Contents

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1.Introduction
2. Cleaning agent for metal masks
 2-1 Purpose of metal mask cleaning
 2-2 Ultrasonic cleaning technology effective for cleaning metal masks
 2-3 Low VOC metal mask cleaner
3. Defluxing chemical
 3-1 Purpose of flux cleaning
 3-2 Ultrasonic cleaning technology effective for flux cleaning
 3-3 One-liquid, low-VOC flux cleaner
4 Closing remarks
Source

1.Introduction

 Cleaning in the electronics mounting industry is a process of separating and removing a flux residue after soldering, a residual solder paste on a metal mask, and other foreign matter from an object to be cleaned by a physicochemical action such as wetting, penetration, dissolution, peeling, and diffusion by a cleaning agent.

 Even if the object to be cleaned is simply immersed in the cleaning agent, it is possible to dissolve the object to be cleaned, but it is common to warm the cleaning agent and apply a physical action by liquid flow, ultrasonic wave, or the like in order to shorten the time and improve the quality.

 However, due to the remarkable development of the IT-related industry in recent years, miniaturization and thinning of mounted components, high-density fine pitch of substrate circuits, and concomitant miniaturization and narrowing of the pitch of metal mask openings used in the paste printing process are progressing, and furthermore, the difficulty of cleaning is increasing together with the difficulty of dissolving the flux and solder paste by composition change and improvement.

 Under such circumstances, the ultrasonic cleaning method is one of very effective cleaning means. This paper introduces our metal mask cleaner, flux cleaner, and cleaning system, which have many market experiences in ultrasonic cleaning applications.

2. Cleaning agent for metal masks (1)

2-1 Purpose of metal mask cleaning

 Examples of the solder paste and paste flux supplying method to the circuit board include a dispensing method, a printing method (metal mask), and a pin transfer method. Among them, a printing method using a metal mask having high productivity is still a mainstream at present. However, after printing, solder particles and polymer additives such as and thixotropic agents in flux remain on the end surface of the metal mask opening, resulting in the following three problems.

① Poor supply of solder paste
When the solder paste is deposited on the end surface of the mask opening, clogging occurs, and a prescribed amount of the solder paste cannot be supplied.

② Excess supply to area that the solder paste shouldn't be supplied.
The clogged solder paste wraps around the back surface of the mask (substrate side) and adheres to unnecessary portions such as insulating portions between terminals, thereby causing bonding failure of solder bridges and the like.

③ Contamination of other solder paste
When the type of solder paste is changed, if the type of solder paste before switching remains on the mask, there is a possibility that a paste having a different solder particle diameter or metal composition may be mixed after switching.

 Heretofore, in order to avoid the occurrence of such a problem, metal mask cleaning has been performed.

 In recent years, with the popularization of wearable products and the like, chip components mounted on a substrate have been increasingly miniaturized from the 0402 type to the 0201 type, and the opening of a metal mask has tended to be further miniaturized in order to cope with the miniaturization.

 Since solder paste having a smaller solder particle diameter needs to be used in order to carry out solder bonding corresponding to miniaturization of pitches, solder paste having a solder particle diameter Type4 (20 to 38μm) to Type6 (5 to 15μm) has recently been developed.

 While the solder paste containing such fine particle diameter solder powder has good mask releasability, the paste tends to be pulled into the back surface of the mask at the time of printing, and thus the need for cleaning has increased more than before.

 There is a need for a cleaning process that can more reliably maintain the print quality of the solder paste while coping with an increase in the degree of difficulty in cleaning.

2-2 Ultrasonic cleaning technology effective for cleaning metal masks

 The metal mask may still be cleaned by hand wiping with a highly volatile and flammable cleaning agent such as IPA.

 However, with the miniaturization of solder particles and the miniaturization of metal mask openings, it has been difficult to maintain the cleanliness of masks by hand cleaning recently, and an increasing number of companies have introduced cleaning machines from the viewpoints of productivity, safety, and the environment.

 In an ultrasonic cleaning machine, there is "immersion method" in which the entire mask is immersed in a cleaning agent to irradiate ultrasonic waves. However, since a sufficient amount of liquid is required to immerse the entire mask, the amount of bath of cleaning agent is large, and when cleaning agents made from dangerous substances are used, it is necessary to pay attention to the chemical quantity management. In addition, since the cleaning agent comes into contact with a portion other than the portion to be cleaned and the ultrasonic wave is irradiated, there has been a problem that the influence on the aluminum frame, the fixing tape adhesive, and the like must be considered.

 On the other hand, in the "direct propagation method" in which ultrasonic waves are directly applied to a metal mask proposed by SAWA Corporation, ultrasonic vibrations are directly transmitted to the mask through a cleaning agent that flows thinly on the mask surface to perform cleaning (FIG. 1). The amount of the cleaning agent to be used can be kept small because only the amount of the cleaning agent to be flowed over the necessary portions on the front and back sides of the mask is sufficient. In addition, by utilizing the difference in the transmission speed of the ultrasonic vibration for each substance, a micro crack is generated on the resin adhering material side at the interface between the mask and the solder paste and the adhering material is peeled off and removed by the penetration of the cleaning agent into the resin adhering material side. Therefore, the mask is hardly damaged, and the mask can be cleaned with high accuracy without unevenness even with respect to the fine mask opening.

 In addition, although the ultrasonic wave has a characteristic that energy is attenuated in inverse proportion to the propagation distance, the influence of the distance can be eliminated by directly irradiating the mask with the ultrasonic wave. Therefore, high cleaning power can be obtained even if the output is suppressed, and the consumption amount of the cleaning agent and power consumption can be reduced and the running cost can be suppressed.


Fig. 1 Direct Radio System

2-3 Low VOC metal mask cleaner

 Even if there is a device contrivance, it is not possible to obtain good cleaning quality in cleaning metal masks, which becomes increasingly difficult unless an appropriate cleaning agent is selected for the device and the cleaning object.

 The metal mask cleaning agent HA-4045U (hereinafter referred to as HA-4045U) developed by us is a cleaning agent specialized for use in ultrasonic cleaning equipment. By containing a large amount of water in the cleaning agent, the viscosity and surface tension are low while the propagation property of high ultrasonic waves is high (Table 1), and the entrapment and stagnation of air bubbles in the liquid which inhibit the ultrasonic wave propagation are unlikely to occur, so that the physical action of ultrasonic waves can be effectively exerted for peeling off and removing insoluble substances such as solder particles, and poorly soluble polymer additives such as thixo agents and waxes.

Table 1 General Properties of Low VOC Metal Mask Cleaner
HA-4045U
Specific gravity (20℃) 1.0
Boiling point ℃ 100 or more
Flash point ℃ Not observed
Surface tension mN/m (20℃) 34
Viscosity mPa·s (20℃) 2.7
Ozone depletion potential (ODP) 0
UN Class 8
UN No 3267
Packing group

 

 In addition, by the formulation design that enhances the ability to dissolve and diffuse dirt in the liquid, dirt peeled off from the mask surface is diffused, and re-attachment to the mask by precipitation and aggregation of dirt in the liquid is unlikely to occur.

 In recent years, volatile organic compounds (VOCs), which are strongly problematic to be released into the atmosphere, can be reduced to 30% or less of conventional VOCs. Since a non-volatile component such as a surfactant is not included and a single substance has good dryness, a water rinsing step after washing is unnecessary, and the load on the environment can be further reduced.

 And has features with extremely low odors. Therefore, it eliminates the need for explosion-proof structures and countermeasures against surrounding odors.

 Photograph 1 shows the external appearance of the metal mask opening after ultrasonic cleaning is performed by the "Metal Mask Cleaning Device SC-AH100-LV" (hereinafter referred to as SC-AH100-LV) manufactured by SAWA Corporation using HA-4045U.


Photo 1 Metal Mask Cleaning Equipment SC-AH100-LV (left) and Cleanability with HA-4045U (right)

 The SC-AH100-LV is a cleaning device in which an ultrasonic cleaning head and vacuum drying nozzles are incorporated in one unit, thereby enabling the cleaning process and the drying process to be performed simultaneously (Fig. 2).


FIG. 2 Construction of the metal mask cleaning unit SC-AH100-LV

 By combining HA-4045U and a SC-AH100-LV specialized for use in ultrasonic cleaning, a series of processes from cleaning to drying can be performed in about 5 to 6 minutes per plate even for masks to which lead-free solder is attached.

 The SC-AH100-LV employs a method in which the front and back surfaces of the metal masks are sandwiched by ultrasonic cleaning heads and the cleaning liquid is supplied only to the cleaning head portion, so that the consumption of the cleaning liquid is extremely small at 50 to 70 ml/plate, and damage to the frame parts does not occur.

3.Defluxing chemical

3-1Purpose of flux cleaning

 The flux is an auxiliary material used to remove an oxide film on the surface of the printed wiring board land, the component electrode, and the solder surface when soldering and bonding in the mounting process, and to secure the wettability of the solder. (2) The flux is exposed to a high temperature of 200° C. or higher during the soldering process, and remains on the electronic circuit as a flux residue through a complicated chemical reaction. The flux residue is preferably removed because it may cause product defects such as inhibition of curing of the coating resin in a later step, or may affect circuit characteristics by moisture absorption due to aging or the like. The purpose of cleaning is to remove these flux residues and solder balls from the surface of the solder joint after mounting and to clean the surface.

3-2 Ultrasonic cleaning technology effective for flux cleaning (3)

 In recent years, lead-free and halogen-free solder pastes have been widely used. Though the flux of the non-cleaning design is the mainstream of these pastes, there are still many demands for cleaning the flux residue after mounting in order to secure the product reliability such as the wire bonding property in the post-process and the resin adhesiveness in the sealing process.

 The object to be cleaned is finely bonded with higher density wiring due to the modularization of the product function and the miniaturization of the mounted components, and there is a tendency that the object to be cleaned is difficult to clean with the improvement of the corresponding solder paste.

 Recently, flux improvement by solder manufacturers has been promoted for the purpose of reducing generation of voids in die bonding paste and suppressing flux scattering during reflow. The improvement of the flux relates to the countermeasures against air bubbles inside the solder generated at the time of melting the solder, and there are measures to increase the active force of the flux and measures to which a flexible resin is added. The increase of the reaction salt with the metal and the blending of the poorly soluble polymer resin causes the poorly cleaned products to be cleaned after soldering.

 Under these circumstances, the ultrasonic method is one of very effective cleaning means. In ultrasonic cleaning in flux cleaning, a cleaning object fixed to a cleaning jig is usually immersed in a cleaning liquid and irradiated with ultrasonic waves in a low frequency range (50kHz or less), whereby an impact force due to a cavitation phenomenon is applied to separate and remove flux residue from the cleaning object. In other words, it is possible to physically separate and disperse the flux residue between the narrow gap parts in which the cleaning agent hardly penetrates sufficiently, and the residue component which hardly dissolves in the cleaning agent by the shock wave. In addition, the ultra-high pressure and high temperature conditions caused locally and instantaneously in the liquid by the compression rupture of the cavitation bubbles remarkably accelerate the chemical action of the cleaning agent (4), and the poorly dissolved components in the flux residue can be quickly decomposed and dissolved.

 However, depending on the mounting substrate, there is a possibility that the irradiation of the ultrasonic wave causes resonance breakdown of the quartz oscillator, generation of micro cracks in the fine bump bonding, and the like, and therefore, it is necessary to evaluate in advance whether or not the application is possible.

 In addition, since the cleaning system utilizes the cavitation phenomenon to the maximum extent, the cleaning property is easily affected by the environmental conditions.

 For example, since the standing wave of the ultrasonic wave is hardly generated when the flow of the medium in the cleaning tank is strong, the effect of the cavitation is said to be low. In addition, when the circulating flow rate of the liquid is increased, the amount of dissolved oxygen in the liquid, which becomes an obstacle to ultrasonic wave propagation, increases due to the entrapment of bubbles by the liquid feed pump. Therefore, washing is basically performed while maintaining a state in which dissolved oxygen in the liquid is kept as low as possible with a gentle flow of the liquid.

 However, depending on the type of the flux residue, the viscosity of the resin coating is high, so that the peeling diffusion effect by the ultrasonic wave propagation is weakened, and the supposed cleaning property may not be obtained. In such a case, it is necessary to increase the liquid circulation amount to provide a concentration gradient around the dirt, thereby increasing the flux dissolution rate.

 In order to realize a short time and a higher cleaning property by ultrasonic cleaning, it is important to increase the dissolution rate of flux by utilizing the liquid flow while reducing dissolved oxygen and bubbles in the liquid.

3-3 One-liquid, low-VOC flux cleaner

 The ultrasonic method in the flux cleaning is composed of synergistic effect between physical action of cavitation and the chemical action of the cleaning agent, and the selection of an appropriate cleaning agent is indispensable for the efficiency of the cleaning.

 The MICROCLEAN ECO series, a single-liquid defluxing agent that can be distilled and recycled (hereinafter referred to as the ECO series), which we have succeeded in developing ahead of other companies, is used in a water-rich state. Therefore, not only about 70% of the VOC components in operation can be reduced, but it is highly suitable for ultrasonic cleaning applications.

 Water has a higher specific gravity than organic solvents used as raw materials for defluxing chemical, and has a higher impact force caused by cavitation phenomenon. Further, since air is harder to dissolve than organic solvents, and the amount of dissolved oxygen which inhibits ultrasonic wave propagation can be suppressed to about 1/5 to 1/10, the physical action of ultrasonic waves can be efficiently utilized by containing a large amount of water.

 While the chemical action of the cleaning agent generally increases as the temperature increases, the effect of the cavitation phenomenon decreases when the temperature exceeds a certain temperature because the increase in the number of cavitation cavities due to the temperature increase is offset by the impact force decreasing action due to the increase in the pressure of the gas in the cavity. (6)

 Therefore, it is necessary to change the set temperature of the cleaning agent depending on whether the chemical action is important or the physical action is important. In the case of water, it is known that the effect of the cavitation phenomenon becomes the highest at about 50° C.

 Generally, a water-containing flux detergent is used to secure a solubility, and the temperature is set to about 60 to 70° C., while the ECO series exhibits a high cleaning power at 50° C., so that both the chemical action and the physical action can be utilized to the maximum in the ultrasonic method.

 The ECO series is formulated so as to separate into an aqueous phase and an oil phase at the time of dilution with water (Photograph 2). The oil phase component (upper layer) has the role of dissolving the resin and diffusing it into the washing solution, and the aqueous phase component (lower layer) has the role of dissolving the ionic substance and penetrating the interface between the flux residue component and the circuit board to swell and separate the resin (patented). Since the separated two phases having different roles alternately and continuously come into contact with the flux residue at the time of clean, it becomes possible to collectively remove the resin component such as the lipophilic denatured rosin and the hydrophilic activator component. Due to such characteristics, it is possible to exhibit high detergency at a lower temperature even for a poorly soluble lead-free solder flux residue.


Photograph2 MICROCLEAN ECO-3002U

 The typical properties of the ECO-series product "MICROCLEAN ECO-3002U" are shown in Table 2. Since the surface tension of each liquid phase is much lower than that of water and is close to the solvent system, the liquid penetrates into the fine gaps, and the flux residue can be washed. In addition, since it has good drainability and dryness, a rinsing agent is not required, and washing to drying can be performed with one liquid. Further, since nonvolatile components such as surfactants are not used at all, there is no deterioration in circuit characteristics due to surface hydrophilicity caused by their remaining on the substrate surface, and good cleaning quality can be ensured after cleaning.

 In addition, they do not fall under the main laws and regulations, such as the Fire Defense Law (dangerous substances), the Industrial Safety and Health Law, and the PRTR Law, and do not include 100 substances subject to the Ministry of the Environment's VOCs (Table 2).

Table 2 General Properties of Low VOC, Single Liquid Deflux Cleaners
MICROCLEAN ECO-3002U
Stock solution 3-fold dilution by water
Specific gravity (20℃) 0.89 -
Boiling point ℃ 160 or more 100 or more
Flash point ℃ 62 94
Surface tension mN/m(20℃) - Oil phase 29 Aqueous phase 29
Ozone depletion potential (ODP) 0 0

 

"MICROCLEANER ECO" is a cleaning system developed to maximize the performance of these distinctive ECO series. So far, we have accumulated a lot of market experience with a cleaning system (MICROCLEANER) incorporating a continuous distillation recycler of hydrous alcohol. We have developed a distiller (patented) capable of stably and continuously distilling even non-azeotropic multicomponent cleaners containing water by newly incorporating vacuum distillation technology, while inheriting the features and advantages of this technology. This makes it possible to perform final cleaning with a clean liquid at all times, and to maintain high cleaning quality. Further, it is unnecessary to regularly replace the entire amount of the liquid, so that the liquid can be used only by pouring a reduced amount of the new liquid, and both the amount of the liquid used and the amount of the waste liquid generated can be reduced. In addition, unlike the conventional "MICROCLEANER", since it is a one-liquid cleaning system, a rinsing process is unnecessary, the number of tanks is reduced, and the design is more compact (Photograph 3).


Photograph 3 Low VOC Single Liquid Batch Cleaning System "MICROCLEANER ECO"

4 Closing remarks

 This paper introduces our low VOC metal mask cleaner and single liquid flux cleaner effective for ultrasonic cleaning.

 In the field of cleaning in the electronics mounting industry, we have been developing products that meet market needs, such as lead-free solder and low VOC. Going forward, we will continue to develop technologies that contribute to quality, cost, and the environment, without satisfying the current situation, and will continue to contribute to the workplace of manufacturing.

Keita Ishihara

【Reference】

(1)Editorial Committee on Microjoining and Assembly Technology: Microjoining and Assembly Technology, pp. 323-333, pp. 432-439
(2)The Japan Welding Engineering Society, Microsoldering Education committee: Standard Microsoldering Technology 3rd Edition, Nikkan Kogyo Shimbun, pp. 1-21, 71-78 (2011)
(3)Akamatsu: Ultrasonic technology, 26(1), pp.57-61 (2014)
Hatanaka: Journal of the Acoustical Society of Japan, Vol. 72, No. 4, pp. 193-200 (2016)
(5) Akamatsu: Electronic Materials, July, pp. 90-94 (2009)
(6) Watanabe: Air Cleaning, Vol. 9, No. 8, pp. 44-54 (1988)

【Source】

From the January-February 2019 issue of Ultrasonic Technology

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