The impulse sets up acoustic vibrations (~1MHZ), which can be analysed electrically at the terminals. Phase shifts versus vibration magnitude are greater when cracks are present.
“Industrial screening, such as automated visual inspection, X-rays and acoustic microscopy, may not find subsurface cracks, especially near corners under capacitor end caps, where stress can be highest,” said NIST. “This non-linear approach: focusing on frequency shifts relative to signal strength rather than the frequency shifts alone, is especially useful because it is not affected by slight variations in size of the capacitors.”
The technique only works for ceramics that are highly non-linear – they get less stiff and their resonant frequency drops when they vibrate more strongly.
In the research, 41 1210-size (~2x3mm) multi-layer barium-titanate ceramic capacitors were measured before and after heating to 189°C and quenching in ice water – which caused visible cracks in 27 of them.
“The non-linear acoustic results were strongly correlated with the presence of visible cracks,” said NIST. “Measurements on 25 of the 27 visibly cracked capacitors yielded results that were outside the range of those for capacitors without cracks.
The study, published as ‘Time-domain analysis of resonant acoustic nonlinearity arising from cracks in multilayer ceramic capacitors‘ at the 2015 Review of progress in quantitative nondestructive evaluation, concluded the technique is promising, and that further work should quantify the level of detection.
The research grew out of an International Electronics Manufacturing Initiative (iNEMI) consortium working group.
While NIST invented the technique, the crack study was carried out with the University of Maryland, NASA Goddard Space Flight Center, and Colorado State University.
According to NIST: A study of US Food and Drug Administration data for several million pacemakers and defibrillators implanted over 1990-2002 found that about one in 150 failed, about one quarter of these failures were battery or capacitor abnormalities, and 61 people died due to device malfunctions.
Image: Top: capacitors studied. Bottom: cracks similar to the one shown in this NASA photo were being hunted.