# Partial Discharge Inception and Breakdown Voltage Analysis in PCB-Based Electrode Structures
## 4 Results of the Breakdown Test Procedure
### Fe_0.5 mm Sample  
Figure 13 shows the PRPD patterns recorded during the breakdown test of a representative FE_0.5 mm sample (one of the eight tested samples) with a PDIV of 5.5 kV, as shown in Figure 13a. The results indicated that the discharge quantity in the positive half-cycle was relatively low, whereas the negative half-cycle exhibited a greater and more-concentrated discharge. Most of the discharge occurred below 15 pC, with a slight decrease in density observed between 15 and 20 pC.  
Figure 13b shows the PRPD pattern at an RPDIV of 6.5 kV. Compared to Figure 10b, the positive half-cycle showed a higher but slightly sparse discharge quantity, whereas the negative half-cycle, although lower in magnitude, was more concentrated and denser, resembling typical surface discharge patterns.  
Figure 13c shows the final PRPD pattern recorded immediately before unintentional breakdown. This sample reached a maximum discharge of nearly 30 nC. Similarly to Figures 7c and 10c, this sample consistently experienced breakdowns at unintended locations after repeated testing. Two methods were used during the testing:  
* Stepped voltage increase: In this method, the voltage was gradually increased to induce breakdown. However, upon reaching a certain voltage level, breakdown occurred either along the surface or in the air between the electrodes, causing the  
Figure 12: RE sample breakdown test results.  
Figure 13: Breakdown test results of the FE_0.5 mm sample.  
circuit to exceed its load capacity and triggering the protection mode of the power supply. Visual inspection confirmed the absence of internal breakdown.
* Extended high-voltage exposure: This method involved maintaining a mid-to-high voltage for an extended period, spanning several hours, to encourage degradation, leading to breakdown. Despite this extended exposure, internal breakdown did not occur, resulting in the conclusion of the test. Ultimately, none of the eight samples achieved internal breakdown.  
The discharge activity and voltage timing diagram for this test is shown in Figure 13d, with the pressure application lasting approximately 35 min.  
Figure 14 shows the images of the FE_0.5 mm sample before and after the testing, which ultimately did not experience breakdown during the test. None of the eight samples experienced breakdown; instead, a whitening effect was observed near the proximity of the two electrodes.  
Figure 15 shows the statistical results of the breakdown tests conducted on the eight FE_0.5 mm samples. The average PDIV was 5.08 kV and the average RPDIV was 6.53 kV. As none of the samples experienced internal breakdown, the results did not include the breakdown voltage or average breakdown voltage, as shown in Figure 15.  
The findings from the FE_0.5 mm sample, particularly the surface discharge observed at various voltage levels, emphasize the need for careful design considerations in high-voltage PCB applications. Even though no internal breakdown occurred, surface discharges can contribute to cumulative insulation degradation over time. Therefore, optimizing surface treatments, selecting appropriate materials, and ensuring proper insulation gap geometry can help mitigate surface discharge risks and improve the overall reliability of PCBs in high-stress environments.