The practical operation of cleaning PCBA

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, the phenomenon of moisture absorption and whitening of residues may occur, which will directly affect the appearance of circuit boards. The use of large numbers of leadless chips, micro BGAs, chip scale packages (CSPs) and 0201 components in printed circuit board assemblies will continue to shrink the distance between components and boards. This will result in a smaller board size with the same number of components, resulting in an overly high component density problem. 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 the ionic contaminants are not cleaned, the following problems may occur: the surface resistance value of the circuit board is too low, the copper layer is corroded, and the conductive surface residues will form dendrites (dendrites) on the surface of the circuit board, causing circuit Local short circuit. Dendritic diffuse dendrites caused by ionic contamination lead to short circuits.

Improper cleaning of non-ionic contaminants can also cause a series of problems: poor adhesion of circuit board masks, poor contact of connectors, physical interference with moving parts and plugs and poor adhesion of conformal coatings, non-ionic contaminants encapsulating ions Contaminants are in it, 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, traces on the circuit board can be shorted.

To use PCB conformal paint, it must be cleaned

If the circuit board needs the protection of the three-proof paint, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 three-level standard. Resin residues that are not cleaned off prior to surface coating can lead to delamination of the protective layer, or cracks in the protective layer; 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 means that the residue on the circuit board is chemically safe, does not have any effect on the circuit board, and can be left on the circuit board. Detection of corrosion, surface insulation resistance (SIR), electromigration, and other specialized detection methods are mainly used to determine the halogen/halide content, and thus the safety of the 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 must ensure that the parameters of the soldering temperature curve, cleaning process setting parameters, 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. It is also possible that the wave soldering temperature profile is unreasonable, and if the preheat temperature is too high, the flux will vitrify, making it unusable as a fluxing agent and creating an unacceptable layer of contamination 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, etc. have been eliminated from the market, replaced by new cleaning agent technologies such as chlorine-free solvents and water-based cleaning agents and new cleaning equipment.

To deal with this challenge, many companies use chlorofluorocarbon trichlorotrifluoroethane (CFC-113) in solvent. It has the advantages of high degreasing efficiency, strong solubility for flux residue, volatile, non-toxic, non-flammable and non-explosive, non-corrosive to electronic components and PCBs, and stable performance. 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, use an anti-static brush to dip the cleaning agent on the PCB, tilt the PCB at an angle of 45°, and use a brush to brush from top to bottom, so that 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

In the actual cleaning process, it often occurs that the board surface of the hand-soldered PCBA is whitened after being placed, and the white marks are scattered around the solder joints and are 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 surface of the PCB before and after the soldering process are confirmed by the infrared spectrum analysis results of this 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 cleaning is not clean, the white residue may be a crystalline powder formed by the rosin after the solvent has evaporated. When the PCB is stored under high humidity conditions, when the absorbed moisture reaches a certain level, the rosin will gradually change from a colorless and transparent glass state to a crystalline state, which is a white powder from a viewing angle. 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. Artificial resins usually do not react with metal oxides below 100°C, but react rapidly when the temperature is higher than 100°C, they volatilize and decompose quickly, and have low solubility in water.

Rosin Modified

This is a substance produced by the reaction between rosin and flux during the soldering process of the PCB board, and the solubility of this substance is generally poor, so it is not easy to be cleaned, and stays on the board to form a white residue. However, these white substances are all organic components, which can still ensure the reliability of the board.

Organometallic salts

The principle of removing oxides on the welding surface is that organic acids react with metal oxides to form metal salts soluble in liquid rosin, which form a solid solution with rosin after cooling, and are removed together with rosin during cleaning. If the welding surface and parts have a high degree of oxidation, the concentration of the products after welding will be high. When rosin is too oxidized, it may remain 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, it is very likely that a halogen-containing flux was used for the electronic auxiliaries. 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. These strong acids begin to react with the oxide layer on the surface of the solder joints to form acid salts, which is the white substance seen.

The cleaning agent is used for cleaning. Since the cleaning agent with poor quality contains a certain amount of water, the board will absorb moisture when it is used in a large amount of cleaning. The combination of these factors is why the surface of the board is still white after washing.