Selection of flux in SMT production
I. selection principle
Due to the wide variety of welding agents, the selection of product needs, technological processes and cleaning methods should be based on the general selection principles as follows:
1. For electronic products that are not intended to be cleaned after welding, the first choice should be free of cleaning flux.It has the characteristic of low residue, but we should pay attention to the match between the flux and PCB precoated flux, and the adaptability to foaming process.
2. For consumer electronic products, rosin flux with low solid content and medium solid content can also achieve the purpose of no cleaning after welding.However, the selection should pay attention to whether the flux meets the requirements after being wet, and it should be no less than that. Generally, this kind of flux has good soldering performance, strong process adaptability and can adapt to different coating methods.
3. If cleaning is required after welding of electronic products, flux should be selected according to the cleaning process.If using water to clean, you can choose water-soluble flux, if using semi-water cleaning, you can choose rosin type flux, such as organic amine saponification agent, to the PCB to be cleaned for welding.Generally, no cleaning flux is used, its welding performance is not good, the price is more expensive, and sometimes the use of non-loose formula will also bring difficulties to cleaning.
4. If voc is used as a non-cleaning flux, it shall pay attention to its matching with the equipment, such as whether the equipment itself is resistant to corrosion, whether the preheating temperature is appropriate, and whether the PCB substrate is appropriate to increase in temperature. For example, some of the substrate is highly absorbent, which means bubble defect.
5. No matter which type of flux is selected, the quality of the flux itself and the adaptability of the wave solder, especially the preheating temperature of PCB, should be paid attention to, which is the primary condition for ensuring the function of the flux.
6. For the foaming process, the welding function and density of the welding machine shall be tested frequently. For those with excessive acid value and water content, new flux shall be replaced.
II. Development direction of flux
Flux is produced along with the welding process. It has been more than 50 years since the invention of wave solder.The flux helps to solder the electronic products to bring people convenience, but also to the human living environment has brought the harm.As people’s awareness of environmental protection increases, how to eliminate or reduce these hazards has been put on the agenda.In the 1970s, the popularization of reflow welding technology, especially the application of reflow welding technology for through-hole components, also brought challenges to flux.In addition, the wave soldering method without flux has been studied at home and abroad, and some progress has been made. Therefore, the flux, especially the solvent flux with high solid content, will be gradually introduced to the market, and the anti-cleaning flux and the VOC flux will be more widely used.
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