The assembly of general electronic products PCBA needs to go through the SMT+THT process, which includes wave soldering, reflow soldering, manual soldering, and other welding processes. Regardless of the welding method used, the assembly (electrical assembly) process is the main source of assembly pollution.
The definition of pollutants is any surface sediment, impurity, slag inclusion, and adsorbed substance that reduces the chemical, physical, or electrical properties of PCBA to an unacceptable level. There are mainly the following aspects:
(1) The components that make up PCBA, as well as the pollution or oxidation of the PCB itself, can cause surface pollution of the PCBA board;
(2) During the production and manufacturing process of PCBA, solder paste, solder wire, etc. need to be used for welding. The flux generated during the welding process will cause residue on the PCBA board surface to form pollution, which is the main pollutant;
(3) Hand marks generated during manual welding process, as well as wave soldering claw foot marks and welding tray (fixture) marks, may also exist on the PCBA surface to varying degrees of other types of pollutants, such as blocking adhesive, residual adhesive from high-temperature tape, hand marks, and flying dust;
(4) Dust from the workplace, steam from water and solvents, smoke, small particulate organic matter, and pollution caused by electrostatic charged particles adhering to PCBA.
Why clean PCBA
In the past, people's understanding of cleaning was not sufficient, mainly because the PCBA assembly density of electronic products was not high, and it was believed that the residual flux was non-conductive and benign, which would not affect the electrical performance. Nowadays, the design of electronic assembly components tends to be miniaturized, with smaller devices, smaller spacing, pins and pads getting closer and closer, resulting in smaller gaps. Pollutants may get stuck in the gaps, which means that relatively small particles may cause potential defects such as short circuits if left between two pads.
According to the analysis and statistics of PCBA electrical fitting quality problems provided by the Reliability Research and Analysis Center of China Saibao Laboratory, the short circuit, open circuit and other late use failures caused by corrosion and Electromigration account for 4%, which is one of the major killers of product reliability.
Pollution may directly or indirectly cause potential risks to PCBA, such as organic acids in residues that may cause corrosion to PCBA; During the electrification process, the electric ions in the residue may cause the movement of electrons due to the potential difference between the two solder pads, which may form a short circuit and cause product failure; Residues can affect the coating effect, causing problems such as inability to apply or poor coating; It is also possible that it cannot be detected temporarily, and after changes in time and environmental temperature, the coating may crack and warp, leading to reliability issues.
