hydrohalide salts of organic amines, such as aniline hydrochloride, react with copper on the substrate during welding to form CuC 1 and copper complexes. As a result, the generated copper compound mainly reacts with tin in the molten solder to generate metallic copper, which immediately melts into the solder. Through these reactions and the melting of copper in solder, solder flows on the copper plate. The reaction is as follows:
Cu+2C6H5NH2。 HCl - CuC 12+2C6H5NH2+H2
CuCl2+2C6H5NH2。 Copper hydrochloride [C6H5NH3]2Cl4
4) Combined use of organic amine and acid
Organic amines themselves contain amino groups. NH: It is active, and adding 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 organic amine is usually selected as the corrosion inhibitor. When organic acids and organic amines are mixed, neutralization reaction will occur, resulting in neutralization products. This neutralization product is unstable, and will decompose rapidly at welding temperature, and regenerate organic acids and organic amines, thus ensuring the original activity of organic acids. After welding, the residual organic acid will be neutralized by organic amine, reducing the acidity of the residue and reducing corrosion. Therefore, after adding organic amine, not only the acidity of flux can be adjusted, but also the solder joint can be bright, and the post-weld corrosion can be minimized without reducing the activity of flux.
At present, it is most suitable for the combination of organic amine and organic acid with strong wetting ability. For example, Xue et al. introduced a compound flux with aliphatic dibasic acid, aromatic acid or amino acid as active components in the patent.
In addition, adding a small amount of glycerol to the flux not only contributes to the storage stability of the flux, but also contributes to the activity of the activator. Zhang Mingling added dibromosuccinic acid, dibromobutanediol and dibromostyrene to the flux to enhance its activity.
Carboxylic acid (including dicarboxylic acid) has moderate low-temperature activity and obviously improved high-temperature activity: organic phosphate, sulfonic acid, organic amine (including hydrazine) halide or organic acid salt has high activity; The activity of halogenated compounds and their substituted acids depends on their specific structures.