Ultrasonic cleaning technology in foreign countries is a very mature cleaning technology, widely used in machinery, electronics, and other fields. The cleaning of the circuit board after welding is one of the most important applications. Domestic applications in this area are also more common. However, it can not be denied that many manufacturers do not have a clear understanding of the basic principles of ultrasonic cleaning. Therefore, there is a very irregular aspect in the application of ultrasonic cleaning technology, it doesn't even work like an ultrasonic cleaning. Some manufacturers, nominally use ultrasonic cleaning, but only as a general immersion cleaning tank use, because the ultrasonic cleaning process is not reasonable, and simply can not clean the circuit board. Not only does not play the ultrasonic cleaning quality, and high efficiency the advantages, on the contrary, it may damage the circuit board and its components.
Selection of ultrasonic cleaning process and parameters for circuit board
Ultrasonic cleaning has several important technological parameters: frequency, power, time, cleaning solvent, circuit board placement, and ultrasonic intensity. The wrong ultrasonic cleaning process is precisely in these issues did not grasp.
The importance of frequency
When the working frequency is very low (within the range of human hearing), the noise will be generated. When the frequency is below 20 kHz, the noise at work not only becomes very high but may exceed the limits of safe noise under the Occupational Safety and Health Act or other regulations. For applications requiring high power to remove dirt without considering surface damage, the lower cleaning frequency from 20 kHz to 30 kHz is generally preferred. The cleaning frequency in this frequency range is often used to clean large, heavy parts or high-density materials of the workpiece. High frequencies are often used to clean smaller, more precise parts or to remove minute particles. High frequencies are also used in applications where the surface of the workpiece does not permit damage. The use of high frequency can improve cleaning performance in several ways. As the frequency increases, the number of cavitation bubbles increases linearly, creating more and denser shock waves that allow them to enter smaller gaps. If the power remains constant and the cavitation bubble becomes smaller, the energy released by the cavitation bubble is correspondingly reduced, thus effectively reducing the damage to the surface of the workpiece. Another advantage of high frequencies is the reduction of the viscous boundary layer (the Bernoulli effect), which allows ultrasound to "detect" very small particles. This is similar to the way small stones can be seen at the bottom of a stream as the water level drops. Circuit board because there are a variety of components and even more delicate devices, and the circuit board surface is uneven, there are many small slits, so the board of course must be cleaned at a higher frequency. Foreign data suggest frequency in 33kHz ~ 66kHz. But in reality, some people think that the higher the frequency is more likely to cause damage to the circuit board, and low frequency is often safe, this is a completely wrong view.
Selection of chemical solvent
Steam pressure, surface tension, viscosity, and density should be the most significant factors considering the effect of the physical properties of cleaning solution on ultrasonic cleaning. Temperature can affect these factors, so it also affects the efficiency of cavitation. Four factors should be considered when selecting a cleaning solution:
(1) cleaning efficiency: experiment to select the most effective cleaning solvent.
(2) simple operation: the liquid used should be safe and non-toxic, simple operation, and long service life.
(3) cost: the cheapest cleaning solvent is not necessarily the cheapest to use. Use must take into account the solvent cleaning efficiency, and safety, a certain amount of solvent can be cleaned how many parts, and the highest utilization factor. Of course, the selected cleaning solvent must achieve the cleaning effect and should be compatible with the workpiece material to be cleaned. Water is the most common cleaning fluid, so the system using the water-based solution is easy to operate, low-cost, and widely used. However, some materials and dirt are not suitable for aqueous solutions, so there are many solvents available. One of the relatively high cost-performance ratios of alcohol solvent.
(4) the amount of chemical solvent: the amount of solvent general ultrasonic cleaning machine instructions will be stated, in general, must be more than two-thirds of the capacity of the cleaning tank. To save costs, some people often use very little solvent or even just cover the circuit board lying flat at the bottom of the tank. This is not much different from the air vibration, not only can not play the role of ultrasound, but on the contrary, it may damage the circuit board and components.
Power and time of ultrasonic cleaning
The power and time of ultrasonic cleaning must be determined by experiment. With too much power and too long cleaning time, the circuit board and components may be damaged.
Treatment of cleaning circuit board
Another consideration for ultrasonic cleaning is the design of the loading, loading, and unloading of the PCB, or the tooling for placing the PCB. When the circuit board is in the ultrasonic cleaning tank, neither the circuit board nor the circuit board basket shall touch the bottom of the tank. The total cross-sectional area of the circuit board shall not exceed 70% of the cross-sectional area of the ultrasonic groove. Rubber and non-rigid plastics absorb ultrasonic energy, so care should be taken when using these materials in tooling. The efficiency of even the best ultrasonic cleaning system can be greatly reduced by the improper design of the work basket or the weight of the workpiece. Any material, if the mesh is higher than 50 mesh, for the ultrasonic performance of the entity, the ultrasonic reflection back. When the mesh size is larger than 0.635 cm, the open-type material can be used for ultrasonic waves. Hooks, shelves, and beakers are used to support the circuit board. In practice, the circuit board is often stacked randomly in the cleaning tank of the ultrasonic cleaning machine, which can not play the unique role of the ultrasonic cleaning, the ultrasonic cleaning machine is equivalent to the ordinary immersion cleaning tank.
Ultrasonic intensity
Ultrasonic intensity is the ultrasonic power per unit area. The effect of ultrasonic cleaning depends on cavitation, but cavitation is related to ultrasonic intensity. The greater the ultrasonic intensity, the more obvious the cavitation, that is, the better the cleaning effect. In addition, according to different cleaning objects, choose the appropriate ultrasonic intensity. Such as cleaning circuit boards, ultrasonic intensity can be lower, and for cleaning machinery parts ultrasonic intensity can be higher. Some people blindly believe that the higher the ultrasonic intensity, the better, to clean there is no need at all, and the higher the intensity, the greater the possible harm to the circuit board.
The ultrasonic cleaning process is a low-cost, safe and efficient cleaning process. Because many manufacturers are not familiar with the principle of ultrasonic cleaning, it leads to many misunderstandings in use, not only can not clean the circuit board, on the contrary but may also cause damage to the circuit board. The satisfactory cleaning effect can be obtained only by carefully considering the technological parameters of ultrasonic cleanings, such as frequency, power, time, cleaning solvent, placing mode of the circuit board, and ultrasonic intensity.
