## 5 Experimental Results and Discussion  
To confirm the proposed mechanism, a Thermo Scientific ELITE VX system was used to identify the hot spot of the thermal emissions and a 3D X-ray was performed using ZEISS Xadia 520 Versa on the hot spot. Figure 4 shows the enlarged X-ray micrograph at the hot spot in the burn-out area and we can clearly see that the short-circuit was due to copper particles at the edge of the PCB along the de-penalized line. From the cross-sectional view of the X-ray micrograph, one can see that the trace of copper is indeed from the V\({}_{\text{IN}}\) (anode) toward the GND (ground). Figure 4: A 3D X-ray of the PCB that shows the cross-section of the PCB. The propagation of the cracks and the movement of copper from the V\({}_{\text{IN}}\) layer to the GND layer due to the presence of the water layer and electric field can be seen: (**a**) shows the formation of crack lines before the resistance between them begins to decrease; (**b**,**d**) are the plane of the GND and V\({}_{\text{IN}}\) layer, respectively, as extracted from the 3D X-ray; (**c**) is the plane in the middle of the GND layer and V\({}_{\text{IN}}\) layer as extracted from the 3D X-ray where the white arrows represent copper particles; (**e**) side view of the PCB showing the short point in between the V\({}_{\text{IN}}\) layer and GND layer. To further analyze and characterize the cracks formed in the PCB, as shown in Figure 4, the PCB area with the cracks was dissected out and examined under an X-ray Micro CT (computed tomography) scanner system (model number CTLab-HX130-2-E). The dissection was needed due to the size limitation of the X-ray system. For a better view of the cracks, the image dataset was reoriented with a new image plane, as shown by the pink color plane in the top left side image in Figure 5. In Figure 5 (top left), 'a', 'b', and 'c' are marked to show the front, top, and left sides, respectively, and their respective 2D images are shown in the same figure. A total of 78 cracks were identified, and these cracks are shown by their individual colors in Figure 6. The histogram of the volume of these cracks is shown in Figure 7a, which shows that the majority of the cracks (25) had a volume less than \(1\times 10^{5}\) (\(\upmu\)m\({}^{3}\)). No crack had a volume greater than \(4\times 10^{5}\) (\(\upmu\)m\({}^{3}\)). Figure 7b shows all the identified cracks and the cracks with the same volume are shown with the same colors in the 3D view. It could be observed that large cracks occurred at the edge of the PCB and they progressed toward the inner parts of the PCB along with the GND and V\({}_{\text{IN}}\) layers. All these cracks were due to excessive high strain at a high strain rate induced during the V-cut de-panelization as measured experimentally with the results shown in Figure 8. The experimentation is discussed in the next section. The large cracks observed were concentrated in a certain area of the PCB instead of being distributed uniformly. This non-uniform distribution of the crack sizes was believed to be due to the inhomogeneous strain distribution along the edge of the PCB during the V-cut de-panelization as simulated using a finite element analysis, which will be discussed in the next section. There were also large cracks present away from the edge but they were closed to the largest cracks, indicating the weakening of the mechanical integrity of the board after the large cracks occurred. Figure 5: Image dataset reoriented with a new image plane as shown by the pink color in the 3D image (top left side). The 2D images are taken at the: front side (**a**); top side (**b**); left side (**c**). To further confirm the proposed mechanism, 5 samples of the PCB that had crack lines due to de-penalization were subjected to 65 \({}^{\circ}\)C/95% RH. Table 1 summarizes the test results. The test condition was selected so that a continuous film of water could be formed in the PCB.