The application of ultrasonic cleaning technology in China has achieved good results. 1. Cleaning of mechanical parts before and after electroplating or before spraying, and cleaning of disassembled and repaired parts require high cleanliness, such as cleaning of oil pump fittings, bearings, brakes, fuel filters and valves. The second is the cleaning of printed circuit boards, silicon wafers, wafers, component housings, seats, railway system signal control relays, components, connectors, picture tubes and electric vacuum devices. The third is the cleaning and sampling of glass sheets in optical systems such as glass, microscope, telescope and sight. The fourth is the cleaning of various bottles and cans used in experiments such as medical equipment, food, pharmacy and biochemistry. The fifth is the cleaning of spinnerets, precision molds, precision rubber parts, jewelry and handicrafts.
There are nearly 40 ultrasonic cleaning equipment manufacturers in China, but their distribution is mainly concentrated in the southeast coastal areas. According to statistics, manufacturers in coastal areas account for 85% of the national total, which shows that the application of ultrasonic cleaning technology in economically developed areas is not only a priority, but also extensive and highly popular. At the same time, it also proves that the prospect of ultrasonic cleaning technology in the central and western regions is very broad. As far as the product level is concerned, the technological progress of contemporary products is also very obvious compared with the 1970s and 1980s.
In recent years, due to the technical transformation of the traditional cleaning process of automobile brake production line and refrigerator compressor production line, it is planned to adopt ultrasonic cleaning process. In foreign countries, ultrasonic cleaning of automobile chassis frame and car shell before spraying, rust removal, oxide film removal and phosphating can be cleaned at one time with special cleaning solution, and then painted after drying.
American Advanced Sonic Proctssing SV Systems Company has introduced a series of equipment for cleaning a large number of coal or precious metal minerals, such as cleaning the soil and colloidal substances on the surface of metal granular minerals, so that chemicals can play a better role; Washing pulverized coal to remove ash and sulfur, etc. The processing speed is more than ten tons per hour.
The application report of American Dvpont Company in New Jersey Pharmaceutical Factory pointed out that ultrasonic cleaning can remove the dirt on the surface of reaction tank or chemical treatment barrel shell. Compared with ordinary methods, it saves energy, reduces cost and reduces environmental pollution. The cleaning process is simple. As long as the dissolver is filled with water, heated to 65℃ and treated with 2% surfactant for 2-4 hours, it can be cleaned.
Some European manufacturers have cleaned 9. 1m3 tanks. In the past, methanol was heated to the boiling point for 4-8h at one time and washed five times. In addition, ultrasonic cleaning can meet the requirements only once, which not only saves solvents, improves efficiency, but also reduces environmental pollution.
With the more and more extensive application of ultrasonic cleaning equipment, all kinds of new ultrasonic cleaning equipment that are constantly improved and perfected are replacing the old equipment that has been put on the market.
Megahertz ultrasonic cleaning technology refers to the use of 700 kHz -2 MHz ultrasonic cleaning. Cleaning system generally consists of piezoelectric sensor display, cleaning container and cleaning liquid, high-frequency power generator and control circuit. For some specific cleaning objects, it is sometimes equipped with hot air drying, special cleaning rack (basket) and cleaning liquid filtration and circulation system.
The main characteristics of megahertz ultrasonic cleaning technology are as follows: first, it avoids surface damage of high-finish objects; Second, submicron particles attached to the surface can be removed; Thirdly, when immersed in liquid, the side facing the transducer can be cleaned, so both sides should be cleaned.
At present, there are commercial megahertz ultrasonic cleaning equipment in foreign markets. Verteq, Imtec and ProSys in the United States have all developed this kind of equipment for semiconductor production lines. In the cleaning of 100-300mm silicon wafer, tiny particles as small as 0. 15μ m on the surface of the silicon wafer can be removed, and the cleaning process can be accelerated, thus effectively preventing particles from reattaching to the surface of the silicon wafer. Megahertz ultrasonic cleaning is an indispensable standard equipment in the production process of many large-scale integrated circuit manufacturers abroad.
Action principle of ultrasonic wave
The principle of ultrasonic cleaning is complicated in theory, involving many factors and functions. There are three main points that can reflect the function of ultrasonic cleaning.
(1) hole interaction
When powerful ultrasonic waves are radiated into the liquid, the cleaning liquid changes near the static pressure (standard air pressure). When the pressure is lower than zero pressure, the oxygen dissolved in the liquid will form tiny bubble nuclei, and then countless tiny cavities (holes) close to vacuum will be generated. The tiny cavity under ultrasonic positive pressure is crushed under adiabatic compression. This powerful shock wave can directly destroy pollutants and disperse them in liquid at the moment of crushing, forming a cleaning mechanism. In the experiment, this powerful cleaning effect can corrode aluminum foil into countless small holes in dozens of seconds.
Cavitation cleaning has a good effect on degreasing. The cleaning of other mechanical parts is generally carried out at the frequency of 28KHZ~50KHZ, and the ultrasonic intensity of the cleaning machine is mostly set at 0.5 ~ 1w/cm2.
(2) Acceleration
When the cleaning liquid is irradiated by ultrasonic waves, the liquid molecules vibrate. The vibration acceleration is 65,438+003 times that of gravity at 28KHZ, and will reach 65,438+005 times at 950KHZ. Through this intense acceleration, the contaminated surface can be stripped and cleaned. However, 950KHZ ultrasonic wave does not produce holes, so it is not suitable for degreasing cleaning. Only submicron particle pollution in semiconductor manufacturing in electronic industry can be cleaned.
(3) promoting physical and chemical reactions
Cavitation causes local high temperature and high pressure of liquid (1000 air pressure, 5500℃), and then the agitation caused by vibration promotes the multiplication of chemical or physical effects, and the liquid is continuously emulsified and dispersed, further promoting the rate of chemical reaction.
Determine the depth of the cleaning solution
When ultrasonic waves are in liquid, due to the mutual interference and strong combination of traveling waves and echoes, a "standing wave" phenomenon will be formed (see figure 1). By determining the depth of the liquid that generates the standing wave, the best ultrasonic radiation effect can be obtained. The depth of the liquid generating the standing wave can be calculated by the following formula.
Liquid depth (λ/2) = sound speed/frequency ÷2
The positive multiple of this liquid depth is also the most suitable depth. For example, the liquid depth at 20℃ is 27mm, 54mm, 8 1mm, and the liquid depth at 38KHZ is 21mm. However, different liquids, liquid temperatures and standing waves of ultrasonic vibrators are different. See table 1.
Table 1 standing wave generation comparison
The sound speed of cleaning solution λ/2
Water 20℃1483mm 27mm
Freon at 20℃ 717mm13mm
Ipa 20℃1168mm21mm
Acid and alkali cleaning agent 20℃ 1483mm 27mm
Setting of ultrasonic generation mode and cleaning conditions
The generation mode of ultrasonic wave is shown in Table 2, which can be selected according to different cleaning purposes. At present, a continuous oscillation mode capable of strong cleaning is usually used. Frequency modulation and multi-frequency methods have many uneven cleaning phenomena, which are not suitable for cleaning seriously polluted objects.
Table 2 Generation Mode of Ultrasonic Wave
Square internal capacity characteristic
The amplitude and frequency of continuous oscillation are fixed and can be cleaned forcefully. Standing wave makes cleaning uneven, so it is necessary to increase shaking to achieve cleaning uniformity.
Widening modulation amplitude variation has good degassing effect, good cleaning performance for different objects and high noise.
Frequency modulation (FM oscillation) oscillation frequency can be changed by thousands of hertz to be clean and uniform. Poor cleaning efficiency and low average output power.
At the same time, multi-frequency and multi-frequency occur at the same time, forming a balanced sound field, which is clean and uniform, and it is difficult to obtain powerful ultrasonic waves.
Multi-frequency alternation, multi-frequency cleaning balance appears at each frequency, and it is not easy to get strong cleaning.
Cone radiation cleaning uses a vibrating body made of stainless steel for ultrasonic radiation. Generally, conventional ultrasonic waves are used in occasions where cleaning is insufficient, and the intensity can reach 10 or 20 times, with high performance. But the cleaning surface is small and the noise is high.
The selection and setting of cleaning conditions mainly include the following points.
Cleaning position: the best cleaning effect can be obtained by placing the cleaning object at the position with the maximum standing wave pressure. However, when cleaning objects larger than standing waves, uneven cleaning is easy to occur. At this time, the object should be shaken within tens of millimeters, which is a common method to reduce poor cleaning.
Attenuation caused by mesh: When cleaning small parts, the basket method is often used. Improper mesh size of the basket will cause ultrasonic attenuation and reduce cleaning power. For example, in the case of 28KHZ, the mesh diameter of the basket needs to be greater than 5mm to be cleaned normally. For example, when cleaning small screws, the mesh should be at least1mm. If the attenuation is large, the normal cleaning effect can also be obtained by using 0. 1-0.5 mm thin plate mesh blue.
Frequency: Regarding the cleaning effect related to frequency factors, it can generally be considered that low frequency is used for dirt that is difficult to clean, and high frequency is more suitable for precision cleaning occasions.
Liquid temperature: With the increase of liquid temperature, bubbles living in the liquid will block sound waves and weaken ultrasonic waves, but in the conventional practice, the cleaning ability is increased by increasing the liquid temperature. Appropriate liquid temperature should be determined according to different cleaning liquids and cleaning objects. In general, the liquid temperature is 5060℃.
Cleaning method and cleaning device
The setting of cleaning process should be determined according to the pollution type, pollution degree and processing batch. For example, glass cleaning generally requires 10 process. When using water-based cleaning agents, the most basic procedures are as follows:
Ultrasonic cleaning (water cleaning agent) → ultrasonic cleaning (pure water and tap water) → dehydration (drying)
Drying treatment is very important for the cleanliness of washing items. Common drying methods include hot air drying, ventilation drying, vacuum drying, centrifugal dehydration drying, IPA lifting drying and so on. It can be selected according to the production batch, cost, product accuracy, shape of washing items, etc.
Industrial ultrasonic cleaning machines are mostly single-tank or double-tank, and there are also multi-tank cleaning machines in the form of automatic cleaning. In recent years, the cleaning methods used in semiconductor industry are mostly single-chip "US spray" high-frequency cleaning combined with 950KHZ ultrasonic tank, which can obtain high-performance cleaning effect. The "beautiful spray" method is to use 950KHZ ultrasonic water curtain to carry out ultra-precision cleaning on liquid crystal glass and circuit chips, and the dust particles can be close to zero.
In the future, if the wet cleaning of different products needs to play the role of cleaning agent 100%, higher requirements will be put forward for ultrasonic cleaning devices.