Cleaning agent cleaning principle
The cleaning of flux residues is mainly accomplished by dissolution. Rosin or organic acids and their tin or lead salts have a degree of solubility in the detergent and are removed by transferring the residue from the circuit board to the detergent. The cleaning methods include manual immersion or brushing, ultrasonic cleaning, etc. . Because of the complex composition of flux, cleaning is more difficult, and is not easy to clean. At present, in addition to small-batch production and low-demand occasions using manual cleaning, the vast majority of the use ultrasonic cleaning machine cleaning.
Cleaning principle of ultrasonic cleaning machine
According to the nature of an object cleaning dirt, the use of chemical cleaning methods or mechanical physical methods, of course, can also be carried out in a variety of combinations of ways to clean. Ultrasonic cleaning is a physical force cleaning, if the cleaning fluid in the addition of a cleaning agent, is a combination of cleaning, more obvious cleaning effect. The principle of ultrasonic cleaning is as follows: put the liquid into the cleaning tank, the role of ultrasound in the tank. Because ultrasonic waves and sound wave is a kind of dense vibration waves, the pressure of the medium changes alternately. If a certain point in a liquid is observed, and the pressure at this point is centered on a static pressure (generally an atmospheric pressure), the pressure increases and decreases, and the amplitude of the pressure increases as the intensity of the ultrasonic wave increases in turn, and produce negative pressure. So-called negative pressure, actually negative pressure does not exist, only in some areas of the liquid to produce tearing force. And the formation of a vacuum of the bubble, and be squeezed behind the collapse of the compression. When the sound pressure reaches a certain value, the bubbles will grow rapidly and then close suddenly. When the bubbles close, the liquids collide with each other to produce a strong shock wave, the cleaning process is caused by the following three effects.
(1) because of the strong shock wave produced when the bubble burst, a part of the dirt layer is peeled off under the action of the shock wave, i. e. dispersing, emulsifying, and shedding.
(2) the air bubbles produced by cavitation, the gap and the space between the impingement dirt layer and the surface, because the small air bubbles expand and contract synchronously with the sound pressure, the same physical force as the skin repeated the action of the dirt layer, layer by layer was peeled off, small bubbles continue to move forward, until the dirt layer was peeled off so far. This is the cavitation secondary effect.
(3) the contribution of ultrasonic vibration of cleaning fluid to cleaning.
(4) the cleaning agent also dissolves the dirt and produces the chemical force of emulsifying and dispersing.
Analysis and solution of some problems in ultrasonic cleaning of the circuit board
Whitening occurs after cleaning the circuit board, indicating that the residue on the board has not been cleaned. The white stuff contains rosin, polymer rosin crystals on the one hand, and tin and leads salts of small organic acids on the other. When using ultrasonic cleaning and equipped with a preheating device, the cleaning process meets the requirements. Under normal conditions, the ultrasonic generator may be unable to heat the cleaning agent, the air temperature is lower, and the air humidity is high, therefore, there is a lack of cleaning agent cleaning force, which cause the phenomenon of the whitening circuit board after washing. For cleaning the circuit board with the cleaning agent, spray protective paint on the welding surface after drying. The paint appears non-wetting in an area, forming a water droplet. This may be due to the operation of some boards with the operator's hand sweat so that some board parts appear after spray-painting into the water drop phenomenon. Improve the process flow, and avoid the production of fingerprints, problems can be solved.
Due to its unique function and efficiency, ultrasonic cleaning is now widely used in printed circuit board cleaning. It plays a key role in the reliability of electronic products. At present, the new design of ultrasonic cleaning machine has begun to adopt automatic program control. The comprehensive improvement in all aspects not only greatly saves labor consumption, but also improves the pollution of the working environment. Ultrasonic cleaning has more and more shown its irreplaceable advantages.