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Practical Guide to Water-Based Cleaning Process for Waste Circuit Boards

1 Introduction: Why Cleaning Is a Key Process for Recycling Used PCBs

With the rapid development of the electronics industry, electronic devices are used in almost every sector of the national economy, from consumer electronics to industrial control, from communication base stations to automotive electronics, printed circuit boards (PCBs) are everywhere. At the same time, the acceleration of technological iteration has also led to the generation of a large number of waste electronic products, and electronic waste has become one of the fastest-growing categories of solid waste worldwide.

Currently, in the recycling of used electronic products, the industry basically adopts two technical approaches: first, inspecting and repairing fully functional printed circuit boards or electronic components, then reusing them—this is the so-called "remanufacturing" or "refurbishing" route; second, for severely damaged PCBs or components that cannot be repaired, they are crushed and sorted, and the metals such as copper, gold, silver, tin, as well as materials like plastics and resins, are recovered.

In the recycling process chain for fully functional PCBs or electronic components, the cleaning process is a crucial link connecting past and future. During long-term use and storage, various types of stains accumulate on the surface of waste circuit boards, which differ significantly from those encountered during circuit board manufacturing. Stains in the production process mainly include flux residues, tin balls, oil stains, etc., with relatively simple and predictable compositions; Stains on waste circuit boards, on the other hand, are more complex and mainly include the following categories:

(1) Dust and particulates: long-term accumulation of environmental dust, fibers, metal debris, etc., adheres to the board surface and component gaps;

(2) Oil stains and grease: lubricating oil, hydraulic oil, and human sebum in the operating environment of equipment form a stubborn organic film;

(3) Thermal grease: thermal grease in heat dissipation parts of CPUs, power devices, etc., is extremely difficult to remove once dry;

(4) Conformal coatings and coatings: Some industrial boards coated with acrylic, polyurethane, or silicon-based conformal coatings turn yellow and brittle after aging;

(5) Sugar stains and beverage residues: Common dried residues in consumer electronics such as cola and coffee, with high sugar content and strong stickiness;

(6) Electrolytes and corrosion products: metal oxidation and electrochemical corrosion products caused by battery leakage or humid environments.

If these stains are not thoroughly removed, they will directly affect subsequent appearance inspections, electrical performance tests, rework and soldering, and the final quality of reuse. For example, residual oil stains can cause poor contact with ICT test probes, residual thermal grease affects heat dissipation efficiency after recoating, and sugar stains may cause leakage and corrosion in humid environments. Therefore, the quality of cleaning often determines the yield and final value of recycling and reusing a batch of waste circuit boards.

[Experience Tip] In actual production, the cost of the cleaning process usually accounts for only 5%-8% of the total recycling cost, but subsequent rework and scrap losses caused by poor cleaning can account for more than 30% of the total loss. Thorough cleaning is one of the most cost-effective stages in the recycling process.

2. In-depth analysis of cleaning methods

There are mainly two cleaning methods for waste circuit boards: ultrasonic cleaning and centrifugal cleaning. Each has its own characteristics in terms of mechanism, applicable scenarios, and equipment investment. In actual production, the choice should be based on the characteristics of the cleaning target, stain type, and production capacity requirements, or both can be used together.

2.1 Ultrasonic cleaning method

2.1.1 Mechanism of action

Ultrasonic cleaning is currently the most widely used method for cleaning waste circuit boards. Its core principle is to use three physical effects of ultrasound in liquids—cavitation, acceleration, and direct inflow—to directly and indirectly affect both liquid and contaminants, thereby dispersing, emulsifying, and peeling off the contaminant layer, thereby achieving the cleaning goal.

Cavitation is the core mechanism of ultrasonic cleaning. When ultrasound propagates through a liquid, alternating compression and sparse zones are generated. In the sparse zone, the liquid is pulled apart to form numerous tiny vacuum bubbles (cavitation bubbles); These cavitation bubbles rapidly collapse in the subsequent compression zone, instantly generating local high temperatures (up to thousands of degrees Celsius) and high pressure (up to hundreds of atmospheres), accompanied by strong microjets. This extreme local energy release can directly impact the contaminant layers on the panel surface and component surfaces, causing them to loosen, break, and peel off.

Acceleration refers to ultrasonic vibrations causing liquid molecules to gain high-frequency acceleration. The inertial force generated by this acceleration helps the liquid penetrate into tiny gaps such as beneath components and pin gaps, "flushing" out contaminants hidden in dead spots. Direct flow refers to the directional flow generated by ultrasonic waves propagating through a liquid along the direction of sound wave propagation. This flow can carry stripped debris away from the cleaning surface, prevent secondary deposition, and promote the circulation and renewal of the cleaning liquid.

It is precisely because of the synergy of these three functions that ultrasonic cleaning is especially suitable for cleaning items with micro complex structures—such as circuit boards densely packed with surface-mount devices (SMDs), solder ball gaps beneath BGA chips, and inside connector pins, areas that are difficult to reach by manual brushing or simple immersion.

2.1.2 Standard Process Flow

A complete ultrasonic cleaning process typically includes four steps, with the first two steps optional, and whether to enable them depends on the severity of the stain:

(1) Rough cleaning (optional, not required): Place old circuit boards or electronic components in an ultrasonic cleaner filled with clean water. Using ultrasonic cavitation and mechanical action, most loose dirt such as dust, fibers, and metal debris are removed. The purpose of rough cleaning is to reduce the burden of subsequent fine cleaning, prevent large amounts of dirt from entering the cleaning agent tank, and extend the service life of the cleaning agent. Rough cleaning time is generally controlled at 3-5 minutes, at room temperature.

(2) Soaking (optional, not required): Soak the roughly cleaned old circuit boards or electronic components in an ultrasonic cleaner filled with cleaning agent, allowing the cleaner to fully wet and penetrate into the dirt layer. Soaking time should not be too long, generally controlled at 10-15 minutes. During soaking, ultrasonic vibrations can be used up to 3 times, each vibration lasting no more than 20 minutes to accelerate penetration and emulsification of the cleaning agent. For boards with heavy oil stains and dried thermal grease, the soaking step can significantly improve the efficiency of subsequent fine cleaning.

(3) Fine cleaning: Place the soaked old circuit boards or electronic components into an ultrasonic cleaner filled with cleaning agents for cleaning; this is the core step of the entire process. Fine cleaning time is generally 5-15 minutes, adjusted according to the type and severity of the stain. If conditions permit, appropriate warming (generally 40-60°C) will improve cleaning results—raising the temperature can reduce the viscosity of the cleaning agent, enhance the activity of surfactants, and accelerate the emulsification and dissolution of the dirt. However, the temperature must be determined based on the temperature resistance characteristics of the cleaning object and should not be raised blindly.

(4) Rinsing: Use deionized water (or distilled water) to thoroughly rinse the old circuit boards or electronic components after fine cleaning, removing residual cleaning agents and dirt that has been stripped but not yet washed away. Rinsing is the most easily overlooked yet crucial step in the entire process—if cleaning agents remain, they will form white spots or films on the board surface after drying, affecting appearance and electrical performance. It is recommended to use at least two counterflow rinses: the first rinse with tap or purified water for initial rinsing, and the second with deionized water for final rinsing, ensuring the conductivity of the rinse water is below the specified threshold.

[Tip] The quality of rinse water directly determines the final cleaning effect. After rinsing with tap water, calcium and magnesium ions may leave white spots after drying; If possible, the final rinse must be deionized water, with a recommended resistivity ≥of 10 MΩ·cm.

2.1.3 Applicable Scenarios and Limitations

Ultrasonic cleaning is suitable for cleaning the vast majority of waste circuit boards, especially boards with dense SMD components and stains hidden in the gaps. Its advantages include high cleaning efficiency, good consistency, the ability to handle complex structures, and suitability for mass production. However, it also has certain limitations: for boards containing vibration-sensitive components such as MEMS devices, crystal oscillators, and ultrasonic sensors, high-intensity ultrasound may cause internal structural damage or parameter drift; For boards containing components that cannot be immersed, such as aluminum electrolytic capacitors or button batteries, they need to be removed or protective measures must be taken first. Additionally, the tank size of the ultrasonic cleaner determines the batch size for a single cleaning, so large-area backplanes or server motherboards may require dedicated large-scale equipment.

2.2 Centrifugal Cleaning Method

2.2.1 Working Principle

Centrifugal cleaning technology is a cleaning process that uses centrifugal force to clean electronic circuit components, precision parts, and semiconductor components. Unlike ultrasonic cleaning, which relies on cavitation, centrifugal cleaning uses centrifugal force generated by high-speed rotation to drive the cleaning liquid to penetrate, dissolve, and wash away dirt. After cleaning, centrifugal force is used to achieve drying, achieving excellent penetration, dissolution, and decontamination effects, ensuring component cleanliness without causing mechanical damage.

The working principle of centrifugal cleaning technology is that inside a sealed process treatment chamber, components are driven by a motor to rotate and perform vertical composite movements. The cleaning agent inside the process treatment chamber generates centrifugal force under rotation, which continuously acts on the contaminant layer on the circuit board. Under the force of centrifugal force, the cleaning liquid is forcefully pressed into tiny spaces such as under components and between pins, gradually dissolved, flushed, and separated from dirt, thereby achieving the cleaning goal.

When the component soaked in the cleaning solution begins to rotate, the underlying space of the component and the direction of force are all on the same plane. The centrifugal force and composite centripetal force generated during rotation fill the area around the component to be cleaned with a mixture of cleaning solution. As the rotation direction changes, the solution cleaning agent flows in all directions, and these forces act together on the dirt, causing it to quickly dissolve and wash away.

After cleaning is complete, when the rotating component leaves the solution, the force pushing the liquid into the space at the bottom of the component also shakes off the liquid on top of the component. Then, the component is rotated in hot air to thoroughly remove any residue. This integrated "cleaning-spin-drying-hot air drying" design gives centrifugal cleaning a unique advantage when handling precision devices.

2.2.2 Process Features and Applicable Scenarios

The main features of centrifugal cleaning include: First, it does not rely on the intense energy release of ultrasonic cavitation, making it gentler for sensitive devices (such as MEMS, crystal oscillators, thin chips) without causing vibration damage; Second, centrifugal force can drive the cleaning solution deep into the bottom of components, delivering excellent cleaning results for bottom solder ball areas such as BGA and CSP; Third, after cleaning, it is dried directly by centrifugal spinning with heated air, reducing water stains and the risk of secondary contamination; Fourth, the equipment is usually used for single or small batch processing, suitable for cleaning high-value, precision components.

Centrifugal cleaning is suitable for scenarios with high requirements for cleaning quality and device safety, such as semiconductor components, aerospace-grade circuit boards, medical electronic boards, and precision modules containing MEMS or sensors. Its limitations include small processing volumes per batch, higher equipment costs, and less flexibility for large-area boards than ultrasonic tank bodies. Therefore, in large-scale waste circuit board recycling lines, it is often used as a supplement to ultrasonic cleaning for handling special sensitive batches.

3 Selection of cleaning agents and design of water-based formulations

Choosing the right cleaning agent is one of the key factors determining cleaning effectiveness. Circuit board cleaning can be divided into three types in terms of process: water cleaning, semi-aqueous cleaning, and solvent cleaning, each with its own applicable scenarios and advantages. In recent years, with increasingly stringent environmental regulations and increased occupational health awareness, water-based cleaning has become the preferred solution for more and more recycling companies due to its safety, environmental protection, and cost-effectiveness.

3.1 Comparison of the Three Major Cleaning Process Routes

For easy comparison, the core features of the three cleaning process routes are summarized as follows:

Table 1 Comparison of three circuit board cleaning process routes

3.2 Core Advantages of Water-Based Cleaning Agents

Water-based cleaning processes use water as the cleaning medium. To improve cleaning effectiveness, water-based PCB circuit board cleaners (commonly known in the industry as "water-based board washing water") can be added to water. Water-based cleaning agents are mainly purified water, and by adding surfactants and various cleaning additives, their cleaning ability is enhanced. Compared with solvent-based cleaning agents, water-based cleaners have the following significant advantages:

First, good safety. Water-based cleaning agents are water-based and contain no organic solvents, so they do not pose risks such as fire or explosion, do not require explosion-proof equipment, or special storage conditions, greatly reducing safety management pressure on production sites. At the same time, most water-based cleaning agents are formulated with low toxicity, generally do not endanger workers' health, and do not require complex ventilation and detoxification systems.

Second, low surface tension and good wetting and penetration. By adding appropriate surfactants, the surface tension of water-based cleaning agents can be reduced from about 72 mN/m in pure water to below 30 mN/m, fully wetting the circuit board surface and penetrating tiny spaces such as under components and pin gaps. Water-based cleaning agents can emulsify polar contaminants (such as inorganic salts and electrolyte residues) and non-polar contaminants (such as oil stains and greases) into a stable emulsion, enabling one-time removal.

Third, they are environmentally friendly and can be reused. Water-based cleaning agents contain no organic solvents and have extremely low VOC emissions. After simple flocculation, filtration, and biochemical treatment, waste liquids can meet discharge standards, placing a much less environmental burden on the environment than solvent cleaning. At the same time, during use, water-based cleaning agents filter and remove suspended contaminants, extending the service life of working fluids and further reducing operating costs and waste liquid generation.

Fourth, flexible and diverse formulation design with wide adaptability. Water-based cleaning agent formulations can be tailored to the type of stain—for heavy oil stains, the emulsifier ratio can be increased; for thermal grease, specific dissolving additives can be added; for conformal coatings, pH and solvent systems can be adjusted. This flexibility allows water-based cleaning agents to adapt to the complex and varied types of stains on waste circuit boards.

Because of these advantages, water-based cleaning has significant advantages from the perspective of environmental protection and human health. This is also the fundamental reason why environmentally conscious European and American countries choose water-based cleaning as their preferred process.

3.3 Water-based formulation design approach

Understanding the formulation composition of water-based cleaning agents helps select suitable products based on stain types in actual use, and also aids in identifying problems when cleaning results are unsatisfactory. A typical water-based PCB cleaning agent formula usually includes the following types of components:

(1) Main medium: purified water or deionized water, accounting for 70%-90% of the total formula amount. The quality of the water directly affects the stability of the cleaning agent and the residue after rinsing. Formulation water is usually required to have an electrical conductivity of ≤ 10 μS/cm.

(2) Surfactants: This is the core functional component of water-based cleaning agents, usually accounting for 5%-20%. Surfactant molecules have both hydrophilic and lipophilic ends, which can reduce the surface tension of water, forming a directional arrangement at the oil-water interface to emulsify and disperse oil residue into fine particles suspended in water. Common surfactants include nonionic types (such as fatty alcohol polyoxyethylene ether AEO, alkylphenol ethoxylates TX-10), anionic types (such as sodium dodecylbenzene sulfonate LAS), and amphoteric types. In practice, formulations usually include a combination of multiple surfactants to achieve synergistic effects.

(3) Cleaning additives: usually accounting for 2%-10%, including chelating agents (such as EDTA, sodium citrate, used to complexe calcium and magnesium ions and metal ions in water, preventing saponification and precipitation), corrosion inhibitors (such as benzotriazole BTA, used to protect copper pins and pads from corrosion), defoamers (to control the amount of foam during cleaning), pH regulators (such as ethanolamine, citric acid, used to adjust cleaning agents to appropriate pH), etc.

(4) Functional additives: Specialized ingredients added according to specific stain types, such as polar solvents for thermal grease (such as NMP, ethylene glycol butyl ether), enzyme preparations for sugar stains, and film-forming solvents for conformal coatings.

When selecting models, it is necessary to choose formulations that focus on different types of stains on waste circuit boards. For example, consumer electronics boards dominated by dust and mild oil stains can be replaced with neutral to weak alkali general-purpose water-based cleaning agents; Industrial control boards and server boards dominated by thermal grease and heavy oil stains require enhanced formulations containing polar solvent additives; Boards containing large amounts of aluminum electrolytic capacitors and aluminum alloy structural components require special attention to the corrosion inhibitor performance of cleaning agents to prevent corrosion and blackening of aluminum parts.

[Tip] When choosing water-based cleaning agents, don't focus solely on 'strong cleaning power.' pH, foam characteristics, corrosion inhibition, rinsing ability, and material compatibility are all dimensions that must be considered. It is recommended to conduct small-batch tests on typical boards to be cleaned before mass use, confirming cleaning effectiveness and device safety before going online.

3.4 Synergistic effects of water-based cleaning and ultrasonics

Using ultrasonic cleaning in water-based cleaning processes yields better cleaning results, which is not accidental but a physical inevitability. The strength of ultrasonic cavitation effects is directly related to the density of the cleaning medium—the greater the media density, the less the energy attenuation of ultrasonic waves during propagation, and the greater the energy released when cavitation bubbles collapse. Water's density (about 1 g/cm³) is much higher than common organic solvents (such as isopropanol about 0.79 g/cm³, ethanol about 0.79 g/cm³), so in water-based cleaning, ultrasonic waves can produce stronger cavitation effects and improve cleaning efficiency.

Additionally, surfactants in water-based cleaning agents can reduce the surface tension of liquids, making cavitation bubbles more likely to form and collapse, further enhancing cavitation effectiveness. At the same time, the emulsification of surfactants can rapidly emulsify and disperse oil removed by ultrasonics, preventing them from redepositing onto the panel surface. This synergistic effect of "ultrasonic mechanical peeling + chemical surfactant emulsification" is the core reason why water-based ultrasonic cleaning processes can efficiently remove complex stains.

4 Complete Cleaning Process Practice (SOP)

The previous chapters systematically introduced the principles and selection from the perspectives of principles. This chapter will take the NW610 water-based cleaning agent as an example to present a set of standard operating procedures (SOPs) that can be directly implemented, covering all stages from pre-treatment to final inspection, and offering quick reference for key process parameters.

4.1 List of materials and equipment

The materials and equipment required for water-based ultrasonic cleaning are as follows:

Table 2 List of cleaning materials and equipment

4.2 Standard Operating Procedures

4.2.1 Step 1: Pretreatment and sorting

Sort and pre-process the waste circuit boards to be cleaned. First, visually inspect each board and remove components that cannot be soaked, such as aluminum electrolytic capacitors (electrolyte may leak and contaminate cleaning solution), button batteries, buzzers (with internal vibrating diaphragms), and components containing paper labels. Next, use a brush or compressed air to remove large dirt, loose dust, and metal debris from the board surface, preventing these dirt from accelerating contamination by cleaning agents after entering the cleaning tank. Finally, sort boards by stain type and device sensitivity: clean boards with heavy oil stains and relatively clean boards separately, and handle boards with many sensitive components separately to avoid cross-contamination.

4.2.2 Step 2: Prepare the dilution solution

Dilute NW610 water-based cleaning agent with purified water at a ratio of 1:30 to obtain the cleaning working solution. Specific calculation method: For example, if 31 kilograms of working solution are needed, take 1 kilogram of NW610 stock solution, add 30 kilograms of purified water, and stir evenly. When mixing, add most of the water to the cleaning tank first, then slowly pour in the cleaning agent solution, stirring while pouring to avoid excessive local concentration and excessive foam. The pH value of the diluted working solution is generally between 9-11 (mildly alkaline), please refer to the product manual for details.

[Tip] 1:30 is a generally recommended ratio; in actual use, adjustments can be made according to the severity of the stain. Boards with heavy oil stains or dried thermal grease can be increased to 1:20 or even 1:15; boards with mild stains can be reduced to 1:40-1:50 to save costs. However, too high a concentration may cause difficulty in rinsing and component corrosion; too low concentration means incomplete cleaning, so a balance point needs to be found in practice.

4.2.3 Step 3: Ultrasonic cleaning

Pour the diluted working liquid into the ultrasonic cleaner. The liquid level should at least cover the heating tube and transducer area. Generally, the liquid level should reach more than two-thirds of the tank height. Running ultrasound at low levels or without load is prohibited to avoid damaging the transducer. Place the circuit board into the cleaning basket, keeping an appropriate spacing between boards (generally 5-10mm) to avoid board surface adhesion that prevents cleaning liquid from flowing. Place the cleaning basket into the cleaning machine, ensuring the board is completely submerged in the working fluid.

Start ultrasonic cleaning at room temperature for 10 minutes. During cleaning, observe the turbidity of the working fluid and the cleanliness of the board surface. If obvious stains remain after 10 minutes, you can appropriately extend the cleaning time to 15 minutes or raise the working liquid temperature to 40-50°C before cleaning. For boards with particularly severe stains, it is recommended to soak them in the cleaning agent for 10-15 minutes (during which ultrasonic can be activated 2-3 times, each session lasting 5-10 minutes), then proceed to the formal fine cleaning process.

4.2.4 Step 4: Rinsing the Wash

After cleaning, remove the circuit board from the cleaning machine, drain the cleaning agent solution from the surface, and immediately switch to the rinse process. It is recommended to use at least two countercurrent rinse rinses: the first is initial rinsing with tap water or purified water to remove most of the cleaning agent residue and suspended debris from the board; the second rinse uses deionized water (or distilled water) for final rinsing to ensure ionionic residues are thoroughly removed. Each rinse lasts no less than 2 minutes. During rinsing, you can gently shake the cleaning basket or activate the blow/overflow function of the rinse tank to accelerate the dispersion of residues.

The endpoint of rinsing can be determined by measuring the conductivity of the rinse solution—when the difference between the conductivity of the last rinse and fresh deionized water is less than 5 μS/cm, rinsing can be considered sufficient. If there is no conductivity meter, a visual inspection can also be done: after removing the board from the rinse water, observe whether the water film on the board surface is uniform and continuous. If the water film breaks or water droplets accumulate, it indicates oil stains or cleaning agent residue on the board and need to be continued.

4.2.5 Step Five: Drying

After rinsing, remove the circuit board from the rinse tank, drain surface moisture, and then perform drying. There are various drying methods, commonly including: hot air circulation drying (temperature set to 60-80°C, 30-60 minutes, suitable for most boards), centrifugal spin-drying + hot air drying (suitable for precision parts and boards where water easily accumulates at the bottom of BGAs), isopropanol displacement and dehydration + natural drying (suitable for temperature-sensitive boards, but introduces organic solvents and is not very environmentally friendly).

The drying temperature should be determined based on the temperature resistance characteristics of the components on the board, generally not exceeding 80°C. For boards containing plastic connectors, film capacitors, and LED devices, the temperature should be kept below 60°C. Drying must be thorough, especially in areas prone to water accumulation, such as under BGA chips, inside connectors, and under shielding covers. If moisture remains, it may cause leakage, corrosion, and short circuits during subsequent storage and use. After drying, the board should be placed in a dry, clean environment to cool naturally to room temperature, followed by subsequent inspection and packaging.

4.2.6 Step Six: Inspection

After cleaning and drying, each board needs to be inspected to ensure the cleaning quality is qualified. Inspection includes two aspects: appearance inspection and electrical performance inspection. Appearance inspection is conducted under natural or white light desk lamps, and if necessary, a 5-40x magnifying glass or stereomicroscope is used to check the board surface for oil residue, white spots, water stains, solder balls, component damage, pin discoloration, and other abnormalities. Focus on areas prone to dirt accumulation, such as the BGA, inside connectors, and under shielding covers.

Electrical performance testing mainly measures the insulation resistance of the circuit board to ensure there is no leakage or short circuit caused by residual electricity after cleaning. Using an insulation resistance tester, apply a 500V DC (or select an appropriate test voltage based on the board's operating voltage) between the key power supply and ground of the board to measure the insulation resistance value. Generally, the insulation resistance after cleaning should not be less than 100 MΩ; for boards with high reliability requirements, it should not be less than 1 GΩ. If the insulation resistance is relatively low, it indicates that ionic residues remain on the board surface (such as surfactants in cleaning agents or minerals in tap water), which needs to be rinsed and dried again.

4.3 Process Parameter Quick Reference Table

To facilitate on-site operation and quick reference, the key process parameters in the above process are organized as follows:

Table 3 Quick reference table of water-based ultrasonic cleaning process parameters

5. Factors affecting cleaning effectiveness and optimization of parameters

In the recycling of waste electronic products, the effectiveness of the cleaning process is influenced by multiple factors, among which the three core factors are cleaning temperature, cleaning time, and cleaning power. There is a complex interaction among these three factors, and properly adjusting these three parameters is key to achieving the best cleaning results. This chapter will analyze the mechanisms of each factor one by one and provide methodologies for parameter optimization.

5.1 Cleaning temperature

The temperature of the cleaning agent directly affects its activity, as temperature significantly affects the movement of surfactant molecules, chemical reaction rates, and liquid viscosity. At low temperatures, surfactant molecules move slowly, have weaker penetration and emulsification abilities, and higher liquid viscosity, which is unfavorable for the flow of cleaning solution through tiny gaps. Therefore, removing the same amount of dirt requires a longer cleaning time.

As the temperature rises, the activity of the cleaning agent gradually increases, improving cleaning efficiency. Generally speaking, within the range of 40-60°C, most water-based cleaning agents can achieve optimal cleaning performance. However, if the cleaning temperature is too high, it can easily cause a series of adverse effects:

First, the cleaning agent's lifespan is shortened. High temperatures accelerate the decomposition and failure of surfactants, and also speed up the evaporation of moisture in the cleaning solution, causing the working fluid concentration to fluctuate constantly, requiring frequent replenishment and replacement.

Second, markings on the surface of electronic components may change color or peel off. Many components use ink printing for screen markings, which can soften, discolor, or even peel off at high temperatures, affecting component recognition.

Third, metal structural parts treated with surface treatment may discolor or even corrode. Some metal structural parts treated with galvanizing, nickel plating, anodizing, and other surface treatments may undergo coating discoloration, oxidation, or even corrosion in high-temperature alkaline cleaning solutions.

Fourth, if the cleaning agent contains a high amount of alkaline components, high-temperature conditions can cause discoloration of device leads (especially copper leads), forming blue-black or deep black oxidation stains, which not only affect the product's appearance but may also impact soldering and electrical performance.

Therefore, the cleaning temperature should be comprehensively adjusted according to the characteristics of the target to achieve the best cleaning effect. For industrial boards mainly composed of oil stains and thermal grease, where the device has good temperature resistance, the temperature can be appropriately increased to 50-60°C; For consumer electronic boards containing plastic parts, LEDs, film capacitors, and other thermal devices, it is recommended to clean within the range of room temperature up to 40°C.

[Tip] When heating and cleaning, always wait until the temperature stabilizes before placing the board in. Do not gradually increase the temperature after placing it in a cold state—uneven temperature during heating and local overheating may damage the device. At the same time, the temperature controller of the cleaning machine should be calibrated regularly to avoid large deviations between the actual temperature and the displayed temperature.

5.2 Cleaning time

Cleaning time is an important parameter in the cleaning process. Mechanically, as the cleaning time increases, the cavitation effect of ultrasound and the chemical action of cleaning agents continue, gradually reducing the amount of stains left on the circuit board components. However, this reduction is not linear—initially, stains are removed quickly, but as easily removed stains are removed, the remaining stubborn stains take longer to be removed, and eventually, the amount of stains retained tends to a balanced value and no longer decreases significantly over time.

Generally, based on experience, when cleaning time exceeds a certain duration (e.g., 10-15 minutes), the amount of stains left will no longer change significantly. This means that indefinitely extending cleaning time cannot further improve cleaning results and may instead cause the following problems: First, production efficiency decreases, and excessively long cleaning times per batch affect the overall capacity of the recycling line; Second, for vulnerable and sensitive components, extending cleaning time means longer ultrasonic vibration and chemical immersion, which may affect the reliability of electronic components, such as causing MEMS device parameter drift, crystal oscillator frequency shifts, and internal stress changes in thin chips; Third, extending cleaning time accelerates the consumption and aging of cleaning agents, increasing operating costs.

Therefore, cleaning time should be reasonably determined based on cleaning requirements, target items, and production cycles. It is recommended to start experimenting with a 10-minute baseline time and gradually adjust based on the cleaning effect: if stains are basically removed after 10 minutes, try shortening to 8 minutes to improve efficiency; If obvious residue remains after 10 minutes, extend to 15 minutes, or increase temperature and concentration before retesting. Finding the shortest time that "just cleans thoroughly" is the optimal cleaning time.

5.3 Cleaning power

Cleaning power refers to the output power of the ultrasonic cleaner, usually measured by power density (W/L, i.e., power per liter of cleaning solution). Generally, the higher the cleaning power, the more cavitation bubbles produced per unit time, the greater the cavitation intensity, and the better the cleaning effect. For stubborn stains such as heavy oil stains and dried thermal grease, higher power can significantly shorten cleaning time.

However, higher cleaning power is not always better. First, the higher the cleaning power, the higher the equipment cost—high-power ultrasonic transducers and driver power supplies are more expensive, and the equipment also has higher energy consumption. Second, excessive power can also negatively impact vulnerable and sensitive components. High-intensity ultrasonic cavitation generates micro-jets and localized high pressure on component surfaces, which may cause surface passivation layer damage to thin chips, microstructure fractures in MEMS devices, frequency characteristic drift in crystal oscillators, and loosening of aluminum wire bonding points. For boards containing these sensitive components, excessively high cleaning power may actually cause device failure.

In practical selection, the power density for cleaning general waste circuit boards is generally in the range of 5-15 W/L. For industrial and server boards mainly composed of through-hole components and robust structures, higher power density (10-15 W/L) can be chosen to improve cleaning efficiency; For consumer electronics and communication boards with dense SMD components and sensitive components, it is recommended to choose lower power density (5-10 W/L), and prioritize cleaning effectiveness by increasing temperature and extending the time, rather than blindly increasing power.

5.4 Multi-factor interaction and tuning methodology

Temperature, time, and power are not independent factors; there is a clear interaction between them. For example, increasing temperature can shorten the time required to achieve the same cleaning effect; Increasing power can achieve the same cleaning effect in a shorter time; Whereas at high temperatures, excessively high power may accelerate device damage. Therefore, parameter tuning should not focus on a single factor but requires a comprehensive consideration.

In actual production, the following optimization methodologies are recommended:

Step one: Define constraints. Define hard constraints such as the upper temperature resistance limit of the device to be cleaned, type of sensitive device, maximum allowable cleaning time, and maximum power of the equipment. These are the boundaries that parameter tuning cannot cross.

Step two: Set the benchmark parameters. Starting from the general recommended parameters—temperature, room temperature, time 10 minutes, power density 8-10 W/L, dilution ratio 1:30—conduct the first round of cleaning tests.

Step three: single-factor variable adjustment. If the cleaning effect is unsatisfactory, single-factor adjustment is performed in the order of "temperature first, then time, then power." Temperature adjustment is prioritized because temperature improves cleaning efficiency most significantly and is cost-effective; Second is adjusting time, as time adjustment does not require additional equipment; Power adjustment is considered last, as power is limited by equipment and may affect device safety.

Step four: Validation and solidification. After identifying parameter combinations that stably meet cleaning quality requirements, conduct at least three batches of repeated verification to confirm consistency and stability of results, then finalize the parameters into the SOP and establish a regular review mechanism.

Step five: continuous optimization. Record cleaning parameters and effects for each batch during production, accumulate data, and gradually optimize parameter combinations. For recycling businesses with large changes in stain types, it is recommended to establish a "stain type-parameter combination" correspondence table to quickly match optimal parameters for different batches.

6. Safe operation and environmental compliance

Although water-based cleaning is much safer than solvent cleaning, it still involves chemical agents, electrical equipment, and waste liquid treatment, so comprehensive safety operation standards and environmental management systems must be established. This chapter covers four aspects: personal protection, equipment safety, waste liquid treatment, and environmental compliance.

6.1 Personal Protective Equipment (PPE)

Operators must wear the following personal protective equipment during cleaning tasks:

(1) Acid and alkali resistant gloves: Choose long-sleeve gloves made of nitrile rubber or neoprene, at least 10cm above the wrist to prevent the cleaner from contacting the skin. Although water-based cleaning agents are low in toxicity, long-term exposure may cause skin dryness, degreasing, and allergies.

(2) Goggles or protective screens: Prevent cleaning liquid from splashing into your eyes. Pay special attention to protection when pouring cleaning agents or removing boards from the cleaning machine.

(3) Waterproof aprons or chemical-resistant clothing: Prevent cleaning agents from splashing onto clothes, protecting skin and clothing.

(4) Non-slip waterproof shoes: The floor in the washing area is prone to water stains, so non-slip shoes can prevent slips and falls.

(5) Optional: Noise-canceling earplugs. High-power ultrasonic cleaners may generate relatively high noise during operation (especially low-frequency models at 28kHz). Prolonged exposure may affect hearing, so it is recommended to wear noise-canceling earplugs when operating near the device.

6.2 Safe operation of equipment

The following points should be noted for the safe operation of ultrasonic cleaners:

First, grounding protection. Ultrasonic cleaners are high-power electrical equipment and must be reliably grounded, with a grounding resistance not exceeding 4Ω. Check the power cord and plug before use; do not use damaged power cords.

Second, liquid level requirements. Before starting the ultrasonic system, confirm that the liquid level in the cleaning tank has reached the specified level (generally at least covering the heating tube and transducer area, reaching more than two-thirds of the tank height). Operating ultrasound at low liquid levels or without load is prohibited, otherwise it may cause the transducer to overheat, damage, or even cause a fire.

Third, heating safety. For cleaning machines with heating functions, the heating tube must be completely submerged in the liquid to start heating; dry burning is prohibited. The thermostat should be calibrated regularly, and the over-temperature protection device should be tested regularly.

Fourth, do not reach your hand into the operating cleaning tank. The cavitation effect of ultrasound in the liquid may cause skin damage (manifested as redness and tingling); plates must be removed after the ultrasound is turned off or using dedicated cleaning baskets and fixtures.

Fifth, the washing basket and fixtures should be made of acid- and alkali-resistant materials (such as 304 stainless steel, PP plastic). Iron or easily corrosive containers are prohibited to prevent rust from contaminating the cleaning solution and the board surface.

6.3 Waste liquid collection and treatment

The waste liquids generated from water-based cleaning mainly include: ineffective cleaning agent working fluids, rinsing wastewater, and oily sewage from pretreatment. These wastes contain surfactants, emulsified oil stains, heavy metals (such as copper, lead, tin dissolved from circuit boards), and suspended solids. They cannot be discharged directly into municipal sewage pipelines and must be treated in compliance with regulations.

The basic process of waste liquid treatment includes:

(1) Classified collection: Collect cleaning agent waste liquid and rinsing wastewater separately. The cleaning agent waste liquid has a high concentration and heavy pollutant load, requiring separate treatment; Rinsing wastewater has a low pollutant concentration and can be partially reused after simple treatment.

(2) Demulsification and oil removal: Adding demulsifiers (such as calcium chloride, aluminum sulfate, PAC, etc.) to cleaning agent waste liquids disrupts the emulsification state, causing oil contamination to float and separate, and the recovered waste oil is disposed of as hazardous waste.

(3) Coagulation and precipitation: adding flocculants (such as PAM) and pH regulators to form floc precipitates between suspended matter and heavy metal ions, removing most of the suspended matter and heavy metals.

(4) Biochemical treatment: The pretreated wastewater enters a biochemical treatment system (such as A/O, MBR, etc.), where microorganisms degrade organic matter in the water (mainly surfactants).

(5) Filtration and discharge: Biochemical effluent must be filtered and disinfected, and only discharged after testing to meet the "Integrated Wastewater Discharge Standard" (GB 8978) or local discharge standards.

For small and medium-sized recycling enterprises that cannot afford to build their own wastewater treatment systems, waste liquids should be entrusted to qualified hazardous waste treatment units for disposal, and the hazardous waste transfer manifest system must be strictly enforced, with proper ledger records.

[Tip] Recycling rinse water is an effective way to reduce wastewater generation. The second rinse water (which is less polluting) can be filtered and reused for the first rinse run, which is then filtered and reused for rough washing, forming a countercurrent rinse + recycling water system. This can reduce fresh water consumption by more than 50% while also decreasing wastewater generation.

6.4 Key points for environmental compliance

Water-based cleaning processes themselves have good environmental attributes, but in actual operation, the following environmental compliance points must still be considered:

First, the choice of cleaning agent. Priority should be given to water-based cleaning agents certified by environmental protection (such as RoHS, REACH), and avoid those containing ODS (ozone-depleting substances), high VOCs, or highly toxic ingredients. Pay attention to phosphorus content in cleaning agents, as phosphorus-containing agents can cause eutrophication of water bodies and have been restricted in some regions.

Second, VOC emissions. Although water-based cleaning emits VOCs much less than solvent cleaning, some enhanced formulations may contain small amounts of alcohol ether solvents, which may volatilize slightly during heated cleaning. It is recommended to install a local exhaust device above the cleaning machine to collect exhaust gases and treat them with activated carbon adsorption.

Third, hazardous waste management. Waste oil, waste cleaning agent filters, oily sludge, and other materials generated during cleaning are classified and stored, labeled, and disposed of according to the requirements of the National Hazardous Waste Catalogue. It is strictly forbidden to mix in household waste or dump it arbitrarily.

Fourth, clean production. Actively promote the concept of clean production, optimizing process parameters, increasing rinsing water reuse rates, and selecting low-foam, easy-rinsing cleaning agents to reduce pollutant generation at the source, rather than relying solely on end-of-pipe treatment.

7 Frequently Asked Questions (FAQ)

During actual cleaning, various problems are often encountered. This chapter summarizes the six most common types of issues in water-based cleaning of waste circuit boards, analyzes possible causes, and provides troubleshooting and solution ideas for on-site technical personnel to reference.

7.1 White spots or residues remain on the board surface after cleaning

Phenomenon: After cleaning and drying, white spots, thin film, or water stains appear on the board surface, and when touched, it feels powdery or sticky.

Possible causes and solutions:

(1) Insufficient rinsing: surfactants and additives in the cleaning agent remain on the board surface, forming white spots after drying. Solution: Increase the number of rinsing passes (at least 2), extend the rinsing time for each pass, and the last rinse must be done with deionized water.

(2) Poor rinsing water quality: Using tap water causes calcium and magnesium ions in the water to dry and form water-stained white spots. Solution: The final rinse uses deionized water or distilled water to control the conductivity of the rinse water.

(3) Excessive concentration of cleaning agent: The concentration of the working fluid exceeds the recommended ratio during preparation, making rinsing difficult. Solution: Reformulate at a standard ratio of 1:30, or appropriately reduce the concentration.

(4) Improper drying method: When drying naturally, water evaporation is uneven, causing solute to concentrate and precipitate locally. Solution: Switch to hot air circulation drying to ensure uniform temperature and thorough drying.

7.2 Component silkscreen or markings falling off

Phenomenon: After cleaning, the model screen, polarity markings, color rings, etc., on the surface of the components become blurred, discolored, or peel off.

Possible causes and solutions:

(1) Excessive cleaning temperature: High temperatures soften and expand the screen printing ink, causing it to fall off under ultrasonic action. Solution: Lower the cleaning temperature and control boards containing ink screen printing elements at room temperature to 40°C.

(2) Cleaning agents are too alkaline: Cleaning agents with high pH can corrode certain types of screen printing ink. Solution: Choose mild neutral or mildly alkaline cleaning agents, or shorten the cleaning time.

(3) Excessive cleaning time: Prolonged ultrasonic vibration and chemical immersion accelerate screen screen wear. Solution: Shorten cleaning time and find the shortest time to "clean just enough."

(4) Excessive ultrasonic power: High-intensity cavitation microjets directly impact the component surface, causing the silkscreen to be "knocked off." Solution: Reduce ultrasonic power density or switch to a 40kHz high-frequency model (cavitation is milder).

7.3 Low cleaning efficiency and inability to remove grease stains

Phenomenon: After cleaning, obvious oil stains, thermal grease, or other stains remain on the panel surface, indicating that the cleaning effect does not meet requirements.

Possible causes and solutions:

(1) Insufficient cleaning agent concentration: The dilution ratio of the working solution is too high, resulting in insufficient concentration of active ingredients. Solution: Increase the cleaning agent concentration by adjusting from 1:30 to 1:20 or 1:15.

(2) Low cleaning temperature: At room temperature, the detergent is inactive and emulsifies and dissolves heavy oil stains slowly. Solution: Appropriately heat to 40-60°C to enhance the detergent's activity.

(3) Insufficient ultrasonic power: low power density and insufficient cavitation strength make it impossible to effectively remove stubborn stains. Solution: Increase ultrasonic power or check whether the transducer is aging or attenuating.

(4) Short cleaning time: Stubborn stains require longer duration. Solution: Extend the cleaning time to 15-20 minutes, or add a pre-soaking step.

(5) Incorrect choice of cleaning agent: General-purpose cleaning agents lack sufficient cleaning power against specific stains (such as dried thermal grease or aged conformal coatings). Solution: Choose specialized enhanced cleaning agents for such stains, or add an appropriate amount of polar solvent additives to the cleaning agent.

(6) Working fluid aging and failure: Cleaning fluid has been used for too long, surfactant is depleted, and the oil load is saturated. Solution: Replace with fresh cleaning solution and establish regular replacement and filtration maintenance systems.

7.4 Excessive foam overflow of cleaning agent

Phenomenon: During ultrasonic cleaning, a large amount of foam is generated, and foam overflows from the cleaning tank, affecting normal operation.

Possible causes and solutions:

(1) Surfactant selection issues: Some high-foam surfactants produce a large amount of stable foam under ultrasonic action. Solution: Choose low-foam or defoaming water-based cleaning agent products.

(2) Excessive cleaning agent concentration: The higher the concentration, the greater the foam generated. Solution: Dilute according to the recommended ratio; do not arbitrarily increase the concentration.

(3) Too low temperature: Foam is more stable at low temperatures and less likely to burst. Solution: Appropriately increase the cleaning temperature; high temperatures help eliminate foam.

(4) Mechanical air entrapment: Inserting the cleaning basket too quickly, or introducing a large amount of air during the cleaning liquid circulation. Solution: Insert the cleaning basket slowly to avoid vigorous stirring; Check whether the circulation system is leaking air.

(5) Temporary emergency: A small amount of defoamer (such as silicone defoamer) can be sprayed on the surface of the cleaning solution, but be aware that defoamers may affect cleaning results and subsequent rinsing, so use them with caution.

7.5 Discoloration and blackening of metal pins

Phenomenon: After cleaning, the copper pins, pads, or metal structural parts of the component discolor, appearing blue-black, dark brown, or grayish, affecting appearance and solderability.

Possible causes and solutions:

(1) High-temperature oxidation of alkaline components: Cleaning agents contain high levels of alkaline components, which react with copper pins at high temperatures to form blue-black or deep black copper oxide/copper hydroxide stains. Solution: Lower the cleaning temperature or choose cleaners with low alkalinity and copper corrosion inhibitors.

(2) Cleaning agent pH too high: Strong alkaline cleaning agents are corrosive to metals. Solution: Choose weakly alkaline cleaning agents with a pH between 9 and 10, and avoid strongly alkaline products with a pH >12.

(3) Insufficient or ineffective corrosion inhibitors: Insufficient copper corrosion inhibitors (such as BTA) in cleaning agents, or prolonged use of working fluids leading to corrosion inhibitor consumption. Solution: Replace with fresh working fluids or choose cleaners with better corrosion inhibition performance.

(4) Not drying promptly after rinsing: The wet panel surface containing alkaline residue naturally oxidizes in the air. Solution: Immediately dry with hot air after rinsing; do not leave it damp for extended periods.

7.6 Failure of sensitive components after cleaning

Phenomenon: Boards containing MEMS devices, crystal oscillators, ultrasonic sensors, thin chips, and other sensitive components show parameter drift, abnormal functionality, or complete failure after cleaning.

Possible causes and solutions:

(1) Excessive ultrasonic power: The mechanical energy of intense cavitation causes MEMS microstructure fractures, oscillator frequency drift, and loose bonding points. Solution: Reduce ultrasonic power density to below 5 W/L, or switch to high-frequency models above 40kHz.

(2) Excessive cleaning time: Prolonged ultrasonic vibration accumulates damage. Solution: Shorten cleaning time and keep boards with many sensitive components within 5 minutes.

(3) Excessive cleaning temperature: High temperatures cause internal stress changes in components and deformation of plastic packages. Solution: Clean at room temperature, do not heat up.

(4) Cleaning agent seeping into the device: Some MEMS devices and sensors have ventilation holes or microchannels, so once the cleaning agent seeps in, it cannot be completely discharged, leading to device failure. Solution: Before cleaning, confirm whether the device can be soaked. Components that cannot be soaked should be removed or sealed for protection.

(5) Fundamental solution: For high-value boards densely packed with sensitive components, it is recommended to switch to centrifugal cleaning to avoid the impact of ultrasonic vibration on the device. Centrifugal cleaning is gentler on sensitive components and directly spins dry after cleaning, reducing the risk of liquid seeping into the device.

8 Summary and Best Practices

The cleaning process in recycling waste electronic products is a systematic project that requires comprehensive consideration of cleaning methods, cleaning agent selection, and process parameters to effectively clean fully functional PCBs or electronic components, improving the efficiency and quality of recycling and reuse. From the systematic introduction in this article, the following best practice conclusions can be drawn:

First, regarding the choice of cleaning method, ultrasonic cleaning is highly efficient, suitable for complex structures, and suitable for mass production, making it the main method for cleaning waste circuit boards; For high-value precision boards containing vibration-sensitive components such as MEMS and crystal oscillators, centrifugal cleaning is a safer choice. The two can be flexibly combined according to product type, forming a cleaning capability layout of "ultrasonic-dominated, centrifugal as supplementary."

Second, when selecting cleaning agents, water-based cleaning agents should be prioritized. Water-based cleaning agents have significant advantages such as safety, non-flammability, low toxicity and environmental protection, low cost, and flexible formulations, making them the optimal choice that meets industry development trends and environmental regulations. When selecting specific products, formulas should be selected based on the main types of stains on waste circuit boards (oil stains, thermal grease, sugar stains, conformal coatings, etc.), with a focus on key indicators such as pH value, corrosion resistance, foam characteristics, and rinsability.

Third, in process specification optimization, temperature, time, and power are the three core variables. Temperature improves cleaning efficiency the most significantly, but due to device temperature resistance limitations, there is a "balance point" in time; longer power is not always better; higher power is better but may damage sensitive components. System optimization should be carried out according to the methodology of "setting temperature first, then adjusting time, and finally fine-tuning power," combined with the characteristics of the cleaning object, and finalizing the optimal parameters into the SOP.

Fourth, rinsing and drying are the most overlooked yet crucial steps in the entire cleaning process. Insufficient rinsing can lead to residual cleaning agents and white spots, while incomplete drying can result in water retention and subsequent corrosion. It is recommended to use at least two counterflow rinses, with deionized water for the last and control the rinsing endpoint through conductivity testing; Drying uses hot air circulation to ensure thorough drying of dead areas such as below BGAs and inside connectors.

Fifth, safety and environmental protection are the baseline for the sustainable operation of cleaning processes. Although water-based cleaning is safer than solvent cleaning, it still requires personal protection, safe equipment operation, and compliant waste liquid handling. Actively promoting clean production by recycling rinsing water, selecting low-foam cleaning agents, and optimizing process parameters to reduce pollutants at the source is both a compliance requirement for environmental protection and an effective way to reduce operating costs.

Finally, optimizing the cleaning process is a process of continuous improvement. It is recommended to establish a cleaning batch record system to record each batch of stain types, cleaning parameters, cleaning effects, and abnormal situations. Through data accumulation, gradually improve the correspondence between "stain type and optimal parameters," continuously enhancing cleaning quality and efficiency, and providing solid process support for high-value recycling of waste electronic products.