Cleaning Problem Of PCB Printed Circuit Board
With the development of the electronics industry, many enterprises at home and abroad have adopted wave soldering and reflow soldering technology and equipment, which not only improves production efficiency, but also improves welding quality. However, due to the existence of various pollution factors, the corrosion and short circuits of the printed circuit board rails and components have seriously affected the reliability of the printed circuit board. Therefore, strict cleaning of printed circuit boards and printed circuit components (PCA), especially military products is necessary.
PCB pollution in the assembly and welding process mainly comes from handling, use of flux, welding process and working environment, causing different environmental pollution. Ambient temperature and humidity will exacerbate pollution hazards. The degree of risk depends on the storage time and the length of the storage environment.
1. Component lead contamination
The most common pollution of component lead is surface oxidation and fingerprint pollution, such as oxide film on nickel-plated lead surface, copper plating or some types of tin-plated oxide film on component lead surface. When oxides form on the surface of the coating, the coating darkens, reducing the solderability of the component lead. There are many factors that form oxides. In addition to the manufacturing process of the parts themselves, the main factors are the storage time and environment of the parts. The main components of the fingerprint are water, skin oil and sodium chloride, as well as hand products and cosmetics, which react to a certain extent with the substrate, thereby reducing the solderability of the lead of the device.
2. Contamination from printed circuit board assembly operations
During the assembly of the printed circuit board, some parts need to be protected with a mask, and some parts need to be fixed or sealed with silicone rubber, epoxy resin, etc. The mask is used to prevent the surface of some parts from being "run" by solder or plastic compounds. wet. Masks commonly used in assembly are tape, thermoplastic polymer, butyl ester or modified butyl ester, ammonia latex, silicone rubber, and polymer liquid dissolved in high vapor pressure solvent. Under the action of high temperature welding, the adhesion of the tape will become a kind of contaminant that is difficult to remove. In hot solvents and solvent vapors that are insoluble or insoluble in water, colloids are formed. Thermoplastic impregnant will remain on the surface of the component, etc., resulting in the formation of pollution.
3. Flux contamination
After PCB components are soldered, there are two kinds of contaminants on the substrate: one is that the contaminants in the flux are diffused by the solder on the PCB, and the other is the contaminants generated on the PCB by the flux itself. There are three types of flux: inorganic flux (including inorganic acids, salts and inorganic gases, etc.), rosin or resin-based organic flux, non-rosin or non-resin type organic flux.
The flux diffuses from the metal surface to the oxide through physical and chemical action, while promoting the wetting of the metal by the liquid solder. During this process, the contaminants change chemically and diffuse into the entire flux residue. The role of rosin or resin-type welding pair will make the metal oxide into rosinate or metal soap. If the halide is used as an activator of rosin flux, as a conventional component of water-soluble inorganic and organic flux formulations, it can convert metal oxides to metal halides. Therefore, the fluoride, chloride or hydroxide used in the flux formulation can convert ordinary tin, lead and copper oxides to chlorides. This chloride is a very corrosive pollutant. When flux residues become inclusions in the solder, especially high vapor pressure compounds, degassing occurs under reduced pressure, generating a large number of bubbles and trachoma, thereby reducing the quality of the solder joints.
