Resonant Acoustic Frequency Shifts Associated With Cracks in Multilayer Ceramic Capacitors

Resonant ultrasound spectroscopy (RUS) was used in this paper to measure shifts in resonant frequency that arise from the presence of cracks in barium-titanate-based multilayer ceramic capacitors (MLCCs). This paper was motivated by an industrial need for a nondestructive quality-control technique with enhanced capabilities for detecting dielectric cracks that have no initial effect on electrical characteristics of MLCCs. In this quality-control application, no information will generally be available on acoustic spectra of capacitors prior to the introduction of cracks during manufacture or mounting in devices. However, information on the distribution of frequencies of a specified resonant acoustic mode of a population of uncracked MLCCs could serve as a reference for determining whether the resonant frequency of an individual MLCC is within an acceptable range. In this paper, acoustic spectra were obtained from sets of MLCCs before and after thermal quenching that generated surface-breaking cracks in a fraction of the specimens. Three resonant modes below 1000 kHz were studied. The largest of these three resonant peaks, in the range of 575 kHz to 595 kHz in MLCCs before heat treatment, was found to be most sensitive to the presence of visible cracks. An analysis of Gaussian fits of the frequency distributions for this mode before and after heat treatment shows that approximately 71% of the visibly cracked MLCCs and less than 1% of the uncracked quenched MLCCs are rejected when the criterion for rejection is that the frequency is more than two standard deviations different than the mean frequency before heat treatment. These results show that RUS is a promising approach for nondestructive screening for the presence of cracks in MLCCs.