## 4 Discussion
### Analysis of Surface Discharge  
Surface discharge generally occurs in a gas (or liquid) with a tangential electric field on the surface of a solid dielectric. Surface discharge occurs only when the applied voltage reaches a certain value, i.e., the surface discharge voltage. The surface discharge voltage is closely related to the surface condition of the solid (such as contamination, moisture absorption, and surface roughness) [34; 35]. When the relative humidity of the environment is higher than the critical relative humidity of the soluble salt on the insulation surface, the pollutants dissolve due to the adsorption of moisture, and the electrolyte is ionized. The movement of ions in the electric field is affected by the electric field force, which is linearly related to the electric field strength. The electric field strength affects the movement time of ions in the electrolyte. The strength of the electric field depends on voltage, geometric parameters (such as distance and shape), dielectric properties, and space charge. For parallel planes, when space charges and their synergistic effects are not considered [3],  
\[E_{mean}=\frac{U}{d} \tag{5}\]  
where \(E_{mean}\) is the average electric field, \(U\) is the applied voltage, and \(d\) is the gap length.  
At a constant \(d\), the applied voltage promotes the movement of ions. At the same time, temperature also plays a role in promoting the movement of ions.  
Under the action of a surface electric field and ionic contamination, a large leakage current flows along the surface of the PCB [34; 35]. The thermal effect of the leakage current causes electrolyte in some micro-regions to begin to evaporate, forming local dry areas. The formation of a local drying zone causes a sudden interruption of the electric current, followed by a spark discharge and the release of more heat. Multiple discharges degrade the surface of PCB, forming a carbonization channel, that is, an electric trace [29]. With the development of discharge time, the carbonization channel formed through the whole electrode, and the insulation performance of PCB surface, will fail. Figure 20 shows the carbonization channel through a PCB with an ionic contamination concentration of 100 \(\upmu\)g/cm\({}^{2}\) within 10 min under a 7 kV applied voltage and 85 \({}^{\circ}\)C/85% RH.  
In addition, the hydrophilicity of the material surface also changes during repeated surface discharges. This can be confirmed by an infiltration test of the sample surface after the test. The water contact angle indicates wetting between the water and PCB [36], Figure 21 shows the contact angle of an uncontaminated sample, and 50 and 20 \(\upmu\)g/cm\({}^{2}\) samples. Figure 22 shows the change in the curve of the contact angle on the surface of different contaminated samples. The higher the concentration of ionic contaminants, the smaller the droplet angle. This is because, under the synergistic effect of electrical and thermal stress in the process of discharge, the organic polymer chains on the surface of  
Figure 20: Carbonization channel through a PCB with an ionic contamination concentration of 100 \(\upmu\)g/cm\({}^{2}\) within 10 min under a 7 kV applied voltage and 85 \({}^{\circ}\)C/85% RH.  
the PCB decompose and break off, resulting in electrical erosion changes. The number of surface hydrophobic groups gradually decreases, and the roughness gradually increases, which further reduces the hydrophobicity of the PCB.