PCB Technical - How to choose a cleaning agent for PCB circuit boards

In PCB printed circuit board components, there are three main ways of bonding or attaching contaminants and components. They are the bonding between molecules, also called physical bonding; the bonding between atoms, also This is called chemical bonding; contaminants are embedded in materials such as solder masks or electroplated deposition in the form of particles, so-called "inclusions."

The core of the cleaning mechanism is to destroy the bonding force of the chemical bonds or physical bonds between the pollutants and the PCB printed circuit board, so as to achieve the purpose of separating the pollutants from the components. Since this process is an endothermic reaction, it must be supplied to achieve the above purpose.

Using an appropriate solvent to provide energy through the dissolution reaction and saponification reaction between the pollutants and the solvent can destroy the binding force between them, so that the pollutants are dissolved in the solvent, so as to achieve the purpose of removing the pollutants.

In addition, you can also use specific water to remove the contaminants left by the water-soluble flux to the components.

Because PCB printed circuit board components are polluted differently after soldering, the types of pollutants are different, and different products have different requirements for the cleanliness of the components after cleaning, so there are many types of cleaning agents that can be used. So, how to choose the right cleaning agent? The following smt processing plant technicians will introduce some basic requirements for cleaning agents.

(1) Wettability. To dissolve and remove contaminants on SMA, a solvent must first wet the contaminated PCB, expand and wet the contaminants.

The wetting angle is the main factor that determines the degree of wetting. The best cleaning situation is that the PCB spontaneously expands. The condition for this situation is that the wetting angle is close to 0°.

(2) Capillary action. Solvents with good wetting ability may not guarantee the effective removal of pollutants. The solvent must also be easy to penetrate, enter and exit these narrow spaces, and be able to circulate repeatedly until the pollutants are removed. That is, the solvent is required to have a strong capillary effect so that it can penetrate into these dense gaps. The capillary permeability of common cleaning agents. It can be seen that the capillary permeability of water is the largest, but its surface tension is large, so it is difficult to discharge from the gap, resulting in low exchange rate of cleaning water and difficult to effectively clean. Although the capillary permeability of the chlorinated hydrocarbon mixture is low, the surface tension is also low. Therefore, considering its two properties, this type of solvent has a better cleaning effect on component pollutants.

(3) Viscosity. The viscosity of the solvent is also an important performance that affects the effective cleaning of the solvent. Generally speaking, when other conditions are the same, the viscosity of the solvent is high, and the exchange rate in the gap on the SMA is low, which means that more force is required to discharge the agent from the gap. Therefore, the low degree of solvent helps it to complete multiple exchanges in the seam of SMD.

(4) Density. Under the conditions of meeting other requirements, high-density solvents should be used to clean the components. This is because, during the cleaning process, when solvent vapor condenses on the components, gravity helps the condensed solution flow downwards, improving the cleaning quality. In addition, the high density of the solution is also conducive to reducing its emission to the atmosphere, thereby saving materials and reducing operating costs.

(5) Boiling point temperature. The cleaning temperature also has a certain effect on the cleaning efficiency. In most cases, the temperature of the solvent is controlled at its boiling point or a temperature range close to the boiling point. Different solvent mixtures have different boiling points, and the change of solvent temperature mainly affects its physical properties. Steam condensation is an important part of the cleaning cycle. The increase in the boiling point of the solvent allows higher temperature steam to be obtained, and higher steam temperature will lead to a larger amount of steam condensation, which can remove a large amount of pollutants in a short time. This relationship is most important in an in-line conveyor belt wave soldering and cleaning system, because the speed of the cleaning agent conveyor belt must be consistent with the speed of the wave soldering conveyor belt.

(6) Solubility. When cleaning the SMA, because the distance between the component and the substrate, between the component and the component and the I/O terminal of the component belt is very small, only a small amount of solvent can contact the contaminants under the device. Therefore, solvents with high dissolving power must be used, especially when cleaning is required to be completed within a limited time, such as in an online conveyor belt cleaning system. However, it should be noted that solvents with high dissolving power are also highly corrosive to the parts to be cleaned. Rosin-based fluxes are used in most solder pastes and dual-wave soldering. Therefore, when comparing the solubility of various solvents, special attention should be paid to the residues of rosin-based fluxes.

(7) Ozone destruction coefficient. With the continuous progress of society, people's awareness of environmental protection is constantly increasing. Therefore, while evaluating the cleaning agent's ability, the degree of destruction of the ozone layer should also be considered. For this reason, the concept of ozone destruction coefficient (ODP) was introduced, which is now based on the destruction coefficient of CFC-113 (trioxytrichloroethane) to ozone, that is, ODPCFC-113=1.

(8) The lowest limit value. The lowest limit value represents the highest limit value that the human body can bear when in contact with the solvent, also known as the exposure limit. Operators are not allowed to exceed the minimum limit value of the solvent during their daily work.

The above is the selection of cleaning agents in PCBA patch processing plants. In addition to the above-mentioned performance, factors such as economy, operability and compatibility with equipment should also be considered.