The electrochemical migration（ECM） behavior and mechanism of immersion silver processing circuit board（PCB-ImAg）and hot air solder leveling circuit board（PCB-HASL） under the 0.1 mol/L Na2SO4 absorbed thin liquid films with different thicknesses were investigated using stereo microscopy and scanning electron microscopy（SEM）.Meanwhile,the corrosion tendency and kinetics rule of metal plates after bias application were analyzed with the aid of electrochemical impedance spectroscopy（EIS）and scanning Kelvin probe（SKP）.Results showed that under different humidity conditions,the amount of migrating corrosion products of silver for PCB-ImAg was limited,while on PCB-HASL both copper dendrites and precipitates such as sulfate and metal oxides of copper/tin were found under a high humidity condition（exceeding 85%）.SKP results indicated that the cathode plate of two kinds of PCB materials had a higher corrosion tendency after bias application.An ECM model involving multi-metal reactions was proposed and the differences of ECM behaviors for two kinds of PCB materials were compared.
Galvanic corrosion on samples of AZ91D magnesium alloy coupled with 2A12 aluminum alloy during neutral salt spray test was investigated.The variations of the surface potential were measured using scanning kelvin probe（SKP）.The results showed that galvanic effect on the corrosion of AZ91D magnesium alloy is closely related to the potential difference between the anodic and cathodic materials.In the initial period,corrosion only occurred in a narrow area at the coupling interface because of the limited distance galvanic current.Then,the corrosion rate of 2A12 aluminum alloy was accelerated due to its poor stability in strong alkali environment,which was attributed to the strong alkalization caused by the corrosion of AZ91D magnesium alloy.With the increase of the potential of 2A12 aluminum alloy as a result of the continuous covering of corrosion products,the potential difference between the two materials was enlarged,which enhanced the galvanic corrosion.
The effects of relative humidity (RH) on a printed circuit board finished with electroless nickel immersion gold (PCB-ENIG) under an adsorbed thin electrolyte layer (ATEL) were investigated in situ via the measurement of cathodic polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy to clearly elaborate the corrosion behavior of PCB-ENIG in the atmospheric environment. Results indicated that the cathodic process of PCB-ENIG under ATEL was dominated by the reduction of dissolved oxygen, corrosion products, and H2O. The cathodic current density of PCB-ENIG increased progressively with increasing RH. Moreover, its cathodic current density in the solution was greater than that under ATEL. This result demonstrated that the diffusion process was not the controlling step during the limiting reduction of cathodic oxygen. When the polarization potentials were located in a more negative region, the cathodic polarization current density gradually decreased under 75% and 85% RH. Notably, the anodic process became the controlling step in the extremely thin liquid film during the remainder of the experiment.
