Concept of No-cleaning
⑴What is No-cleaning [3]
No-cleaning refers to the use of low-solid content, non-corrosive flux in electronic assembly production, welding in an inert gas environment, and the residue on the circuit board after welding is extremely small, non-corrosive, and has an extremely high surface insulation resistance (SIR). Under normal circumstances, no cleaning is required to meet the ion cleanliness standard (the US military standard MIL-P-228809 ion contamination level is divided into: Level 1 ≤ 1.5ugNaCl/cm2 no pollution; Level 2 ≤ 1.5~5.0ugNACl/cm2 high quality; Level 3 ≤ 5.0~10.0ugNaCl/cm2 meets the requirements; Level 4 > 10.0ugNaCl/cm2 is not clean), and can directly enter the next process. It must be pointed out that "clean-free" and "no cleaning" are two absolutely different concepts. The so-called "no cleaning" refers to the use of traditional rosin flux (RMA) or organic acid flux in electronic assembly production. Although there are certain residues on the board surface after welding, the quality requirements of certain products can be met without cleaning. For example, household electronic products, professional audio-visual equipment, low-cost office equipment and other products are usually "no cleaning" during production, but they are definitely not "clean-free".
⑵ Advantages of no cleaning
① Improve economic benefits: After achieving no cleaning, the most direct benefit is that there is no need to perform cleaning work, so a large amount of cleaning labor, equipment, site, materials (water, solvent) and energy consumption can be saved. At the same time, due to the shortening of the process flow, working hours are saved and production efficiency is improved.
② Improve product quality: Due to the implementation of no cleaning technology, it is required to strictly control the quality of materials, such as the corrosion performance of flux (halides are not allowed), the solderability of components and printed circuit boards, etc.; in the assembly process, some advanced process means need to be adopted, such as spraying flux, welding under inert gas protection, etc. The implementation of the no-clean process can avoid the damage of cleaning stress to the welding components, so no-clean is extremely beneficial to improving product quality.
③ Beneficial to environmental protection: After adopting the no-clean technology, the use of ODS substances can be stopped, and the use of volatile organic compounds (VOC) is greatly reduced, which has a positive effect on protecting the ozone layer.
Material requirements
⑴ No-clean flux
In order to make the PCB board surface after welding reach the specified quality level without cleaning, the selection of flux is a key. Usually, the following requirements are imposed on the no-clean flux:
① Low solid content: less than 2%
Traditional fluxes have high solid content (20-40%), medium solid content (10-15%) and low solid content (5-10%). After welding with these fluxes, the PCB board surface has more or less residues, while the solid content of the no-clean flux is required to be less than 2%, and it cannot contain rosin, so there is basically no residue on the board surface after welding.
② Non-corrosive: Halogen-free, surface insulation resistance>1.0×1011Ω
Traditional soldering flux has a high solid content, which can "wrap up" some harmful substances after welding, isolate them from contact with the air, and form an insulating protective layer. However, due to the extremely low solid content, the no-clean soldering flux cannot form an insulating protective layer. If a small amount of harmful components remain on the board surface, it will cause serious adverse consequences such as corrosion and leakage. Therefore, no-clean soldering flux is not allowed to contain halogen components.
The following methods are usually used to test the corrosiveness of soldering flux:
a. Copper mirror corrosion test: Test the short-term corrosiveness of soldering flux (solder paste)
b. Silver chromate test paper test: Test the content of halides in the soldering flux
c. Surface insulation resistance test: Test the surface insulation resistance of the PCB after soldering to determine the reliability of the long-term electrical performance of the soldering flux (solder paste)
d. Corrosion test: Test the corrosiveness of the residue on the PCB surface after soldering
e. Test the degree of reduction in the conductor spacing on the PCB surface after welding
③ Solderability: expansion rate ≥ 80%
Solderability and corrosiveness are a pair of contradictory indicators. In order for the flux to have a certain ability to eliminate oxides and maintain a certain degree of activity throughout the preheating and welding process, it must contain some acid. The most commonly used in the no-clean flux is the non-water-soluble acetic acid series, and the formula may also include amines, ammonia and synthetic resins. Different formulas will affect its activity and reliability. Different companies have different requirements and internal control indicators, but they must meet the requirements of high welding quality and non-corrosive use.
The activity of the flux is usually measured by pH value. The pH value of the no-clean flux should be controlled within the technical conditions specified by the product (the pH value of each manufacturer is slightly different).
④Meet environmental protection requirements: non-toxic, no strong irritating odor, basically no pollution to the environment, and safe operation.
⑵No-clean printed circuit boards and components
In the implementation of the no-clean welding process, the solderability and cleanliness of the circuit board and components are the key aspects that need to be controlled. To ensure solderability, the manufacturer should store it in a constant temperature and dry environment and strictly control its use within the effective storage time, provided that the supplier is required to guarantee solderability. To ensure cleanliness, the environment and operating specifications must be strictly controlled during the production process to avoid human pollution, such as hand marks, sweat marks, grease, dust, etc.
No-clean welding process
After adopting no-clean flux, although the welding process remains unchanged, the implementation method and related process parameters must adapt to the specific requirements of no-clean technology. The main contents are as follows:
⑴ Flux coating
In order to obtain a good no-clean effect, the flux coating process must strictly control two parameters, namely the solid content of the flux and the coating amount.
Usually, there are three ways to apply flux: foaming method, wave crest method and spray method. In the no-clean process, the foaming method and the wave crest method are not suitable for many reasons. First, the flux of the foaming method and the wave crest method is placed in an open container. Since the solvent content of the no-clean flux is very high, it is particularly easy to volatilize, which leads to an increase in the solid content. Therefore, it is difficult to control the composition of the flux to remain unchanged by the specific gravity method during the production process, and the large amount of solvent volatilization also causes pollution and waste; second, since the solid content of the no-clean flux is extremely low, it is not conducive to foaming; third, the amount of flux applied cannot be controlled during coating, and the coating is uneven, and there is often excessive flux remaining on the edge of the board. Therefore, these two methods cannot achieve the ideal no-clean effect.
The spray method is the latest flux coating method and is most suitable for the coating of no-clean flux. Because the flux is placed in a sealed pressurized container, the mist flux is sprayed out through the nozzle and coated on the surface of the PCB. The spray amount, atomization degree and spray width of the sprayer can be adjusted, so the amount of flux applied can be accurately controlled. Since the flux applied is a thin mist layer, the flux on the board surface is very uniform, which can ensure that the board surface after welding meets the no-cleaning requirements. At the same time, since the flux is completely sealed in the container, there is no need to consider the volatilization of the solvent and the absorption of moisture in the atmosphere. In this way, the specific gravity (or effective ingredient) of the flux can be kept unchanged, and it does not need to be replaced before it is used up. Compared with the foaming method and the wave crest method, the amount of flux can be reduced by more than 60%. Therefore, the spray coating method is the preferred coating process in the no-clean process.
When using the spray coating process, it must be noted that since the flux contains more flammable solvents, the solvent vapor emitted during spraying has a certain risk of explosion, so the equipment needs to have good exhaust facilities and necessary fire extinguishing equipment.
⑵ Preheating
After applying the flux, the welded parts enter the preheating process, and the solvent part in the flux is volatilized by preheating to enhance the activity of the flux. After using the no-clean flux, what is the most appropriate range for the preheating temperature?
Practice has proved that after using no-clean flux, if the traditional preheating temperature (90±10℃) is still used for control, adverse consequences may occur. The main reason is that no-clean flux is a low-solid content, halogen-free flux with generally weak activity, and its activator can hardly eliminate metal oxides at low temperatures. As the preheating temperature increases, the flux gradually begins to activate, and when the temperature reaches 100℃, the active substance is released and reacts quickly with the metal oxide. In addition, the solvent content of no-clean flux is quite high (about 97%). If the preheating temperature is insufficient, the solvent cannot be fully volatilized. When the weldment enters the tin bath, due to the rapid volatilization of the solvent, the molten solder will splash and form solder balls or the actual temperature of the welding point will drop, resulting in poor solder joints. Therefore, controlling the preheating temperature in the no-clean process is another important link. It is usually required to be controlled at the upper limit of the traditional requirements (100℃) or higher (according to the supplier's guidance temperature curve) and there should be enough preheating time for the solvent to fully evaporate.
⑶ Welding
Due to the strict restrictions on the solid content and corrosiveness of the flux, its soldering performance is inevitably limited. To obtain good welding quality, new requirements must be put forward for the welding equipment-it must have an inert gas protection function. In addition to taking the above measures, the no-clean process also requires stricter control of the various process parameters of the welding process, mainly including welding temperature, welding time, PCB tinning depth and PCB transmission angle. According to the use of different types of no-clean flux, the various process parameters of the wave soldering equipment should be adjusted to obtain satisfactory no-clean welding results.