Solder paste creeping corrosion phenomenon

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Creeping corrosion is a corrosion phenomenon on the surface of the circuit, which is generated by the reaction of corrosive gases such as sulfide or chloride in the environment in contact with bare copper. Creeping corrosion is characterized by solid corrosive material along the circuit and soldermask or encapsulation material surface migration growth, the formation of a structure similar to a spider's web, resulting in electrical shorts or impedance changes between adjacent pads or circuits, affecting the performance and reliability of electronic products.

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Creeping corrosion occurs from the exposed Cu surface, Cu surface in the role of sulfur-containing substances will generate a large number of black Cu sulfide, and in the exposed surface of Cu and its surrounding diffusion, buildup.
The oxide of Cu is insoluble in water. However, the sulfides and chlorides of Cu are soluble in water and have high surface mobility driven by concentration gradients. The products will diffuse from the high concentration zone to the low concentration zone. Sulfides are semiconducting in nature and do not cause an immediate short-circuit, but as the concentration of sulfides increases, their shopper decreases and causes a short-circuit failure.

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In addition, the resistance value of the corrosion product will change drastically with the change of temperature, can drop from 10M¦¸ to 1¦¸. Humidity (water film) will accelerate this creeping corrosion: sulfide (such as sulfuric acid, sulfur dioxide) dissolved in water will generate a weak acid, the weak acid will cause the decomposition of the copper sulfide, forcing the clean Cu surface to be exposed, which continues to occur corrosion. Obviously the increase in humidity will accelerate this creeping corrosion. This corrosion occurs very quickly, some PCB veneer or even run less than a year on the failure.
The main factors affecting the occurrence of creeping corrosion phenomenon are: atmospheric environment (such as exhaust gases in the atmosphere and humidity); PCB substrate and plating materials; pad definition; PCBA assembly in the welding and other factors.
As the atmospheric environment to promote the corrosion of electronic equipment elements and gases, are listed as: SO2, NO2, H2S, O2, HCI, Cl2, NH3, etc., corrosive gas components of the indoor concentration, accumulation rate, the source of the occurrence of the impact and susceptibility of the material and the allowable concentration of the table shows. As soon as the above gases are dissolved in water, corrosive acids and salts are easily formed.
Table 1. Indoor concentrations, accumulation rates, sources, effects and susceptible materials and allowable concentrations of other components of corrosivity

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Humidity: According to the dissolution/diffusion/deposition mechanism of creeping corrosion, an increase in humidity should accelerate the occurrence of sulfide corrosion.Ping Zhao et al. concluded that the rate of creeping corrosion is exponentially related to humidity.Carig Hillman et al. found in the experimental study of mixed gases that the rate of corrosion increases dramatically and parabolically with the rise of relative humidity. Cu, for example, when the humidity increases from 60% RH to 80%, the corrosion rate of the latter for the former 3.6 times.
In order to prevent the occurrence of creeping corrosion, the following measures can be taken:
The use of three-proof coating, that is, in the circuit surface coated with a protective film to isolate the external corrosive gases;
Design and process to reduce the probability of PCB and components of exposed copper, select the surface treatment process with strong corrosion resistance, such as lead-free hot air leveling, tin dipping, etc., to avoid the use of chemical silver, organic solder protection, etc.;
The assembly process should minimize thermal shock and contamination ion residue, choose the appropriate flux, and effective cleaning;
The whole machine design to strengthen the control of temperature and humidity, to avoid the formation of water film and the dissolution of corrosive substances;
The location of the machine room should avoid obvious sulfur pollution, such as industrial areas, volcanic areas, swamps and so on.