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As the performance of precision electronic devices such as optical modules (including cameras and sensors) improves and demand for them increases, the technology for removing microscopic foreign substances (particles) that occur or adhere to products during the manufacturing process is becoming increasingly important. Meanwhile, legislation on chemical substances is becoming stricter, and the organic fluorine compounds that have been commonly used for particle removal are now being restricted in their manufacture and use, mainly in Europe and the United States, due to concerns about water pollution and the harmful effects they have on the human body. In this paper, we will introduce the particle removal cleaning agents “MICROCLEAN FPS-4700” and MICROCLEAN FPS-4712, which have a number of excellent features such as high cleaning power, safety, and low VOC, and the particle removal system that contributes to reducing running costs while maximizing the cleaning performance, which were newly developed by us, a manufacturer of flux cleaning agents and cleaning equipment, based on our know-how.
Particles are a general term for microscopic particles of foreign matter in the order of nanometers to micrometers that are generated and mixed in during the manufacturing process of electronic devices and components, and which have the potential to adversely affect product quality and yield. The substances that make up particles are diverse, including cutting dust generated during processing and molding, dust in the air, and metal and resin shavings generated by contact and wear of the moving parts of manufacturing equipment, regardless of whether they are organic or inorganic.
Among these, the workers themselves in the manufacturing process are one of the biggest sources of particle generation. The human body is covered in a large number of foreign substances such as microscopic particles and fiber waste, and if skin flakes or skin surface secretions fall off or scatter due to the worker's movements, there is a high risk of contaminating the product surface. In some cases, this includes salt from sweat, iron oxide and titanium oxide from hair products and cosmetics, and there is a risk of damaging the electrical reliability of the product due to metal contamination. For this reason, workers are required to wear dust-proof clothing and masks that meet the cleanliness requirements of the workplace, and are required to follow a number of rules, but as long as people are involved, it is difficult to eliminate the generation of particles completely.
At present, the installation of collision damage mitigation brakes and rear-view cameras is being phased in for new domestic vehicles, and the spread of advanced driving support systems is progressing rapidly. In 2021, the world's first commercially available vehicles with Level 3 (automatic driving under specific conditions) functions will be launched, and development and legal reform aimed at achieving Level 4 (fully automatic driving in limited areas) is also progressing in various countries.⑴。
The key to improving the safety and functionality of automobiles is sensors that accurately recognize the environment around the vehicle. A wide range of camera modules and optical sensors are used in combination, including viewing cameras that monitor blind spots for the driver, sensing cameras that check the distance between vehicles and the safety of the surrounding area, and LiDAR (Light Detection and Ranging). In the manufacturing process for these sensors, particles that adversely affect detection accuracy are a major enemy. If particles remain on the control board or actuator, they can contaminate important components such as image sensors and optical elements from the inside due to unit operation or vibration after assembly. As a result, this can lead to a decrease in image processing accuracy and image quality, and cause serious accidents due to false detection and operational failure.
In addition to in-vehicle applications, there has been a marked increase in demand for camera modules, particularly for mobile device cameras, and there has been a significant increase in functionality. In addition to the increasing popularity of multi-lens cameras for smartphones and other small devices to accommodate a wide range of focal lengths and shooting environments, high-end models are now appearing with ultra-high resolution cameras of over 100 million pixels. In proportion to the improvement in camera functions, smaller particles of less than 10µm in diameter in smaller quantities are also becoming a problem for product quality, and while demand for cleaning is increasing, there is also a demand for even higher levels of cleaning quality than before.
Most particles are solid materials that are insoluble or difficult to dissolve in various solvents, such as metal particles and proteins, so they require “separate-type cleaning”, where they are peeled off from the object to be cleaned and removed. When the particle size is less than the order of millimeters, the forces that pull the particles away from the surface of the base material, such as gravity and electrostatic repulsion, are dominated by the forces that attract the particles to the base material, such as van der Waals forces and solid-liquid bridging forces, and the relative adhesion increases, making removal difficult⑵. Therefore, it is difficult to remove particles by immersion or liquid flow alone, and it is important to select a cleaning method that effectively applies physical force. In wet cleaning, the ultrasonic method is one of the most effective cleaning methods for particles. The synergistic effects of the physical and chemical actions of ultrasonic irradiation in liquids, such as cavitation, molecular acceleration, and reaction promotion, can quickly detach and disperse various particles. The degree of contribution of each action changes depending on the frequency, so better cleaning quality can be obtained by setting conditions based on the particle size and material of the object to be removed.
Fluorine-based cleaning agents such as HFE (hydrofluoroether) and HFC (hydrofluorocarbon), as well as aqueous cleaning agents and pure water, have been widely used as particle removal cleaning agents. HFE and HFC are non-flammable and have excellent drying properties, and they are highly compatible with resin and metal components. Compared to most organic solvents and water, they have a specific gravity that is around 1.5 times higher, and the physical action of ultrasonic cleaning exerts a large amount of drag on the object being cleaned, and is excellent at peeling particles off the surface of the base material. However, they have limited oil and fat dissolving power and tend to be selective about the objects they clean. In addition, these fluorine compounds are collectively referred to as PFAS (per- and polyfluoroalkyl substances), and because they are extremely difficult to break down in nature and have a high bioaccumulation rate, it has been pointed out that they can cause various health problems, such as an increased risk of cancer, when they are taken into the human body⑶. In Europe and the United States, it is expected that the manufacture and use of all PFAS, which number over 10,000, will be regulated by 2025, and in Japan, too, measures are being considered in response to the discovery of PFAS contamination in tap water in some areas and the detection of PFAS in the blood of local residents.
In the case of aqueous cleaning agents, formulations containing surfactants and alkaline ingredients are generally used for particle removal. As the main ingredient is water, both VOC (volatile organic compounds) and GWP (global warming potential) are almost zero, and as it has no flash point and is highly safe, water is easily able to generate cavitation, and high cleaning effects can be obtained using an ultrasonic wave. On the other hand, in the rinsing process, a large amount of wastewater is continuously generated by the continuous flow of pure water, so there are significant costs involved in installing and operating various treatment facilities. There is also a risk that surfactants that cannot be rinsed away with water will remain on the surface of the object being cleaned, and in the cleaning of electronic devices, there is a concern that this will cause moisture to be absorbed by the surface of the transmission path, increasing the dielectric loss of the electronic circuit.
As environmental problems, such as global warming, become more serious, laws and regulations surrounding chemical substances, not just PFAS, are being reviewed worldwide, and stricter management and operation are being demanded. Industrial cleaning, which uses large amounts of chemicals and pure water, is always in close proximity to environmental problems, and while meeting the high cleaning quality requirements, there is a desire for the development of systems that are even more safety- and environment-conscious.
MICROCLEAN FPS-4712 (hereinafter referred to as “FPS-4712”) is a weakly alkaline cleaning agent for particle removal that is based on glycol ether and contains approximately 70% water. When used under heated conditions, it separates into two phases and can be used in a uniform, white emulsion state to remove all types of particles, both organic and inorganic. The removal rate and appearance of various particles after being applied to a glass plate and cleaned with FPS-4712 are shown in Figure 1. FPS-4712 was able to remove almost all of the inorganic particles, which have relatively weak adhesive properties, as well as foreign substances of human origin, which are difficult to remove with other cleaning agents.
| Object to be cleaned | Before | MICROCLEAN FPS-4712 | MICROCLEAN FPS-4700 | Fluorine-based | Water-based |
|---|---|---|---|---|---|
| Sebum + skin fragments Average particle size: 20μm |  |  |  |  |  |
| ◎ | ◯ | △ | ◯' | ||
| Aluminum Average particle size: 3μm |  |  |  |  |  |
| ◎ | ◎ | ◎ | ◯ | ||
| Talc Average particle size: 0.5μm |  |  |  |  |  |
| ◎ | ◯ | △ | ✕ |
Removal rate evaluation criteria ◎：99％ or more／◯：98 ～ 90％／◯'：89 ～ 70％／△：69 ～ 40％／✕：39％ or less
Cleaning conditions: Ultrasonic 45kHz/300W, 50°C/1min cleaning
Figure 1: Comparison of particle removal performance
One of the reasons why the FPS-4712 has high particle removal performance is that it has an independent oil phase in the aqueous phase. The two phases, which have very different polarities, come into contact with the particle surface in an alternating and continuous manner, and this effectively dissolves various polar and non-polar contaminants such as salts and oils that cause solid-liquid bridging. In addition, the phase with a high affinity for the particle surface spreads preferentially, and the other phase plays a role in peeling them off the base material surface and dispersing them, thereby providing a powerful separation and removal effect. Furthermore, the alkaline components in the cleaning agent deprive protons (H+) on the surfaces of the immersed objects to be cleaned and particles, causing them to become negatively charged. The cations in the liquid then gather around them in a cloud-like formation (electric double layer), increasing the repulsion (electrostatic repulsion) between the surfaces and, as a result, promoting the detachment and dispersion of particles while preventing them from reattaching.
For applications where further reduction of component impact and VOC reduction is desired, the MICROCLEAN FPS-4700 (hereinafter referred to as “FPS-4700”) is also available. With a VOC content of less than 100 g/L, it is classified as a “low VOC content semi-aqueous cleaning agent” in the mandatory national standard (GB38508-2020) enforced in China in 2020, and can be used without any problems in China. The FPS-4700 has the same feature of phase separation by heating as FPS-4712, and although it is slightly inferior to FPS-4712, it has good removal properties for all types of particles that are equal to or better than fluorine-based and water-based cleaning agents (Figure 1). Furthermore, because it has no effect on almost all metal and resin materials in terms of formulation design, it can be used for a wide range of applications without being limited to the base material or the object to be cleaned.
The general properties of the FPS-4700 and FPS-4712 are shown in Table 1. In addition to the fact that neither of these products has a flash point and is therefore not covered by Japan's Fire Service Act (Hazardous Materials), they are also not covered by the PRTR system or the Organic Ordinance of the Industrial Safety and Health Act, etc., so they can be used safely regardless of the method of operation or cleaning. In particular, because both cleaning agents are used in an emulsion state with water as the dispersant, they are superior to general solvent-based cleaning agents in terms of the propagation of the ultrasonic waves, and it is possible to effectively apply the impact force of cavitation to the particles. In addition, because they do not contain non-volatile components, they can be rinsed and dried with a single liquid, just like fluorine-based cleaning agents, and when used in combination with our particle removal system, they can be used for continuous in-line distillation and recycling.
Table 1: General Properties of MICROCLEAN FPS-4712 and MICROCLEAN FPS-4700
| Product# | MICROCLEAN | |
|---|---|---|
| FPS-4712 | FPS-4700 | |
| Specific Gravity | 0.98 | 0.99 |
| Surfae tension mN/m（25 ℃） | 30 | 32 |
| boiling point ℃ | ≧100 | ≧100 |
| Flash point ℃ | None | None |
| ODP | 0 | 0 |
| Chinese VOC restriction （GB38508-2020） | Semi-aqueous | Low VOC Semi-aqueous |
In this section, we will introduce an overview of the batch-type ultrasonic cleaning equipment for particles that we have newly developed based on the know-how we have accumulated over many years as a top manufacturer of flux cleaning agents and cleaning equipment. It is based on a two-tank configuration of a cleaning tank with a capacity of approximately 50 L and a drying tank, and in addition to a multi-stage filter mechanism, it also has a built-in compact Distiller(Distillation and recycling device).
In particle removal, the cleanliness control of the cleaning agent is the most important point. As the particle concentration in the liquid increases, the risk of re-contamination of the base material surface also increases, so it is necessary to install a filter in the liquid circulation line to collect foreign matter and maintain a concentration that does not affect the cleaning quality. In addition, in order to reliably remove particles larger than the particle size to be removed from the liquid within the cleaning time and prevent re-adhesion, it is necessary to select a filter structure and filtration accuracy that match the object, and to design the filtration flow rate and liquid circulation line based on the tank capacity.
While these insoluble substances can be removed by filtration, the non-volatile components that are soluble in the cleaning agent, such as oils and fats and some resins that are carried into the liquid along with the particles, are not collected, and so they remain in the liquid and accumulate, becoming a factor that causes the surface of the object being cleaned to become re-contaminated. Therefore, our company has made it possible to maintain the cleanliness of the liquid at a level close to that of new liquid by removing non-volatile contaminants that are soluble in the liquid and very fine particles that could not be removed by filtration using the distillation and recycling technology we have cultivated in the flux cleaning business (Figure 2). Both the FPS-4700 and FPS-4712 are non-azeotropic mixed liquids, but by using our Distiller(Distillation and recycling device), it is possible to maintain cleanliness in-line without changing the composition.The dirt is concentrated in the Distiller as waste liquid, and only the waste liquid needs to be discharged periodically, so the amount of waste liquid is extremely small, and running costs can be greatly reduced compared to water-based cleaning agents. Because the Distiller is designed to be compact, the total space required for the entire system, including associated equipment, is about 30% less than that required for a single-tank cleaning machine for fluorine-based cleaning agents.
Figure 2: Particle removal system - Liquid circulation flow diagram.
In ultrasonic cleaning, when turbulence causes the liquid surface to ripple and bubbles to form, the impact force of cavitation is absorbed and the cleaning effect is greatly impaired. On the other hand, when there is almost no liquid flow, the removed particles and contaminants remain in the vicinity of the object to be cleaned, reducing cleaning efficiency and possibly recontaminating the surface of the object to be cleaned. Therefore, this system is equipped with an overflow and degassing mechanism in the cleaning tank to quickly eliminate removed particles from the cleaning liquid surface, while at the same time achieving a uniform and gentle laminar flow without bubble biting, thereby maximizing cleaning efficiency by ultrasonic waves and cleaning agents. The appropriate amount of dissolved gas in the cleaning agent is always maintained by the degassing mechanism, preventing unevenness in cleaning due to inhibition of ultrasonic wave propagation (Figure 3).
【Measurement conditions】
Cleaning agent: Microclean FPS-4712
Ultrasonic conditions: 28 kHz/600 W, liquid temperature 50°C
Figure 3 Correlation between circulating flow rate and average sound pressure with and without degassing
In addition, it is generally said that there is a correlation between the size of particles and the frequency of ultrasound suitable for removal⑷, and it is important to use the right frequency for the type of object to be removed or cleaned. In the frequency range below 75 kHz, the resonance bubble diameter, which is the nucleus of cavitation, increases in proportion to the amplitude of the sound wave, increasing the impact force, making it suitable for relatively large particles and strongly adherent objects, but it also causes significant damage to the materials. In the frequency range above 100 kHz, the contribution of cavitation decreases while the acceleration motion of molecules increases, and cleaning proceeds by the force of collision of the constituent molecules of the cleaning agent with the object to be cleaned, making it suitable for the removal of very small objects. In addition, the wavelength of the standing wave that occurs in the liquid for each frequency of the ultrasonic waves that are irradiated is fixed, and the optimal position for cleaning where cavitation occurs intensively changes, so if the object to be cleaned is fixed in place, problems such as uneven cleaning and increased component impact due to the installation position are likely to occur. This system incorporates an oscillating mechanism in the cleaning process to achieve high cleaning quality without irregularities even in the low-frequency range (Figure 4). The number of cleaning tanks can be increased and each tank can be equipped with an ultrasonic unit of a different frequency to meet various specifications according to the required cleaning quality and production volume.
【Test condition】
Object to be cleaned : Camera morule
Cleaning agent：FPS-4712
Cleaning condition：Ultrasonic 28kHz/600W、Liquid temp. 50℃/1 min cleaning
Figure 4: Comparison of average particle removal rates (n=5) with and without oscillation
In this paper, we have introduced the cleaning agent for particle removal that our company has newly developed to meet the increasingly sophisticated particle removal requirements, as well as the particle removal system that maximizes the removal performance of the cleaning agent. As a leading manufacturer of flux cleaning agents and cleaning equipment in the electronics field, we have many customers who use our cleaning systems. We will continue to strive to develop cleaning systems that meet a wide range of needs, with an eye on the increasingly stringent demands for reducing environmental impact and the growing demand for advanced cleaning to improve product performance.
【Source】
From the January/February 2024 issue of Ultrasound Techno Magazine
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