Preface

As CFC (HCFC) and 1.1.1-trichloroethane with cleaning ability, compatibility, use of safe, fast drying device, etc., so has been considered an ideal cleaning agent. Over the years, the electronics industry has been used in the cleaning components of these products, especially after cleaning SMT solder components. However, since the late 80s, it was discovered that CFC and other ODS cleaning agent in the destruction of the ecological balance of ozone depleting substances, a serious threat to people's lives. To this end, the development of alternative technologies in the global expansion ODS substances. Thus, the use of SMT technology using no-clean soldering materials is the trend and achieve consistent green manufacturing is a global voice. However, the effect of no-clean how is it? And CFC-free cleaning materials can match it? By no-clean materials, processes and equipment of the no-clean technology future trends.

Late 80s early 90s, industrial countries have developed the following four main alternative technologies, and has been widely used in current electronic cleaning process.

1 Solvent Cleaning
2 half-water cleaning
3 water cleaning
4-clean
However, the search for a suitable alternative technologies is not easy, any kind of alternative technologies are not completely replace all the properties of ODS must be for different products, different processes use different alternatives. Also, consider the materials, equipment, technical compatibility and other issues. Proved several alternatives in the above three is just a transitional period before the technology, as the phase-out process. In the long run, or no-clean technology should be used to achieve the ultimate purpose does not use ODS. Because of the abolition of the cleaning process will significantly reduce manufacturing costs. To achieve this, the surface of the plate after welding residue must be reduced to a minimum or no residue. In addition, this technique is for the one-step alternative ODS technology, though, now require some investment, but it brings advantages and economic benefits are significant. But make no-clean technology to be successful, must address the ball, bridge, drain welding and other process defects. If the flux residues are corrosive, then, the product life will be affected. In other words, to solve these problems is to achieve no-clean soldering technology key. Currently, the United States, Japan and other industrialized countries the main research and development on the issues from a number of new technologies, new processes.

1. No-clean materials
1.1 The no-clean flux
In ultra fine pitch assembly, flux cored welding wire in the electronic plug-in plays a decisive role. Flux from the initial stage of any print flaws, has a great influence on the pass rate. Pre-production process because any error will be run through the entire production process. To this end, the flux the following requirements:

* Through the template of good printing performance of fine cracks
* A small slump
* The formation of a good ball
? Good wetting properties of the components
* No corrosion after reflow
* Low content of solvent residue
Flux mainly composed of two components: flux powder and solvent, solvent, including rosin (resin, synthetic resin), activator (no halide, containing halide) solvent and certain additives. No-clean solder flux is a low-solid, solid content of about 1.5 to 3% range. The solvent flux is mainly isopropanol, ethanol or methanol. These little or no residue flux residue, so you can not clean after welding. When using no-clean flux to emphasize the following points:

* In the same time ensuring minimal residues, also called flux with sufficient activity.
* The proportion of difficult to control (monitoring and maintenance of solid flux content).
* Suction effect of solid content than the average flux significantly.
* Foam difficult.
* Processed using the general scope than the flux of solid content of the small range of processing time.
Solids no-clean flux ratio than traditional flux much less. When the solids content of less than 5%, the GDP will be significant changes in the sensitivity.

1.2 The non-volatile compounds (VOC) flux

U.S. restrictions on VOC emissions is very strict, and conduct a detailed study of a variety of different. The current problem is that most no-clean flux in the high VOC content, some of the flux in the ethanol content of up to 99%. Some U.S. companies are now a new generation of flux, reaching a low or no emissions of VOC emissions purposes. The flux in the preparation of water instead of alcohol solvent, that is, we often say that water-soluble base flux, water-soluble flux based almost no VOC. The flux in addition to environmental pollution, but also non-combustible, spraying the heated environment is very safe.

Water-soluble flux for wave soldering agent-based warm-able to propose new requirements. Figure 1 and Figure 2 shows the two warm-up curves, the two curves identified through the use of reading and the transmission speed preheater formed. Figure 1 shows the application of 2% solid content of the alcohol based flux effect. Figure 2 shows with 2% soluble solids content of the warm-based no-clean flux curve. If contact with the crest of the component, water does not evaporate, it will splash flux, the formation of solder balls and solder joints may be formed via holes. The Process is how the difficulties of overheating in the case of non-PCA, evaporated water. Experimental results show the strong convection heating to improve the preheating temperature, which can effectively solve this problem.

1.3 No-clean solder paste
No-clean solder paste can be divided into four categories: normal, low residue, low residue and low residue solder paste. The paste can be further divided into the rosin-based solder paste and two non-rosin-based solder paste (see Table 1).
Table 1 Classification of no-clean solder paste
Cleaning-free solder paste
Type of solid content (weight) atmosphere
3.5 ~ 5% of ordinary common gas
2.9 ~ 3.4% of low-residue containing 500ppm of O2 air nitrogen can improve the wetting
2.1 ~ 2.8% of very low residual O2 content of air is sometimes less than 500ppm of nitrogen can improve the wetting
Low residue <2.0% O2 less than 100ppm requires the use of nitrogen atmosphere
1.3.1 General-clean solder paste
General no-clean solder paste is a typical rosin its flux. The proportion of solid content of 3.5 to 5%, the residue products from different manufacturers have different content and colors, but the common denominator is the transparent color - yellow between. From the outside, transparent residue is better, but the residue color has nothing to do with corrosion and reliability.

General no-clean solder paste does not require special atmosphere, such as; for reflow soldering with a good weldability nitrogen. Although no-clean solder paste wetting is a problem, but the problem is usually caused by component and the substrate due to poor solderability. As the ordinary no-clean rosin-based solder paste most of them, the viscous properties of the number and printing classes with RMA and OA compared paste. The color and amount of residue no-clean solder paste is a common major defects. From the exterior, users do not like the yellow residue, which residue will prevent too much e-test.

1.3.2 Low residue no-clean solder paste

Low residue no-clean solder paste is not a rosin-based solder paste, is made of synthetic materials. In general, the least amount of solder paste residue is made of synthetic materials. The main advantage of the low residue paste is less residue, so the appearance of problems does not exist, but also reduce or eliminate the need to detect problems. Low residue solder paste composition the proportion of solid content of about 3.4%, low residue solder paste in the proportion of solid content of 2.1 to 2.8%, low residue solder paste of solid content of less than 2%. In the preparation of low, low, low residue solder paste, the main metal content is increased to 91 ~ 92% (flux levels down to 20%). Design flux paste is also a process of development. As most of the flux in the reflow will break down or volatile, so a reduction of the formation of residue.

And sometimes as high metal content in the solvent content is high flux of these two factors, low residue no-clean solder paste viscosity number and the service life of the RMA and the OA than the average life span is short. Low residue no-clean solder paste composition than most commonly used chemical corrosion OA RMA and small.

2. No-clean flux coating process and the inert atmosphere
2.1 The flux coating method
Currently, the use of the coating there are two main methods: foam and spray. According to many literature reports shows that the method for the foam containing more than 5% of solid content of the flux is, that is practical, but also considerable economic benefits available. Can be monitored continuously or intermittently to control the proportion of solid content and is not used in the corresponding range of flux used. However, no-clean flux and flux residues are less than 5% solid content, the majority of no-clean flux through the solid content of 2% low-flux solid foam is not full, this is not suitable for foam application . Therefore, the foaming method can only meet the general requirements of welding. In addition, the last closed flux allows rapid evaporation of ethanol solvent, the flux of solid content increased, so that the proportion of testers do not need to control the flux of solid content, because the proportion of flux, and vary considerably thinner, so required titration to monitor the flux .

Foam coating process is mainly used rosin based flux, so that the compressed air through the foam tube into the bubble so that the flux coating on the PCB, its advantage is not modified wave soldering machines, saving investments, the disadvantage is coated uneven residues on PCB, can not control the welding dose, need to frequently monitor the flux of change and often change flux, flux consumed in large quantities.

Spray flux no-clean soldering process is in a popular method. It can precisely control the deposition flux. Flux spray system can be designed as single-channel system, from a single channel system in a closed container supply of non-recycled flux. Do not need to monitor the flux of solid content.

Spray coating process is the use of spray devices, the flux to the PCB on the atomized spray, wave after preheating. Spray coating process as a coating evenly, with less weight without any titration or monitoring, periodic emissions of the old without flux, the flux can be controlled on-board deposit and closed system, eliminating the problem of flux stains etc., were recognized by many users, but it takes a certain spray equipment funds, as in the original foam equipment to transform the investment is comparatively less. Can be seen by comparing the spray method has many advantages than the foam method. Many foreign companies do this type to spray the foam flux coating device is a breakthrough. Way of using the spray coating of no-clean flux has become in the future direction of development.

2.2 Inert Atmosphere Welding

As noted above, no-clean soldering process in the existence of ball, bridge, drain welding, oxidation, poor solderability, wetting, poor, and many other defects, especially in the general atmosphere, wave soldering and reflow soldering , solder oxidation is particularly prominent. Currently, the United States, Japan and other countries are based on an inert atmosphere to overcome these problems. Which should be used to conduct a comprehensive measure of the atmosphere, not only to take into account the cost of gas, but also consider the security and other physical properties. Per unit volume of hydrogen and helium is much more expensive than nitrogen. More expensive than nitrogen, hydrogen 5 to 10 times, helium 20 to 30 times more expensive. At the same time, hydrogen is a flammable gas, explosion in the atmosphere limits 4 to 75%. If the reflow furnace using hydrogen, on the need to reprovision the safety systems, such as interlock devices, exhaust gas combustion devices and and the corresponding process steps. Although carbon dioxide, nitrogen slightly cheaper price than some, but it may cause some oxidation problems affect solderability. Through the evaluation and experimental applications, to create awareness of the inert gas nitrogen is the real, its low price, and in the course of no major safety problems, for people of all ages.

Use of inert gas, as opposed to the ordinary gas has obvious advantages, can significantly reduce the residues of the board. However, the use of different solder paste and reduce the effect of residues is different. Although can not eliminate the residue, but, if you can control the gas environment solder paste and flux of chemical technology and new developments in combination they can achieve a new low-residue soldering technology.

To promote the use of disposable rapid welding process, commonly used method is to use nitrogen so that the whole or part of the wave soldering machine inert. Much of the information that, under the conditions in the absence of oxygen will increase the wet strength of welding and reduce wetting time. Also observed that when the peak inertia of welding electrode  , it will not solder burr. Formed in the solder joints under nitrogen atmosphere, the appearance of a more beautiful, that is smooth and reduce the rate of leakage welding.

Inertia of the wave soldering process is the best way when not in use flux, solder on the components coated with wave soldering. The nitrogen peak solder improved the surface condition, the contact time and preheat temperature solder the best value has great scope for adjustment. Inertia welding process is very positive side of solder residue decreased significantly (80 ~ 95%), reducing solder consumption, more importantly, reduced rework effort. In many cases, saving money than the welding process in the use of nitrogen to be more money spent.

Recently, the emergence of a new technology that will only peak inertia welding, that is partial deactivation, known as the inert boundary welding. No protective cover and channel, the gas supply in contact with the components the first one for each spot welding peak import and export terminal end, in fact, masked by the peak of the components, so that the welding points and nitrogen content of the peak below 10ppm. This method has the protective cover, the channel is completely inert welding has certain advantages compared with the wave soldering system is relatively common, with good visibility and accessibility. Since no nitrogen sealed door, so do not affect production.

The focus of recent research for the next generation of inert gas products, new processes, including through the use of no-clean technology to replace CFC, 0.5-micron fine pitch devices following the production of environmentally friendly non-VOC assembly process and more BGA and PCMICA popular technology.

3. No-clean welding equipment
3.1 The wave soldering equipment
No-clean wave soldering equipment, there are three: one is the most old-fashioned rotary drum, and the lowest operating costs. This type of equipment is used in stainless steel or plastic mesh drum, and its slot in the flux rotation, the drum is equipped with a hot knife, the flux activation, the wire-line flux is blown on the PCA. Deposition flux is controlled by the drum rotation speed. This simple system has a high consistency and repeatability. Defect is not sealed, so the possible flux of volatile solvents.

"Luxury" of the sealed flux spray equipment can be divided into two categories: ultrasonic nozzle and spray gun type. These two types of flux spray machine is the flux out of pressure vessel, or in a sealed container extraction pump flux. Ultrasonic spray flux of liquid flux machine is transported to the ultrasonic oscillator, the role of the ultrasonic oscillator, the flux is very small droplets rupture, was mist. Atomized droplets from the nozzle into PCA. By changing the flow, ultrasonic nozzle flow rate can be controlled flux deposition flux.

Flux is the use of commercial spray nozzle. Flux from the nozzle is often used in pressure vessel supply. The most popular kind of gun is the flux swing back and forth type of gun running. The system consists of mechanical device installed in a single nozzle structure, it can belt with the wave soldering machine to run back and forth. According to PCA's specifications, the control will be spraying in the corresponding range. Pressure by adjusting the flux tank, deposition flux can be controlled.

Both ultrasonic and spray coating equipment are sealed one-way system, so do not need to monitor the flux of solid content. Although the two coating equipment is expensive, but they quickly get the flux coating device efficiency, this is due to the consumption of flux and thinner reduced, and flux density of coating (50 ~ 3000μg/in2) ability to improve , repeatability is ± 10%, makes this technology is fast application.

In wave soldering, in order to overcome the problem of oxidation of the solder alloy, usually in the wave soldering equipment in nitrogen, making it inert to obtain high-quality welding. Inertia of the wave soldering machine has several different types. Total inertia of the welding machine is very expensive, very complicated. However, the use nitrogen effect is obvious. Usually consumption of 600 ~ 1200CFH (standard cubic inch) range. Completely inert welding equipment replacement costs are lower, in the general wave soldering machine, fitted with an inert gas protection equipment, including solder bath or solder pre-heater tank. Another advantage of this technology can retrofit existing welding. This type of welding consumption of nitrogen in the range of 1400 ~ 2400CFH.

In order to solve the problem of solder oxidation, in addition to nitrogen, the wave soldering machine in the formation of the structure on the crest of the vents a lot of transformation, creating a double-peak like, Ω wave, λ wave, O wave, T wave, W waves and the combinations peaks form.

3.2 Reflow soldering equipment

Infrared heating reflow welding machine tunnel furnace structure, with 2 or 3 warm-up area, a welding area in the furnace outlet and a cold wind outside the cooling area. PCB with a chain or mesh belt through the furnace, transmission speed can be variable speed according to process needs. To meet the double-sided PCB for soldering, using the up and down while heating.

Infrared radiation heating elements using infrared radiation ceramic plate. Wavelength of 2.5 ~ 5μm furnace temperature profile can be set, the actual temperature of weld curve can be measured, set the curve and the actual measurement of the temperature curve can be displayed on the screen. Accurately measure the weld curve is inconsistent with the ideal soldering temperature profile, set the curve can be adjusted so that the final curve of the actual welding welding curve close to the ideal.

General, reflow welding machine has a hand in the transmission system bodies, in the event of problems, hand-delivery chain, the product can be sent out the furnace, it can take a sudden power failure or failure to immediately remove the furnace products. In addition, in order to prevent oxidation, to ensure the welding quality, some of the nitrogen reflow welding additional protection.

In the more high-end circuit assembly reflow soldering, the use of clean solder paste has been closed and the atmosphere of infrared convection heating N2 reflow soldering equipment. United States not only continue to use the auto welding rods, but also in-depth study. Vapor phase reflow soldering due to production costs make it expensive and application of environmental restrictions and other reasons, this paper does not make for such devices are discussed in detail. In recent years, mainly engaged in the forced convection heating reflow soldering technology and equipment research, to meet the PCMICA, BGA and fine pitch QFP of welding.

With the appearance of fine pitch assembly, nitrogen atmosphere reflow soldering processes and equipment also will be published. This technique improves the quality of reflow soldering and yield, as the direction of development in this area.

Charge N2 reflow furnace atmosphere advantages:v * To reduce oxidation.
* Wet solder joints, components better.
* Non-wetting defects (cold solder joint, lack of weld, solder incomplete, etc.) reduced.
* Reduce the solder ball formation.
* There is no bridge to improve the quality of welding.
Experiments show that the oxygen content of nitrogen in the residue, the lower the solder paste, solder ability to lower, that is, the higher the rate welding defects. Therefore, only by improving the purity nitrogen furnace, in order to play the role of no-clean solder paste. Obviously, no-clean soldering is to achieve high purity N2 atmosphere for the price.

Conclusion:

ODS cleaning technology in the alternative, because of flux, solder paste modified, making the welding process and welding equipment change with it. In particular, the assembly must meet the increasingly high density and quality requirements, making the implementation of no-clean process is very difficult. No-clean flux in the development process, mainly around the reduction of solder oxidation and reduction residue and improve the wetting and other problems started. To overcome these problems, the main countermeasures is filled with inert atmosphere soldering equipment. Despite the manufacturers of welding equipment vary, but generally are in the wave of the waveform, the process to open a breakthrough. There are still no-clean technology in laser welding technology, we can see, no-clean technology has been firmly established, the main trend of development.

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