Electronic products PCBA board cleaning necessity and process requirements!
Date: 2018/8/9 11:13
During PCBA processing, solder paste and flux will generate residues. The residue contains organic acids and decomposable electric ions. Organic acids have a corrosive effect, and electric ions remaining on the pads can cause short circuits. The residue on the PCBA board is relatively dirty and does not meet customer requirements for product cleanliness. Therefore, it is necessary to clean the PCBA board.
What are the PCBA processing pollution
Contaminants are defined as any surface deposits, impurities, slag inclusions, and adsorbates that reduce the chemical, physical, or electrical properties of PCBA to substandard levels. There are the following aspects:
1.The components that constitute PCBA, the pollution or oxidation of PCB itself will cause PCBA board surface pollution;
2. In the production process of PCBA, solder paste, solder, solder wire, etc. are used for soldering, and the flux during the soldering process will generate residues on the PCBA board to form pollution, which is the main pollutant;
3. Hand imprints generated during the manual soldering process, and wave soldering claw foot imprints and soldering tray (fixture) imprints during the wave soldering process. There may also be other types of contaminants on the PCBA surface, such as blockages. Hole glue, residual glue of high temperature tape, handwriting and flying dust, etc .;
4. Dust on the work site, water and solvent vapors, smog, tiny particles of organic matter, and static electricity caused by pollution of charged particles attached to PCBA.
The above shows that the pollutants mainly come from the assembly process, especially the welding process.
During the welding process, as the metal is heated, a thin layer of oxide film will be generated, which will hinder solder infiltration, affect the formation of solder joint alloys, and are prone to false soldering and false soldering. The flux has the function of deoxidation, which can remove the oxide film of the pads and components to ensure the smooth progress of the soldering process. Therefore, a flux is required in the soldering process. The flux plays a vital role in the formation of a good solder joint and a sufficient plated-through-hole filling rate during the soldering process. The role of the flux in soldering is to remove oxides on the soldering surface of the PCB board to achieve the necessary cleanliness of the metal surface, destroy the surface tension of the molten tin, prevent the solder and soldering surface from oxidizing again during soldering, increase its diffusion force, and help heat. Pass to the welding area. The main ingredients of the flux are organic acids, resins and other ingredients. High temperatures and complex chemical reactions alter the structure of flux residues. Residues are often polymers, halides, and metal salts produced by the reaction with tin and lead. They have strong adsorption properties, but have poor solubility and are more difficult to clean.
The purpose of PCBA cleaning, why PCBA cleaning
1.Appearance and electrical performance requirements
The most intuitive effect of the pollutants on PCBA is the appearance of PCBA. If it is placed or used in a high temperature and humid environment, the residue may become hygroscopic and white. Due to the large number of leadless chips, miniature BGAs, chip scale packages (CSP) and 0201 components used in the components, the distance between the components and the circuit board is constantly shrinking, the size of the board is becoming smaller, and the assembly density is increasing. In fact, if the halide is hidden under the component or is not cleaned under the component at all, local cleaning may cause catastrophic consequences due to halide release. This can also cause dendrite growth, which can cause short circuits. If the ionic pollutants are not cleaned properly, they will cause a lot of problems: low surface resistance, corrosion, and conductive surface residues will form dendritic distribution (dendrites) on the surface of the circuit board, causing local short circuits, as shown in the figure.
A major threat to the reliability of military electronic devices is tin whiskers and intermetallic compounds. This problem has always existed. Tin whiskers and intermetallic compounds can eventually cause short circuits. In tidal environments and when electricity is present, if the ion contamination on the assembly is excessive, it may cause problems. For example, due to the growth of electrolytic tin whiskers, the corrosion of conductors, or the decrease in insulation resistance, it may cause short circuit on the wiring on the circuit board, as shown in the figure.
Improper cleaning of non-ionic contaminants can also cause a series of problems. It may cause poor adhesion of circuit board masks, poor contact of connectors, physical interference to moving parts and plugs, and poor conformal coating adhesion. At the same time, non-ionic contaminants may also enclose ionic contaminants, and may cause Some residues and other harmful substances are wrapped and brought in. These are issues that cannot be ignored.
2.Three anti-paint coating needs
To make the three-proof paint coating reliable, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 level III standard. Resin residues that have not been cleaned off before surface coating can cause the protective layer to delaminate or cracks in the protective layer; activator residues may cause electrochemical migration under the coating, resulting in failure of the coating crack protection. Studies have shown that coating adhesion can be increased by 50% through cleaning.
3.No-clean also needs cleaning
According to current standards, the term no-clean means that the residue on the circuit board is chemically safe, does not have any impact on the circuit board, and can be left on the circuit board. Detecting corrosion, surface insulation resistance (SIR), electromigration, and other specialized testing methods are mainly used to determine the halogen / halide content, and then determine the safety of the no-clean assembly after assembly. However, even with low solids no-clean fluxes, there will still be more or less residue. For products with high reliability requirements, no residues or other contaminants are allowed on the circuit board. For military applications, even disposable electronic assemblies are required to be cleaned.
Common PCBA cleaning process
1.Fully automated online cleaning machine
A physical picture of a fully automated online cleaning machine is shown in Figure 7. This cleaning machine thoroughly and effectively cleans organic and inorganic pollutants such as rosin flux, water-soluble flux, no-clean flux / soldering flux, etc. remaining on the surface of PCB after SMT / THT soldering. It is suitable for large-scale PCBA cleaning. It uses safe and automated cleaning equipment on the Denso production line, and completes all processes of chemical cleaning (or water-based cleaning), water-based rinsing, and drying through different chambers online. During the cleaning process, PCBA is cleaned in different solvents through the conveyor belt of the washing machine. The cleaning liquid must be compatible with components, PCB surfaces, metal plating, aluminum plating, labels, handwriting and other materials. Special components need to be considered whether they can withstand cleaning.
The cleaning process is: ---- chemical pre-washing --- chemical cleaning --- chemical partition ---- pre-rinsing --- rinsing --- final spray --- air-cut drying --- drying .
2.Semi-automatic offline cleaning machine
A semi-automated offline physical map is shown in Figure 9. This cleaning machine thoroughly and effectively cleans organic and inorganic pollutants such as rosin flux, water-soluble flux, no-clean flux / soldering flux, etc. remaining on the surface of PCB after SMT / THT soldering. It is suitable for small batch multi-type PCBA cleaning. It can be set anywhere in the production line by manual handling, and complete all processes of chemical cleaning (or water-based cleaning), water-based rinsing and drying in a cavity offline. During the cleaning process, PCBA usually needs to be fixed by a fixture or placed in a basket. The cleaning liquid must be compatible with components, PCB surface, metal plating, aluminum plating, labels, handwriting and other materials. Special components need to be considered whether they can withstand Cleaning. There are certain requirements for the placement density and inclination of PCBA in the cleaning basket. These two factors have a direct impact on the cleaning effect.
3.PCBA online single-sided brush cleaning machine
This equipment is specially designed for flux removal after wave soldering, instead of manual brushing, reducing cleaning costs;
After multiple brushing processes, it can effectively remove the flux residue on the PCBA after soldering without wetting the front components;
The equipment can clean PCBA with a maximum width of 350mm;
The equipment adopts humanized design, and the roller brush is easy to disassemble and assemble, which can realize rapid cleaning and replacement;
Adjustable track width, adjustable transport speed and brushing frequency;
It can be connected to wave soldering to realize automatic online cleaning.
4.Manual washing machine
The manual cleaning machine thoroughly and effectively cleans organic and inorganic pollutants such as rosin flux, water-soluble flux, rosin flux, no-clean flux / soldering flux, etc. remaining on the surface of PCB after SMT / THT PCBA welding. It is suitable for PCBA cleaning of small batches of samples. Through temperature control, it adapts to the manual cleaning process of MPC microphase cleaning agent, and completes chemical cleaning in a constant temperature bath.
Note: Ultrasonic cleaning is also used by some PCBA manufacturers as a solution with low investment and easy implementation. However, the space military has restricted (prohibited) the use of ultrasonic cleaning processes.
Sonic cleaning should not be used to clean electrical or electronic parts, components or components containing electronic components. Protective measures should be taken to prevent damage to the components (US military standard DOD-STD-2000-4A "Electrical and Electronic Equipment" General welding technical requirements "); IPC-A-610E-2010 three-level standard also generally prohibits the ultrasonic cleaning process.
Common cleaning methods
1. Water-based cleaning process: spray or dip
2. Semi-water-based cleaning process: rinse with water after washing with hydrocarbon
3. Vacuum cleaning process: polyol or modified alcohol
4. Gas phase cleaning process: HFE, HFC, nPB (n-bromopropane), azeotrope
PCBA cleanliness detection method, how to detect whether PCBA is clean
1.Visual inspection method
Use a magnifying glass (X5) or an optical microscope to observe the PCBA and evaluate the cleaning quality by observing the presence of solder solid residues, tin slag, beads, non-fixed metal particles and other pollutants. It is generally required that the surface of PCBA must be as clean as possible, and traces of residues or pollutants should not be seen. This is a qualitative indicator. It usually takes the user's requirements as the goal, develops its own judgment criteria, and uses multiples of magnifiers during inspection. The characteristic of this method is simple and easy to implement, but the disadvantage is that it is impossible to check the pollutants at the bottom of the component and the residual ionic pollutants, which is suitable for occasions with low requirements.
2. Solvent extract test method
The solvent extraction test method is also called the ionic contaminant content test. It is an average test of the content of ionic pollutants. The test generally uses the IPC method (IPC-TM-610.2.3.25). It is to immerse the cleaned PCBA in the test solution of the ionization pollution tester (75% + 2% pure isopropanol plus 25% DI water), dissolve the ionic residue in the solvent, carefully collect the solvent, and measure its resistivity. Ionic pollutants usually originate from the active materials of the flux, such as halogen ions, acid ions, and metal ions generated by corrosion. The results are expressed in terms of sodium chloride (NaCl) equivalent per unit area. That is, the total amount of these ionic pollutants (including only those that can be dissolved in the solvent), which is equivalent to the amount of NaCl, does not necessarily exist on the surface of PCBA or only NaCl exists.
3. Surface insulation resistance test method (SIR)
This method measures the surface insulation resistance between conductors on PCBA. The measurement of surface insulation resistance can indicate the leakage of electricity due to pollution under various temperature, humidity, voltage and time conditions. Its advantages are direct measurement and quantitative measurement; and it can detect the presence of flux in local areas. Since the residual flux in PCBA solder paste mainly exists in the gap between the device and the PCB, especially the BGA solder joints, it is more difficult to remove. In order to further verify the cleaning effect, or to verify the safety (electrical performance) of the solder paste used , Usually measure the surface resistance in the gap between the component and the PCB to check the cleaning effect of PCBA. General SIR measurement conditions are 170 hours at 85 * C ambient temperature, 85% RH ambient humidity, and 100V measurement bias.
4. Ionic pollutant equivalent test method (dynamic method)
5. Detection of flux residue
PCBA cleaning precautions
The printed board assembly should be cleaned as soon as possible after assembly and soldering (because the flux residue will gradually harden over time and form corrosives such as metal halide salts) to completely remove the residual flux, solder and other contaminants from the printed board.
When cleaning, prevent harmful cleaning agents from penetrating into the components that are not completely sealed, so as not to cause damage or potential damage to the components. After the printed board components are cleaned, they are baked and dried in an oven of 40 ~ 50 * C for 20 ~ 30 minutes. Touch the device with bare hands before the cleaning parts are dried. Cleaning should not affect components, markings, solder joints, and printed boards. The assembly of general electronic products PCBA must go through the SMT + THT process flow, during which wave soldering, reflow soldering, manual soldering and other soldering processes are performed. No matter what type of soldering, the assembly (electric assembly) process is the main process Source of assembly pollution. Cleaning is a process of dissolving and removing welding residues. The purpose of cleaning is to extend the life of the product by ensuring good surface resistance and preventing leakage.
It can be seen from the evolving electronics market that modern and future electronics will become smaller and smaller, and the requirements for high performance and reliability will be stronger than ever. Thorough cleaning is a very important and highly technical task, which directly affects the working life and reliability of electronic products, and also relates to the protection of the environment and human health. To re-understand and solve the problem of soldering and cleaning from the perspective of the entire production process system, the implementation of the plan must be combined with the use of soldering materials such as flux, solder paste, solder wire, etc. The matching of no-cleaning can effectively remove the residues, making the cleaning cleanliness easier to meet customer expectations.