How to clean your pcb properly with ultrasonic(1)–how to choose an best ultrasonic cleaner ?

Ultrasonic cleaning is a very mature technology in the industry cleaning.it is widely used in machinery, electronics and other fields. Cleaning circuit boards after soldering is one of the very important applications. However, there are so many manufacturers did not figure out the basic principles of ultrasonic cleaning.So there is no specification in the application side of ultrasonic cleaning technology, and even completely contrary to the principle of ultrasonic cleaning. Some manufacturers, nominally using ultrasonic cleaning, in fact, just as an ordinary immersion cleaning tank to use, because the ultrasonic cleaning process is unreasonable, the board can not be cleaned. Not only did not play a good cleaning quality ultrasonic cleaning, high cleaning efficiency advantages, on the contrary, it is possible to damage the circuit board and its components.

NO.1The principle of ultrasonic cleaning

Ultrasound wave like other sound waves, is a series of pressure points, i.e., an alternating compression and expansion waves. If the acoustic energy is strong enough, the liquid is pushed in the expansion phase of the wave, thereby generating bubbles; in phase compression wave , these bubbles burst or implosion moments in liquid, to produce a very effective impact, particularly for cleaning.
This process is called cavitation. Ultrasonic cleaning is based on the cavitation effect, namely the rapid formation of numerous bubbles and rapidly imploding in the cleaning solution. The resulting impact of dirt immersed in the cleaning solution inside and outside surfaces of the workpiece comes off. With the improvement of the ultrasonic frequency, the number of bubbles will increase but the bursting impact weakened, therefore, high-frequency is particularly suitable for cleaning small dirt particles without destroying the surface of the workpiece.

NO.2 parameters of ultrasonic cleaning

Ultrasonic cleaning has several important parameters: Frequency, Power, Time, Cleaning solvent, The placement of the circuit board, The ultrasonic intensity. Error ultrasonic cleaning process is also no real grasp on these issues.

• Frequency
• Solutions
• Ultrasonic power and time
• How to place PCB
• Ultrasonic intensity

1.Frequency

When the operating frequency is low ( in the range of human hearing )it will produce noise. When the frequency is below 20 kH z, working noise only becomes large , and may exceed the limits specified in the Occupational Safety and Health Act or other regulations safe noise.

      In the application of removing dirt and damage without considering the damage of workpiece surface,we will select the 20kH z ~ 40 kH z lower cleaning frequency . This cleaning frequency range are often used for cleaning large, heavy parts or high density materials of the workpiece . High frequency is typically used for cleaning smaller, more sophisticated parts, or remove tiny particles . High frequency is also used to apply surface can not be damaged . The use of high frequency can improve cleaning performance from several aspects . As frequency increases , the number of cavitation bubbles increases linearly , resulting in more and more intense shock wave so that it can enter into a smaller gap. If power is kept constant, the cavitation bubbles is getting small and a corresponding reduction in the release of energy , which effectively reduces the damage to the workpiece surface . Another advantage is to reduce the high-frequency viscous boundary layer ( Park Nuri effect ) , so that the ultrasound can ‘ find’ very small particles. This situation is like reducing the water level in the creek and then you can see the stone clear.
Because of there are various components on the board even more delicate devices and the circuit board surface is uneven, there are many small narrow Joints, so the circuit board must be cleaned with higher frequency. The data suggested that the frequency is 33 kH z ~ 66 kH z. But in reality, some people think that the higher the frequency the more likely to cause damage to the circuit board, and the low-frequency tend to be safe, this is completely wrong point of view.

2.Solutions

Taking into account the impact of the physical properties of the cleaning liquid of ultrasonic cleaning, the vapor pressure, surface tension, viscosity and density should be the most significant among the many factors. Temperature can affect these factors, it can also affect the efficiency of the cavitation effect.

When selecting the cleaning solution, you should consider the following four factors:

(1) Cleaning Efficiency: The most effective cleaning solvent choice by doing experiments.

(2)Simple Operation: liquid used should be safe, non-toxic, easy operation and long life.

(3) Cost: The cheapest cleaning solvent is not necessarily the lowest cost. Must take into account the use of the solvent cleaning efficiency, safety, the number of workpieces certain amount of solvent, the maximum utilization of other factors can be cleaned. Of course, the solvent must be selected to achieve the cleaning effect of the cleaning, and should be compatible with the material of the workpiece being cleaned. Water is the most common cleaning fluid, so easy to use water-based solution of system operation, low cost, wide application. However, for some materials, and are not applicable in an aqueous stain solution, then there are many solvents are available. Where there is a relatively high cost of alcohol solvents.

(4) the amount of chemical solvents: The amount of solvent ultrasonic cleaning machine for general use . The instruction will tell you , in general must be more than two-thirds of the cleaning tank volume. In fact, people often use very little solvent for saving costs, or even only just overshadowed tile in the bottom of the circuit board, which is actually not too much difference with empty vibration , not only can not play the role of ultrasound, the opposite may damage circuit boards and components.

3.ultrasonic cleaning power and cleaning time

Ultrasonic cleaning power and time must be determined by experiment. Too much power and too long cleaning time, there is likely to damage the circuit boards and components.

4.How to place the PCB

Another consideration is that how to place or load the circuit board. when the PCB in the ultrasonic cleaning tank, regardless of the board or the washing basket are not touch the bottom. The total cross sectional area of the circuit board should not exceed 70% of the cross sectional area of the ultrasonic bath. Rubber and non-metallic plastic absorbs ultrasonic energy, it will be held carefully for such materials . Poor basket designed, or too heavy workpieces, even the best ultrasonic cleaning system will be greatly reduced. Any material, if the mesh >5mm, the ultrasonic waves will show normal performance and reflected back. When the mesh is greater than 0. 635 cm, the ultrasound only showed a performance of open materials. Hooks, shelves, and the beaker can be used to support the circuit board. In practice, people often casually stacked circuit boards in the ultrasonic cleaning tank , so it can not play a unique role in ultrasonic cleaning, ultrasonic cleaning machine which is equivalent to the ordinary immersion tank.

5.Ultrasonic intensity 

Ultrasonic intensity means ultrasound power per unit area or “watts per gallon” .The cleaning effect depends on the cavitation , but the generation of cavitation effect related to the intensity of the ultrasonic. The greater the intensity,the cavitation effect is more obvious.it means the better cleaning effect. Further, depending on the cleaning object to select the appropriate intensity ultrasound. Such as cleaning circuit boards, ultrasonic intensity can be lower, or cleaning the mechanical parts,ultrasonic intensity may be higher. Some people blindly think that the ultrasound intensity as high as possible, so as to be washed clean, in fact, there is no need, and the higher the intensity, the greater the potential for harm to the circuit board.

      We suggest that the specific process parameters must be combined with their own specific circumstances to determine by experiment.