PCBA Tech - How to choose and use the cleaning agent for PCB board?

In PCB printed circuit board components, there are three main methods for bonding or attaching pollutants and components. They are the bonding between molecules and molecules, also known as physical bonding; the bonding between atoms and atoms, also known as It is a chemical bond; contaminants are embedded in materials such as welding masks or electroplating deposits in particle form, which is the so-called "doping".

The center of the cleaning mechanism is to damage the bonding force of the chemical bonds or physical bonds between the contaminants and the PCB printed circuit board, so as to achieve the intention of separating the contaminants from the components. Because this process is an endothermic reaction, it is necessary to supply to achieve the above intention.

smt patch processing

Choosing an appropriate solvent and supplying energy through the dissolution reaction and saponification reaction between the pollutants and the solvent can damage the binding force between them, dissolve the pollutants in the solvent, and achieve the purpose of removing the pollutants.

In addition, specific water can be used to remove the contaminants left by the water-soluble flux on 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 a suitable cleaning agent? The following smt processing plant technicians will introduce some basic requirements for cleaning agents.

Wetness

For a solvent to dissolve and remove the contaminants on the SMA, it is first necessary to moisten the contaminated PCB, expand and moisten the contaminants.

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

Capillary effect

A wet solvent may not ensure effective removal of contaminants. The solvent must be easy to penetrate, enter and exit these narrow spaces, and be able to repeat the cycle until the contaminants are removed. That is, the solvent is required to have a strong capillary effect so that it can enter 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, so the two functions are summarized and considered. This type of solvent has a better cleaning effect on component pollutants.

Viscosity

The viscosity of the solvent is also an important function that affects the effective cleaning of the solvent. Generally speaking, under the same other conditions, the viscosity of the solvent is high, and the communication 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 SMD seam original.

density

Under the condition that other requirements are satisfied, a high-density solvent should be used to clean the components. This is because in the cleaning process, when solvent vapor condenses on the components, gravity helps the condensed solution move 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 data and reducing operating costs.

Boiling point temperature

The cleaning temperature also has a certain effect on the cleaning power. In most cases, the solvent temperature is controlled at its boiling point or a temperature scale close to the boiling point. Different solvent mixtures have different boiling points, and the change of solvent temperature mainly affects its physical functions. Steam condensation is an important part of the cleaning cycle. The improvement of solvent boiling point allows higher temperature steam to be obtained, and higher steam temperature will cause more steam condensation, which can remove many pollutants in a short time. This connection is most important in the in-line conveyor 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.

Dissolving ability

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 is very small, only a few solvents can touch the contaminants under the device. Therefore, it is necessary to choose a solvent with high dissolving ability, especially when the cleaning is required to be completed within a constrained time, such as in the online conveyor belt cleaning system. However, it should be noted that solvents with high dissolving ability 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 dissolution capabilities of various solvents, special attention should be paid to the residues of rosin-based fluxes.

Ozone damage factor

With the continuous progress of society, people's awareness of environmental protection continues to increase. Therefore, when evaluating the ability of cleaning agents, the degree of damage to the ozone layer should also be considered. For this reason, the concept of ozone damage factor (ODP) was introduced, which is now based on the damage factor of CFC-113 (trioxytrichloroethane) to ozone, that is, ODPCFC-113=1.

Minimum constraint

The minimum constraint value represents the maximum limit value that the human body can bear when touching 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 functions, factors such as economy, operability and compatibility with equipment should also be considered.