Sunday, January 16, 2022

Test method for surface cleanliness

<p> Cleanliness testing</p> <p> Cleanliness measurement methods are very important for process control, quality assurance, and failure analysis. They are generally used to obtain cleanliness data about measurement subjects such as various mechanical equipment, electronic parts, etc. Detailed process. </p> <p> There are requirements for sampling when testing cleanliness. The basic requirements for sampling are determined by the number of samples and the sampling location. The larger the volume of the part, the larger the surface area, and the lower cleanliness, the number of samples will decrease accordingly. Parts should be randomly selected from the production, and no contamination of the parts should be caused during the sampling process and the subsequent inspection process. </p> <p> Typical types of pollutants</p> <p> When testing cleanliness, firstly, the environment should be clean, and the cleanliness should be compatible with the requirements of the test; secondly, the clothes and hands of the test personnel should be clean; Third, the utensils used must also be clean. Cleanliness measurement methods</p> <p> There are many cleanliness measurement methods, divided into two categories: oil pollution and particulate pollution. The main types are as follows: </p> <p> * Visual inspection method </p> <p> The visual inspection method is to manually inspect the outer surface or the inner cavity surface of the part that can be seen directly under the microscope with the eyes. Adjust the illumination brightness and magnification of the microscope, artificially can judge whether the pollution particles are metal, non-metal, or fiber and size. The visual inspection method can check the relatively large and obvious particles, spots, rust spots and other pollution remaining on the surface of the parts, but the result of the inspection has a lot to do with human factors. </p> <p> * Contact angle method (also called water drop angle method)-measuring grease pollutants</p> <p> The so-called contact angle is the thermodynamic equilibrium held by the liquid on the solid surface Horn. The measurement of the contact angle formed between a solid and a liquid is an analysis technique that is widely known in many similar fields such as surface treatment and polymer surface analysis. It is a surface analysis technique that is very sensitive to changes in a single layer of multiple units. The contact angle of the liquid drop on the solid surface is measured to evaluate the wettability of the surface. If the droplet can wet the surface, the contact angle is small. On the contrary, if the droplet cannot wet the surface, and beads or bubbles tend to form on the surface, the contact angle is large. This is the principle of the “water film residue” test. A large contact angle indicates that the surface is contaminated by hydrophobic dirt (oil/grease, etc.). On the contrary, a small contact angle and droplets are broken or thinned, indicating that the surface is clean. This test method is also greatly affected by the material of the substrate, the roughness of the substrate and human factors, and this method is not easy to identify very light or scattered dirt. Especially some special materials (such as PTFE plastic) have a large contact angle to most liquids even if the surface is very clean. Therefore, the contact angle method is not suitable for testing certain substrates or critically important surface cleanliness. </p> <p> * Fluorescence luminescence method-measuring grease pollutants </p> <p> In many cases, ultraviolet light can be used to detect the cleanliness of the surface of the part. Under the irradiation of ultraviolet light, the pollutant particles on the surface will fluoresce. Because the energy of ultraviolet rays is absorbed by the dirt, the electrons of the dirt particles are excited and jump to the high-level electron layer. The unstable electrons at the high-energy level will then return to the original low-level electron layer. In the process, the original absorbed energy is It is released in the form of heat and light-fluorescence. The frequency of this activation release is thousands of times per second, so the fluorescence under ultraviolet light is not flickering but continuous and stable. According to the fluorescence, the position of the dirt on the surface of the part can be visually detected. The fluorescence intensity can also be used with signal detection equipment. The measurement thus indicates the degree of contamination of the surface. But if you want to identify the composition and other characteristics of the pollutants, you must rely on other analysis methods. The German SITA Surface Cleanliness Meter adopts the principle of fluorescence excitation method and can be used to detect grease and oil pollutants on the surface of parts</p> <p> * Particle size quantitative method-particle pollutants measurement</p> <p> This It is a new method for determining the cleanliness of parts. The basic principle is that the surface to be detected and the pollutant particles have different light absorption or scattering rates. The test method is to clean a certain number of parts under certain conditions, fully filter the filter membrane through which the cleaning solution passes, and collect the dirt on the surface of the filter membrane, then dry the filter membrane with a microscope (the best equipment is with Image recognition and analysis equipment with shooting function) is detected under light irradiation, and the dirt particles are counted according to the particle size and quantity, and the result of solid particle contamination of the measured object parts can be obtained. This is a cleanliness detection method suitable for quantification of precision cleaning, especially for detecting tiny particles and colored impurity particles. But if the filter membrane is white, the recognition of white dirt and bubbles may cause misjudgment. </p> <p> Limit value of particle size quantity method: For parts of specific specifications, when a certain number of samples, inspection frequency, cleaning medium, cleaning parameters and operation process are specified, the particles are counted according to size, and each size The ranges respectively stipulate the maximum allowable number of particles. As long as one item exceeds the standard, the test conclusion is unqualified. </p> <p> * Gravimetric method-measuring particulate pollutants </p> <p> Gravimetric method is the most commonly used cleanliness determination method in industrial production and testing. The measurement principle is to clean a certain number of samples under certain conditions, and then fully filter the cleaned liquid through the filter membrane. The dirt is collected on the surface of the dried filter membrane, and the filter membrane is fully dried again. According to the analysis The balance weighs out the mass of the dried filter membrane before and after the filtration and cleaning, and calculates the added value to be the mass of the solid particle contaminants on the test sample. </p> <p> Typical limit of gravimetric method: For parts of specific specifications, the maximum allowable weight of residual dirt under certain sample quantity, inspection frequency, cleaning medium, cleaning and filtration methods, unit is mg Or ug. </p> <p> The choice of cleaning fluid</p> <p> There are many types of cleaning fluids used to determine and analyze cleanliness. During use, it can be used according to the requirements of the measurement method and the detergency. Corrosion and rust, whether it is harmful to the human body, whether it is flammable and explosive, whether it has a chemical reaction to the filter membrane, whether it can be recycled, the impurity content, the price, whether it is easy to equip and other factors to choose. </p> <p> At present, the commonly used ones at home and abroad are: </p> <p> * NY-120 solvent oil</p> <p> * Anhydrous ethanol</p> <p> * Trichloromethane< /p> <p> * Aviation washing gasoline</p> <p> * 95% ethanol</p> <p> * Trichloroethylene</p> <p> * Isopropanol</p> <p> * Carbon tetrachloride</p> <p> * Distilled water and demineralized water</p> <p> * Liquid detergent without solid particles</p> <p> Choice of filter membrane</p> <p> There are also many kinds of filter membranes used to determine and analyze cleanliness. The basic requirements are: flat and smooth, easy and constant weight, high drying precision, no negative value during filtration, and convenient operation. When in use, it can be selected according to the requirements of the measurement method, whether it has a chemical reaction with the cleaning fluid, the required filtration accuracy (ie, the pore size), whether it is qualified, the price, whether it is easy to equip and other factors. </p> <p> At present, the commonly used filter membrane materials at home and abroad are: </p> <p> * Mixed cellulose ester membrane (MCE) </p> <p> * Polyvinylidene fluoride membrane (PVDF) </p> <p> * Nylon membrane (NYL) </p> <p> * Polycarbonate membrane (PC) </p> <p> * Polytetrafluoroethylene membrane (PTFE) </p> <p > * Polyethersulfone (PES) </p> <p> At present, the commonly used filter membrane pore size (um ): </p> <p> 0.05, 0.22, 0.3, 0.45, 0.65, 0.8, 1.2, 3, 5 , 8, 11, 20, 30, 41, 60, 80, 100. </p>

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