Cleaning technology of printed circuit board

Cleaning technology of printed circuit board

The traditional method of cleaning printed circuit boards is to use organic solvents. The mixed organic solvent composed of CFC-113 and a small amount of ethanol (or isopropanol) has a good cleaning ability on the residue of rosin flux, but because of CFC- 113 has a destructive effect on the atmospheric ozone layer and is currently banned. The non-ODS cleaning processes currently available include water-based cleaning, semi-aqueous cleaning, and solvent cleaning. In addition, no-cleaning processes without cleaning can be used. Which process to choose should be decided according to the electronic products and importance, the requirements for cleaning quality and the actual situation of the factory.

 1 Water-based cleaning

    1.1 Water-based cleaning process

    The water-based cleaning process uses water as the cleaning medium. In order to improve the cleaning effect, a small amount of surfactants, washing aids, corrosion inhibitors and other chemical substances (generally 2%-10%) can be added to the water. According to the specific conditions of different pollution on the printed circuit board, the additive in the water-based cleaning agent can make the cleaning application wider. Water-based cleaning agents have a good dissolving effect on water-soluble dirt, and combined with physical cleaning methods such as heating, brushing, spraying, ultrasonic cleaning, etc., a better cleaning effect can be achieved. Adding surfactants to the water-based cleaning agent can greatly reduce the surface tension of the water, strengthen the penetration and spreading ability of the water-based cleaning agent, and can better penetrate into the gaps between the tightly arranged electronic components. Clean the dirt that has penetrated into the printed circuit board substrate. The use of water dissolution and surfactant emulsification and dispersion can also remove the residues of synthetic active fluxes. Not only can it dissolve and remove various water-soluble dirt, but also can remove synthetic resins, fats and other non-productive fluxes. Soluble dirt removal. For the use of rosin-based flux or water-based cleaning agents, add appropriate saponifiers. Saponifiers are used to saponify organic acids such as rosin acid in rosin and fatty acids in oils when cleaning printed circuit boards. A chemical substance that produces fatty acid salts (soap) that are soluble in water. This is a common ingredient in many cleaning agents used to clean flux and grease on printed circuit boards. The saponification agent is usually a basic inorganic substance such as sodium hydroxide, potassium hydroxide and other strong bases, and it may also be a basic organic substance such as monoethanolamine. Commercial saponification agents generally also contain organic solvents and surfactants to clean and remove residues that cannot be saponified. Since the saponification agent may corrode aluminum, zinc and other metals on the printed circuit board, it is easy to aggravate the corrosion when the cleaning temperature is relatively high and the cleaning time is relatively long. Therefore, corrosion inhibitors should be added to the formulation. However, it should be noted that printed circuit boards with components sensitive to alkaline substances should not be cleaned with water-based cleaning agents containing saponifiers.

    If ultrasonic cleaning is used in the water-based cleaning process, the "cavitation effect" that produces a large number of small air bubbles during the propagation of ultrasonic waves in the cleaning solution can effectively strip insoluble dirt from the electronic junction board. Taking into account the compatibility requirements of printed circuit boards, electronic components and ultrasonic waves, the ultrasonic frequency used in the cleaning of printed circuit boards is generally around 40KHz.

    The water-based cleaning process includes three steps of cleaning, rinsing, and drying. First, use a water-based cleaning agent with a concentration of 2%-10% and physical cleaning methods such as heating, brushing, spraying, ultrasonic cleaning, etc. to clean the printed circuit boards in batches, and then use pure water or ionized water (DI water) for 2 -Rinse 3 times, and finally dry with hot air. Water-based cleaning requires the use of pure water for rinsing is the cause of the high cost of water-based cleaning. Although high-quality water quality is a reliable guarantee for cleaning quality, in some cases first use low-cost deionized water with a conductivity of 5um·cm for rinsing, and finally use high-purity deionized water with a conductivity of 18um·cm A good cleaning effect can also be achieved by rinsing once. A typical water cleaning process is shown in Figure 1. A typical process is: batch cleaning of electronic circuit boards with a water-based cleaning agent at a temperature of 55°C, combined with strong jet cleaning for 5 minutes, then rinse with deionized water at 55°C for 15 minutes, and finally hot air at a temperature of 60°C Blow dry for 20 minutes. In order to improve the utilization of water resources, tap water used in the cleaning process or deionized water used in the rinsing tank, according to the literature, using tap water (hard water with more ions) in pre-cleaning, not only can greatly reduce production costs, And its decontamination ability is no worse than soft water or deionized water.

 2 Semi-aqueous cleaning

    2.1 Semi-aqueous cleaning agent

    Organic solvents and surfactants are generally included in the components of semi-aqueous cleaning agents. For example, the EC-7 semi-aqueous cleaning agent first used in the cleaning of printed circuit boards is composed of terpene hydrocarbon solvents and surfactants. consist of. Most semi-aqueous cleaning agents also contain water in their formulations, but due to the high water content (only 5%-20%), semi-aqueous solvents and solvent cleaning agents are transparent and uniform from the outside. The solution. Different from general solvent cleaners, the organic solvent used in semi-aqueous cleaners has a relatively high boiling point, so the volatility is low. It is not necessary to clean in a closed environment like solvent cleaners, and the cleaning agent does not need to be replaced frequently during the cleaning process. Only need to replenish the cleaning dosage appropriately. The organic solvents used for preparing semi-aqueous cleaning agents for cleaning printed circuit boards mainly include terpene and petroleum hydrocarbon solvents, glycol ethers, N-methylpyrrolidone, etc. The type of solvent should be selected according to the printed circuit board. Contamination of raw materials such as circuit boards, electronic components, etc., and the type of flux used during welding.

    2.2 Semi-aqueous cleaning process

    It also includes three processes of cleaning, rinsing, and drying. The cleaning process is often combined with ultrasonic cleaning to improve the cleaning effect and reduce the cleaning time. Since the use of ultrasonic will increase the temperature of the cleaning agent, it is necessary to strictly control the cleaning temperature and not exceed the flash of the cleaning liquid. Point (generally the cleaning temperature is controlled below 70 ℃). An emulsification recovery tank is added between the cleaning and rinsing processes, and the organic solvent contained in the semi-aqueous cleaning solution has a high concentration. After cleaning, there will still be more cleaning solution stained on the surface of the printed circuit board. After the printed circuit board is directly placed in the water rinsing liquid, the organic solvent on the surface of the printed circuit board will contaminate the rinsing water, which greatly increases the load of the subsequent water treatment process, and it increases between the cleaning and rinsing processes. An emulsification recovery device containing emulsifier water solvent can strip the organic solvent on the surface of the printed circuit board from the surface of the printed circuit board by emulsification and dispersion, and can be used in this emulsification recovery device to filter The device and the oil-water separation device separate and recover organic solvents and dirt deposits. Since there are few organic solvents on the surface of the printed circuit board entering the rinsing tank, the load of the rinsing process is reduced, and the load of wastewater treatment is also reduced. Rinse with deionized water for 2-3 times to remove the dirt. Since semi-aqueous cleaning uses water as a rinsing agent, it has the same drying difficulties as water-based cleaning, and similar measures need to be adopted to increase the drying speed.

    2.3 Advantages and disadvantages of semi-aqueous cleaning process

    The advantages of the semi-aqueous cleaning process are: it has strong adaptability to various welding processes, so the use of the semi-aqueous cleaning process does not need to change the original welding process; its cleaning ability is relatively strong, and it can remove water-soluble dirt and oil at the same time; and Most metal and plastic materials have good compatibility, and are less volatile than solvent cleaning agents. The evaporation loss during use is small. The disadvantages are: the same as water-based cleaning, the need to use pure water to rinse, drying is difficult, and the amount of waste water treatment is large. . The semi-aqueous cleaning process requires a large area and space, and the equipment has a large one-time investment, especially the online cleaning machine. Since semi-aqueous cleaning agents contain more organic solvents, safety measures such as protection against toxic solvents, fire and explosion prevention should be increased. Moreover, semi-aqueous cleaning agents cannot be recycled and reused through distillation like solvent cleaning agents, so the cost is relatively high.

3 Solvent cleaning process

    3.1 Organic solvents used for cleaning printed circuit boards

    The use of organic solvents to clean printed circuit boards is to use their ability to dissolve dirt. After eliminating CFC-113, TCA and other ODS cleaning agents, the current solvent cleaning agents are mainly HCFC, HFC, HFE and other fluorine solvents, and they can also be used. Hydrocarbon solvents, alcohol solvents, etc. In order to improve the cleaning effect of fluorine-based solvents, hydrocarbon solvents, alcohol solvents, etc. are added to form mixed solvents. Some mixed solvents are still azeotropic mixtures with constant boiling points (such as HCFC-141b-141b and methanol, HCFC-225 and Azeotropic solvent cleaning agent formulated with ethanol). Because these fluorine-based solvents also have the advantage of being non-flammable and their performance is very similar to that of CFC-113, the cleaning process and cleaning equipment basically do not need to be changed or only slightly adjusted.

    3.2 Advantages and disadvantages of solvent cleaning

    The solvent cleaning process is relatively simple. It only needs to use the same solvent cleaning agent for cleaning and rinsing. Since the solvent cleaning agent is mostly very volatile, no special drying process is required. After the solvent is used, it can be separated from the dirt by distillation and recycled, which not only reduces the cost, but also relatively simple waste liquid treatment. The cleaning equipment originally used for CFC-113 cleaning can be used without major modification; solvent cleaning is particularly suitable for cleaning printed circuit boards that are sensitive to water and have poor sealing of components. The shortcomings of various alternative solvent cleaning agents have been introduced previously and will not be repeated.

    3.3 Typical solvent cleaning process

    Typical solvent cleaning procedures include the following:

    Ultrasonic and immersion cleaning-spray cleaning-vapor phase rinsing and drying

    Solvent heating immersion cleaning-cold rinsing-spray cleaning-vapor phase rinsing and drying

   Gas phase cleaning-ultrasonic plus immersion cleaning-cold rinsing-gas phase rinsing and drying

    Gas phase cleaning-spray cleaning-gas phase rinsing and drying

 4 No-clean process

    4.1 What is a no-clean process

    No-cleaning process refers to the quality control and process control of raw materials such as printed circuit boards and electronic components. Alternative processes have the characteristics of cost-generation transformation, low production and operation costs, and environmental friendliness. For companies that have a higher degree of automation, a larger scale of production, and less demanding product reliability indicators after welding, it is most suitable to switch to the no-clean process. Moreover, the switch to the no-clean process saves the cost of cleaning equipment, cleaning agent, etc., and can greatly reduce operating costs.

    4.2 The key issues that should be solved by using no-clean technology

    The following three key elements should be fully considered when choosing a no-clean process: the evaluation of the selection of flux/solder paste used; the adjustment and control of the production process; the quality control of the raw materials.

    4.2.1 Selection and evaluation of the flux/solder paste used

    The selection and evaluation of flux/solder paste is the primary task to be solved in the development and implementation of no-clean process. It must be ensured that the residue of flux/solder paste after soldering will not affect the reliability performance indicators of electronic products. Practice has proved that weak organic acid flux with low solid content and rosin flux with low activity and low residual content can meet the requirements of reliability performance indicators of electronic products. Generally, printed circuit boards of electronic products can use RMA type rosin flux with low activity and low residue content. This type of flux does not have special requirements for the soldering environment, but note that there are still many residues after soldering, so it is not It is suitable for use on circuit boards with three-proof coating treatment or surface encapsulation. Circuit boards that require three-layer treatment or other surface protection treatments should use low solid content weak organic acid type flux, because this type of flux has fewer residues after soldering, and it affects the surface coating and the circuit board. The adhesion effect is minimal. However, most of these fluxes have poor anti-oxidation effects on circuit boards and components during the preheating process, so the soldering process should be carried out under nitrogen protection when using these fluxes. Taking nitrogen protection measures can not only prevent the oxidation of circuit boards and components during the preheating process, but also improve the wetting performance of soldering, reduce the formation of solder balls, and improve soldering quality.

    4.2.2 Irregularities and control of the production process

    Due to the use of no-clean flux/solder paste, the soldering process and process parameters will inevitably change, including increasing the use of nitrogen as a shielding gas, adjusting the temperature change curve, and changing the flux coating method (use spray coating instead) , Strengthen the monitoring of flux and lead-tin solder composition, change the installation method of electronic components, the transmission and installation methods of printed circuit boards, etc. For example, in the transmission and installation methods of electronic components and printed circuit boards, mechanized automatic transmission and installation are used instead of manual operations, thereby avoiding the adverse effects of hand sweat and fingerprints on the reliability of printed circuit boards. Through the adjustment of the temperature curve in the wave soldering machine and the reflow soldering machine, the activity of the flux can reach the best state just before the soldering, thereby improving the quality of the candle connection. In addition, it has been introduced above that for printed circuit boards with high reliability requirements, when using low solid content weak organic acid flux, nitrogen protection should also be used during soldering. The spray flux is applied and the amount of flux applied is strictly controlled. Under the premise of ensuring the welding quality, the amount of flux applied is reduced as much as possible, so that the residual flux after welding can be kept to a minimum level. For printed circuit boards with high soldering reliability requirements, the control of process parameters must be more stringent. For example, the temperature curve in the wave soldering machine and reflow soldering machine must be measured before production to meet the process requirements, and the chemistry of lead-tin solder The ingredients must be analyzed at least once, and if found to be non-compliant, they must be replaced immediately. When the flux is foamed and coated, the flux should be monitored in real time. After the completion of the no-clean process welding, the no-clean welding wire must be used when repairing and repairing. Only by adopting an effective production process can we ensure that the use of no-clean flux/solder can achieve good soldering results.

    4.2.3 Quality control of energy raw materials

    The high quality requirements for various raw materials are an important factor affecting the no-clean process. Therefore, the quality of various raw materials must be strictly controlled when the no-clean process is adopted. Such as the control of the cleanliness level and solderability of printed circuit boards and electronic components, the quality and stability of flux/solder, the quality of surface protection materials, the effectiveness of process control and quality management, etc. Because any link in the production process is unqualified, the final product will be unqualified.