PCBA Tech - Selection principle of flux in PCBA SMT production

1. Selection principle of flux in PCBA production

Due to the wide variety of welding agents, the selection of welding agents should be based on the needs of products and the process flow and cleaning methods. The general selection principles are as follows:

1) I. For electronic products that are not intended to be cleaned after welding, non-cleaning flux shall be preferred. It has the characteristics of low residue, but attention should be paid to the matching between the flux and PCB precoated flux and the adaptability to the foaming process during the selection.

2) For consumer electronic products, rosin type flux with low solid content and medium solid content can also be used without cleaning after welding. However, attention should be paid to whether the SIR of the welding flux meets the requirements after being damped, which should not be lower than. Generally, this kind of welding flux has good soldering performance, strong process adaptability, and can adapt to different coating methods.

3) If the electronic products need to be cleaned after welding, the welding flux shall be selected according to the cleaning process. If water cleaning is used, water-soluble flux can be used. If semi-water cleaning is used, rosin type flux, such as organic amine saponifier, can be used to weld PCB to be cleaned. Generally, no-clean flux is not used. Its soldering performance is poor and its price is expensive. And sometimes the use of non-rosin formula will bring difficulties to cleaning.

4) If voc no-clean flux is selected, attention should be paid to the compatibility with the equipment, such as whether the corrosion resistance and preheating temperature of the equipment itself are suitable, generally the requirements are appropriate to increase the temperature, and whether the FR-4 PCB substrate is suitable, for example, some substrates have high water absorption, and are prone to bubble defects.

5) No matter which type of flux is selected, attention should be paid to the quality of the flux itself and the adaptability of the wave soldering machine, especially the PCB preheating temperature, which is the primary condition to ensure the realization of the flux function.

6) For foaming process, the welding function and density of the welding machine should be tested frequently. For those with excessive acid value and excessive water content, a new flux should be replaced.

2. Development direction of flux

Flux is produced with the welding process. It has a history of more than 50 years since the invention of wave soldering flux. Flux not only helps people to weld electronic products, but also brings harm to human living environment. With the improvement of people's awareness of environmental protection, how to eliminate or reduce these hazards has been put on the agenda. The popularization of reflow soldering technology in the 1970s, especially the use of reflow soldering technology for through-hole components, also brought challenges to the flux. In addition, at present, the wave soldering method without flux is being studied at home and abroad, and some progress has been made. Therefore, flux, especially solvent-based flux with high solid content, will be gradually introduced into the market, and no-clean flux and VOC-free flux will be more widely used.

3. Inspection

As printing solder paste is the key process to ensure the assembly quality of SMT, the quality of printing solder paste must be strictly controlled. The inspection methods mainly include visual inspection and SPI inspection. The visual inspection shall be conducted with a 2~5 times magnifying glass or a 3.5~20 standby microscope, and the SPI (solder paste inspection machine) shall be used for inspection at narrow intervals. The inspection standard shall be implemented according to the IPC standard.

Solder paste is a thixotropic fluid. The printing performance of solder paste, the quality of solder paste graphics, and the viscosity of solder paste are greatly related to the viscosity and thixotropy of solder paste. In addition to the mass percentage content of alloy, the particle size of alloy powder, and the particle shape, the viscosity of solder paste is also related to temperature. The change of ambient temperature will cause the fluctuation of viscosity. Therefore, it is best to control the ambient temperature at 23 ℃ ± 3 ℃, At present, solder paste printing is mostly carried out in the air, and the ambient humidity will also affect the quality of solder paste. Generally, the relative humidity is required to be controlled at RH45%~70%. In addition, the solder paste printing workshop should be kept clean, dust-free and free of corrosive gases. At present, the assembly density is getting higher and higher, and the printing difficulty is getting higher and higher. The solder paste must be used and stored correctly, mainly including the following requirements:

1) It must be stored at 2~10 ℃.

2) It is required to take out the solder paste from the refrigerator the day before use (at least 4 hours in advance), and open the container lid after the solder paste reaches room temperature to prevent moisture condensation.

3) Before use, use a stainless steel mixing knife or an automatic mixer to mix the solder paste evenly. When mixing manually, the solder paste should be mixed in one direction. The machine or manual mixing time is 3~5min.

4) After adding the solder paste, the container cover should be closed.

5) Clean free solder paste cannot use recycled solder paste. If the printing interval is more than 1 hour, the solder paste must be wiped off the template and recycled to the container used that day.

6) Reflow soldering within 4 hours after printing.

7) When repairing the plate without cleaning solder paste, if the flux is not used, the solder joint should not be scrubbed with alcohol. However, if the flux is used when repairing the plate, the residual flux that is not heated outside the solder joint must be scrubbed at any time, because the flux that is not heated is corrosive.

8) Products that need to be cleaned shall be cleaned on the same day after reflow soldering.

9) When printing solder paste and mounting operation, it is required to take the edge of PCB or wear gloves to prevent contamination of aluminum PCB.