In the past ten years of FS Technology’s operation, many customers have emphasized the importance of PCB cleaning to us. For this reason, we wrote this detailed article on PCBA cleaning, hoping to help your PCB project.
Why it is necessary to clean the PCB board
The circuit board looks ugly when not cleaned
According to the past sales experience of FS Technology, customers attach great importance to the appearance of the PCBA board obtained. When customers place or use circuit boards in a high temperature and humid environment, moisture absorption and residual whitening may occur.
This is the main factor that affects the appearance of the circuit board. A large number of leadless chips, micro BGAs, chip scale packages (CSP) and 0201 assemblies are used in printed circuit board assemblies. These components will continue to shrink the distance between the component and the board. This will result in a smaller board size with the same number of components, leading to the problem of excessive component density. In the actual manufacturing process, halogen elements are hidden under the electronic components, so it is difficult for us to clean them. If the circuit board is forcibly cleaned, it may cause catastrophic consequences due to the release of halides.
Not cleaning the PCB will cause a short circuit
The contaminants of circuit boards include ionic contaminants and non-ionic contaminants, which we will explain in detail below.
If ionic contamination is not cleaned, the following problems can occur. The surface resistance value of the circuit board is too low. The copper layer is corroded. Conductive surface residues can form dendrites on the board surface. cause a partial short circuit in the circuit. Dendritic diffuse dendrites caused by ionic contamination lead to short circuits.
Improper cleaning of non-ionic contaminants can also cause a range of problems. Board mask adhesion is poor. Poor connector contact. Enables physical interference of moving parts and plugs and poor conformal coating adhesion. Non-ionic contaminants encapsulate ionic contaminants, and may encapsulate and bring in other residues and other harmful substances.
Reduced printed circuit board reliability
For military electronics, reliability is paramount. A significant threat to uncleaned boards is tin whiskers and intermetallic compounds. This problem has always existed, and tin whiskers and intermetallic compounds can eventually cause short circuits. Excessive ionic contamination on the assembly can cause problems in humid environments and with electricity. For example, due to the growth of electrolytic tin whiskers, corrosion of conductors or reduction of insulation resistance. The circuit board traces are shorted mainly for this reason.
To use PCB conformal paint, it must be cleaned
If the protection of conformal paint is required, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 third-level standard. Resin residues that are not removed can cause the protective layer to delaminate, or the protective layer to crack. activator residues can cause electrochemical migration under the coating, resulting in failure of coating cracking protection. Studies have shown that coating adhesion can be increased by 50% by cleaning.
“No-clean” also needs to clean the PCBA board
By current standards, the term no-clean refers to the fact that the residue on a circuit board is chemically safe. We can understand that it will not have any effect even if it is left on the PCB. Professional testing methods such as corrosion testing, surface insulation resistance (SIR), electromigration, etc. are mainly used to determine the halogen/halide content to determine the safety of no-clean assembly after assembly. However, even with low solids no-clean flux, 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 no-clean electronic assemblies require cleaning.
The principle of cleaning PCB
Cleaning is the process of removing contaminants. It mainly adopts solution cleaning method. Through the dissolution or chemical reaction between the contaminants and the solvent, the physical bonds or chemical bonds between the contaminants and the PCB are destroyed, so as to separate the contaminants. for the purpose of removing contaminants from PCBA. Whether it is rosin or organic acids and their tin or lead salts, there is a certain degree of solubility, and the removal of residues is accomplished by the process of transferring from the circuit board to the cleaning agent. During the dissolution process, increasing the temperature of the cleaning agent or supplementing it with ultrasonic waves and brushing will speed up the cleaning speed and improve the cleaning effect.
Cleaning after PCBA soldering is a value-added process. Its main task is to remove flux residues after soldering, adhesive tape residues and other artificial contaminations, in order to improve the reliability of PCBA use. This was considered non-value-added labor in the past, but now it seems to be a misunderstanding.
PCBA cleaning needs to be staged
The cleaning of PCBA is divided into cleaning of placement (SMT assembly section) and cleaning of insertion (THT section). Cleaning removes the build-up of surface contamination during product processing and reduces the risk of product reliability in terms of surface contamination.
To clean PCB, the first thing to make sure is that the cleaning agent matches the residue produced by the circuit board during the soldering process, that is, to solve the compatibility between the flux residue and the cleaning agent, so that the residue can be easily removed and meet the cleanliness goal. . An effective cleaning process includes the following points. 1. The welding temperature curve must be guaranteed. 2, the cleaning process setting parameters,. 3. The parameters of solder paste, solder and flux all reach the best matching range.
For wave soldering, there may be a reaction between the flux and the solder mask after the furnace, resulting in dark stains. The pollutants are obviously sticky when touched by hand, and general cleaning agents cannot be washed off. The wave soldering temperature profile may also be unreasonable. If the preheat temperature is too high, the flux will vitrify. This will cause the flux to not work and create an unacceptable contamination layer on the board.
The latest PCB cleaning solvents
As technology advances and regulations change, cleaning products will face increasing challenges. For example: CEE 648 standard, REACH, which relate to which chemical products can or cannot be used in cleaning agents. In the past few years, cleaning agent technologies such as CFC, ETD, ES, HCFC have been eliminated from the market. Instead, new cleaning agent technologies and new cleaning equipment such as chlorine-free solvents and water-based cleaning agents are used.
To deal with this challenge, many companies use chlorofluorocarbon trichlorotrifluoroethane (CFC-113) in solvent. It has the following advantages: 1. High degreasing efficiency. 2. Solubility to flux residue. 3. Volatile. 4. Non-toxic. 5. Non-flammable and non-explosive. 6. No corrosion to electronic components and PCB. 7, stable performance and other advantages. It is an ideal solvent. However, it has a damaging effect on the atmospheric ozone layer and seriously endangers the living environment of human beings. In 1987, the governments of various countries in Montreal, Canada signed an agreement on the protection of the ozone layer – “Montreal Protocol on Substances that Deplete the Ozone Layer”. So far, there is no solvent that can be completely replaced and is excellent in cleaning ability.
How to clean PBCA in PCBA factory
Most of the small and medium-sized foundries or manufacturing plants use manual brushing cleaning methods based on cost considerations. That is, dip the cleaning agent on the PCB with an anti-static brush, tilt the PCB at a 45° angle, and brush from top to bottom with a brush. This way the cleaning agent dissolves and the residue flows down with the cleaning agent. It is mainly used for local cleaning or cleaning of PCBA with components that cannot be cleaned on some PCBs. Although this process method is simple, it is inefficient and consumes a lot of cleaning agents.
For well-known foundries or large-scale manufacturing plants, the cleaning process is gradually reconsidered. Generally, these factories will be equipped with corresponding online cleaning machines or offline cleaning machines to replace manual cleaning with equipment cleaning to ensure the quality of PCBA cleaning.
The circuit board has white residue after cleaning
When we are cleaning, the surface of the circuit board will be whitened, and the white marks will be scattered around the solder joints, which is abnormally prominent. The wave soldered PCBA board surface has dark stains after cleaning, which seriously affects the appearance acceptance. It does not meet the IPC-A-610E-2010 standard after manual soldering cleaning and placement.
Removal of Soldering Flux Soldering Residues Using Chemical Solvents Most of the dissolution processes rely on alkaline pH cleaning agents. These cleaners contain metal ions that allow the metal ions to facilitate chemical reactions to form lead salts. Some lead salts, Pb(NO)3, are easily soluble in water, while others are insoluble in water. These lead salts aggregate on the surface of PCBA to form white precipitates.
White residues are common contaminants on PCBAs and are generally a by-product of flux. Common white residues are polymerized rosin, unreacted activator, and lead chloride or bromide, which is the reaction product of flux and solder. These substances expand in volume after absorbing moisture, and some substances also undergo hydration reaction with water. The residues are becoming more and more obvious, and it is extremely difficult to remove these residues on the PCB. If the overheating or high temperature takes a long time, the problem will be more serious. The rosin and residues on the PCB surface were confirmed by the results of infrared spectroscopy analysis of the process.
Whether the board has white residue after cleaning, or white matter after no-cleaning board storage, or white matter on solder joints found during repair, there are four cases.
Rosin in Flux
Most of the white substances produced after cleaning, storage, and failure of solder joints are rosin inherent in the pcb flux itself. Rosin is usually a transparent, hard and brittle solid substance without a fixed shape, not a crystal. Rosin is thermodynamically unstable and has a tendency to crystallize. After the rosin crystallizes, the colorless transparent body becomes a white powder. If the PCB is not cleaned properly, the white residue may be rosin crystalline powder formed after the solvent has evaporated. When a PCB is stored in high humidity conditions, it can overabsorb moisture.
If the moisture saturation reaches a critical point, rosin will gradually change from a colorless and transparent glassy state to a crystalline state. At this point, its form should be changed to white powder. Its essence is still rosin, but the shape is different, it still has good insulation and will not affect the performance of the board. The rosin acid and the halide (if used) in the rosin are used together as the active agent. Synthetic resins generally do not react with metal oxides below 100°C. If it is at a temperature above 100 °C, it will react rapidly, volatilize and decompose quickly, and have low solubility in water.
This is a substance produced by the reaction of rosin and flux during PCB soldering. This substance is generally poorly soluble, difficult to clean, and stays on the plate to form a white residue. However, these white substances are all organic components, which can still ensure the reliability of the board.
The principle of removing oxides on the soldering surface of circuit boards is that organic acids react with metal oxides to form metal salts that are soluble in liquid rosin. When it cools, it forms a solid solution with the rosin, which is removed along with the rosin during cleaning. If the welding surface and parts are highly oxidized, the concentration of the product after welding will be high.When the rosin we use is over-oxidized, it may be left on the board along with undissolved rosin oxides. At this time, the reliability of the board will be reduced.
Metal inorganic salts
During the assembly process, many assembly companies use halogen-containing fluxes as electronic aids. Halogen ions (F, Cl, Br, l) remain on the board surface after welding. These ionic halogen residues are not white by themselves, nor are they sufficient to cause whitening of the board surface. Such substances form strong acids when exposed to water or moisture. The solder joints on the surface of the circuit board react with the strong acid to form an acid salt, which is the white substance you see.
We can use cleaning agent to clean the PCB. Because the circuit board will absorb water, so do not use poor quality cleaning agent when cleaning. The combination of these factors is why the surface of the board is still white after washing.