The surface of the failed capacitor capacitor does not find obvious cracks, damage and other abnormal phenomena, excluding the failure caused by factors such as external force impact of the capacitor. Use X-ray to perform perspective inspection of the failure capacitor, and no obvious structural abnormalities were found.

2. Electrical Parameter Testing

The failure capacitance resistance is less than 2kΩ, which is much less than the unused capacitance resistance, indicating that the failure capacitor has obvious leakage phenomenon, as shown in Table 1 of the test result.

3. Thermal EMMI Hotspot Positioning

It was found that there are obvious abnormal highlights inside the failure capacitor, and the highlights are mainly concentrated on the side of the electrode near the capacitance end.

4. Sliced analysis

(1) The position where the NG1 capacitor shows abnormal bright spots in the hot spot has obvious cracks. Further grinding to the inner electrode is revealed, and the crack is found to pass through the intersection of the inner electrode, resulting in a short circuit or low resistance channel between the upper and lower layers of the inner electrode. Compared with the slicing of different locations, with the gradual grinding to the inside of the capacitor, the cracks gradually become smaller, indicating that the origin of the crack expansion should be close to the outside of the capacitor. In addition, the microscopic morphology observes that the cracks are fine, and no melting and burning phenomenon is seen, which is presumed to be mechanical stress cracks.

(2) NG2, NG3 are found on the bottom side of the capacitor crack, and extended to the inner electrode area to cause the capacitor layer between the leakage short circuit, the position and hot spot abnormal bright spot corresponding. The crack originated from the bottom surface end electrode, and its angle and the bottom angle is close to 45 °, which is a typical mechanical stress cracking morphology.

5. Material verification

According to the specification book, the three-point bending test is carried out to confirm the appearance and electricality of the capacitor after the experiment, and no obvious abnormalities are found. The slice analysis of #1 and #2 capacitors has not been found to have obvious cracks, medium layer holes and other defects. Therefore, the material can meet its specifications.

6. Simulation experiment

In order to confirm whether the internal crack of the failure capacitor is a mechanical stress crack, the #3 and #4 capacitors continue to bend simulation test for the three-point bending, and the capacitance is slicing analysis after the test.

The slicing results show that when the substrate is bent at 4mm, there are obvious cracks in the interior of the #3 and #4 capacitors, and its position originates from the end electrode position and is 45° with the welding surface, which is basically the same as the internal cracking morphology characteristics of the failure capacitor. Therefore, it is inferred that the failure capacitor is subjected to a large external stress, and then cracks occur internally.

7. Analysis and Summary

Analysis: The reason for the mechanical stress crack inside the capacitor is that because the capacitive porcelain body is made of hard and brittle ceramic material, this material is more sensitive to the stress generated by the deformation of the single plate. When the capacitor is subjected to additional stress, the crack will be generated at the stress concentration point, as shown in Figure 15, if the device is subjected to upward stress, the bottom weld end and the welding end of the intersection at the junction of the welding end is the stress concentration point, this position becomes a weak link, easy to produce a crack with the welding end of 45° or "Y" font, and vice versa.

Summary: The main reason for the failure of the capacitor is that the cracks that are extended to the inter-internetized electrode by external stress are formed, resulting in a decrease in the resistance value caused by the leakage channel.

8. Recommendation

Check the external stress sources in the process of capacitance assembly, testing, etc.