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Cleaning mechanism of solder joints in SMT processing

source:FAQ release time:2021-04-26 Article author:Victoria Popular:pcb

In the process of SMT patch processing, because there are auxiliary agents such as flux, some pollutants or spots will be generated after oxidation or high temperature, so in the end, many pcbA need to be cleaned to be a perfect product. In terms of cleaning, whether solvent cleaning or water cleaning is used, it must undergo surface wetting, dissolution, emulsification, saponification, etc., by applying different methods of mechanical force to remove contaminants from the surface of the assembly board, and then rinse or rinse. Finally dry. Today, the editor of Wei Enchen will analyze it with you:


1. The surface is wet


The decontamination medium forms a uniform thin layer on the surface of the object to be washed, which wets the surface of the object to be washed and swells the contaminants on the surface of the object to be washed. The condition of surface wetting is that the surface tension of the cleaning medium is required to be less than the surface tension of the washing object. Add to the cleaning medium


These surfactants can significantly improve the wetting ability.


Second, dissolve


The main decontamination mechanism of organic solvent cleaning is dissolution. When choosing solvents, the principle of similar compatibility should be followed, and polar pollutants should be selected.


Polar solvents and non-polar pollutants should be selected non-polar solvents. However, polar solvents and non-polar solvents are often mixed and used in actual production. Adding some surfactants to the organic solvent cleaning can improve the detergent's ability to dissolve the thread residue.


(1) Ionic dissolution


Ionic dissolution refers to the dissociation of pollutants into ions in water.


The dissolution of this ionic substance will increase the conductivity of water. The negative ionic contaminants on the surface assembly board include the active material in the flux, the reactant of the active material in the flux and the metal oxide, the salt in the sweat of the hand, the polymer in the manufacture of printed circuit board, and the post-soldering Salt in the residue, etc. Such contaminants may cause poor electrical performance of electronic products or cause corrosion of solder joints.




(2) Non-ionic dissolution


Non-ionic dissolution means that pollutants dissolve in water without changing the conductivity of the water.


Typical non-ionic contaminants are flux bodies, such as polyvinylidene, dirt on hands, phases in water and similar compounds


These substances do not conduct electricity, but they can absorb moisture, which can also cause a decrease in insulation resistance and corrosion of solder joints in SMT processing.


3. Emulsification


In the water cleaning and semi-aqueous cleaning process, a certain amount of emulsifier can be added to the water. When cleaning synthetic trees, curing and contaminants, these contaminants and emulsifiers emulsify and dissolve in water. Adding surfactants can improve emulsification.


Four, saponification


Saponification is a pure chemical process and also a neutralization reaction


The pen chemical reaction uses fatty acid salt, which makes the rosin and acid saponification reaction, so that the residue on the surface of the washing is dissolved in water


And was cleared. The saponification agent will corrode the surface of aluminum and other metals, so when using the saponification agent, you should add the corresponding corrosion inhibitor


And observe the compatibility of the saponification agent with the components and other materials. The addition of surfactants can promote the saponification reaction.


5. Chelation


Chelation refers to the addition of complexes to dissolve insoluble substances (such as heavy metal salts).


Six, apply mechanical force


The application of mechanical force in different ways can speed up the cleaning speed and improve the cleaning efficiency.


① Scrubbing. There are two methods of manual scrubbing and mechanical scrubbing. Manual cleaning refers to the manual cleaning of the surface assembly board after welding by the operator holding a scrubbing tool. Mechanical scrubbing refers to the vigorous scrubbing performed by scrubbing tools installed on the cleaning equipment.


②Dip washing. Dipping refers to immersing the object to be washed under the surface of the cleaning agent. Different mechanical methods such as stirring, spraying or ultrasonic can be used to improve the efficiency of cleaning.


③Spray in the air. In a closed container, spray washing is performed by increasing the water pressure and flow rate. Spray cleaning has batch cleaning and online


Two methods of cleaning. The energy (pressure and flow) of the water flow directly affects the cleaning effect. Under the same water flow, the high pressure is more helpful for the cleaning of residues. However, if the pressure is too high, the cleaning effect will be affected due to the large splash back when the water hits the surface of the assembly board. The low-pressure water flow can have a soaking effect and is conducive to the dissolution of residues. Therefore, a spray washing method combining high and low pressure water flow is generally used.


④Immersed spray cleaning (jet cleaning). Refers to the flushing of the liquid stream immersed in the liquid surface. The nozzles are installed below the liquid surface and discharged at opposite positions on both sides of the washing machine. In order to ensure that the liquid flow does not look for each other. The formation of a flow hot bath effect. This method is suitable for washing products that are sensitive to sound waves, and for cleaning agents that are flammable, explosive, volatile, and easy to foam.


⑤ Please wash after centrifugation. The principle is to use the torque formed by the power of the motor to generate centrifugal force on the laundry. And made by centrifugal force


Use to keep dirt away from the surface of the assembly board. This cleaning method can make the cleaning liquid and lotion penetrate the parts that are difficult to clean, such as narrow spacing, tiny gaps, holes, and holes, to achieve the cleaning purpose. Centrifugal cleaning is suitable for post-welding cleaning of high-density surface mount boards.


⑥Ultrasonic cleaning. Ultrasonic cleaning relies on cavitation and acceleration to promote physical and chemical reactions. This kind of ultrasound


Cavitation can penetrate into small areas that are difficult to reach by other cleaning methods, and the cleaning efficiency is relatively high.


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