Ceramic Capacitor Reliability Revisited

In the last decade, the volumetric efficiency (capacitance/volume) of ceramic capacitors has been rapidly increasing. To achieve this the dielectric layer thickness has been decreasing to below 1 micron which results in a higher electric field strength across the electrode layers. What effect does this have on reliability? In years past it was expected that if MLCCs are used within the specified temperature and voltage limits that wear-out will not be a concern. Clive Hendricks, Intel Corporation, created considerable interest within the industry with his paper at the 2010 CARTS. In his paper he concluded that the industry standard life testing was not adequate in predicting the reliability of MLCCs. Accelerated life tests were performed on 0402, 2,2uF, X6S, 4V capacitors. Using the familiar (at least in the capacitor community) Prokopowicz-Vaskas equation for MLCC life acceleration with voltage and temperature, Clive Hendricks reported considerable variation with different suppliers. The predicted useful lifetime varied from 509 years to only 8 years. Is this an indication that we should pay more attention to MLCC wear-out concerns?

Two recent published papers have now weighed in on this issue. N. Kubodera, et al, from Murata Manufacturing Co., have published a study in the 2012 CARTS. This study reviewed the MLCC wear-out mechanism It was found that the standard temperature acceleration models were accurate. However this testing showed that the voltage acceleration equation showed two different acceleration factors. At higher electric field strengths the acceleration factor was greater than for lower field strengths. This means that tests at higher voltages may be overly pessimistic concerning predicted field life. Murata then developed a voltage acceleration model that covered a wide range of electric field strengths. The result of this model is a log-normal distribution. One aspect that needs further work is a combined temperature/voltage model that is useful at higher voltage stresses.

David Liu, MEI Technologies, NASA Goddard Space Flight Center, also has been doing extensive research in this area. His study found two separate failure distributions. The earlier failures in this study showed a different failure distribution slope compared to the later failures. It was determined the early failures were due to avalanche-like leakage current breakdown due to defects in the ceramic. It was concluded that the later failures were due to intrinsic causes – the diffusion of oxygen vacancies under an electric field. The early failures due to defects in the ceramic is entirely logical since due to the decreasing dielectric thickness. One positive aspect to the base metal electrode (BME) capacitors, which are now 90% of the capacitors used today, is that the grain sizes are are smaller than the precious metal electrode capacitors.

The story on MLCC wear-out is not yet finished. As the dielectric layer thickness of high capacity MLCCs continues to decrease, the reliability must be assessed. The reliability of decoupling capacitors must be very high since there are many capacitors used per design. MLCC reliability is an area that needs to be closely watched as capacitive efficiency increases.

References:

1. C. Hendricks, et al, Reliability Challenges for CPU Decoupling MLCC, 2010 CARTS

2. N. Kubodera, et al, Study of the Long Term Reliability for MLCCs, 2012 CARTS

3. D. Liu and M. Sampson, Some Aspects of the Failure Mechanisms in BaTiO3-Based Multilayer Ceramic Capacitors, 2012 CARTS