Defects in High-Voltage Ceramic Capacitors Found Acoustically
Source: Power Electronics Magazine
Acoustic imaging of high-voltage ceramic chip capacitors performs a non-destructive cross section through the capacitor—or, since the method is non-destructive, as many cross sections as desired. All of the imaging modes have the same purpose: to prevent flawed capacitors from causing field failures.

Within the body of a high-voltage multi-layer ceramic capacitor, a gap-type defect is beginning its career of destruction. The defect may be a void within the ceramic dielectric, a crack within the dielectric or a knit line delamination between the dielectric and the electrode. Any of these defects can grow until it creates a pathway between two electrodes. When current suddenly arcs from one electrode to the other, the capacitor will fail. It is likely to fail explosively and may in addition send a power surge that will damage the next component downstream. Smaller multi-layer ceramic chip capacitors have roughly similar defects, but those defects are more likely to slowly create numerous cracks in the surrounding dielectric until a typically non-explosive failure occurs.

The risk of electrical failure makes it sensible to inspect high-voltage ceramic capacitors before they are mounted on a board. Acoustic imaging tools such as the Sonoscan C-SAM tools are the weapon of choice here, in part because they are non-destructive, and in part because of the nature of the defects. The tool’s transducer launches a pulse of ultrasound at a given frequency into the top surface of the capacitor. The pulse travels into the capacitor at a speed around 5,000 m/s. There are very few features in the capacitor that can reflect a significant portion of the pulse, with one major exception: the interface between the solid material of the capacitor and the air in the void or other defect reflects very nearly 100% of the pulse to the transducer. None of the pulse crosses the void (an ultrasonic pulse doesn’t travel through air), but the transducer receives a very high amplitude echo.

Read the full article at Power Electronics.