Reliability of High-Voltage MnO2 Tantalum Capacitors CARTS... · Reliability of High-Voltage MnO2 Tantalum Capacitors . Erik Reed, George Haddox . KEMET Electronics Corporation, 2835

Reliability of High-Voltage MnO2 Tantalum Capacitors

Erik Reed, George Haddox

KEMET Electronics Corporation, 2835 Kemet Way, Simpsonville, SC 29681 Phone: +1.864.963.6300, Fax: +1.864.228.4081

e-mail: [email protected]

Abstract While tantalum polymer capacitors are making significant penetration into many applications previously dominated by MnO2 tantalum capacitors, the present reality is that significantly more capacitors are manufactured with MnO2 as the cathode material than are made with conducting polymer. There are reasons other than market inertia for the good market performance of MnO2 capacitors. These include lower cost, good reliability, and robustness against high temperature and humidity. Recent work reported at CARTS focused on the reliability of tantalum polymer capacitors which display very long predicted lifetime. This work featured accelerated lifetesting of capacitors at various voltages and temperatures, and formulation of acceleration models that predict lifetime at normal use conditions. Even while the polymer capacitors were receiving this needed attention, improvements continued to be made in MnO2 capacitors. The most improvement is seen in MnO2 capacitors rated from 25 to 50V. The present work involves accelerated lifetesting of these improved high-voltage MnO2 capacitors. The test strategy is similar to that employed with the polymer capacitors. Time-to-failure data are presented that were collected over a range of voltages and temperatures. Trends in the data are identified and models for voltage and temperature acceleration are discussed. Median lifetime at rated conditions is estimated, and comparisons and contrasts are drawn between MnO2 and polymer capacitors and their underlying degradation mechanisms. Introduction While it is true that the market share of tantalum polymer capacitors in the tantalum capacitor industry is growing, high-volume manufacture of MnO2-cathode tantalum capacitors continues. The number of MnO2 tantalum capacitors manufactured per year still outnumbers that of tantalum polymer capacitors by roughly 6:1 (revenue by about 3:1) and the MnO2 parts are yielding their market share reluctantly. MnO2-cathode tantalum capacitors still cost less, have lower DC leakage current, withstand temperatures above 125oC better, can be more stable in humid environments, and have a proven record of reliability that spans decades. The market share of tantalum polymer capacitors is growing because these parts offer some attractive performance advantages (lower ESR and somewhat higher volumetric efficiency with good reliability). Because they are a relatively new technology, they have received much recent research attention. Beside the obvious need to properly describe their electrical performance advantages, additional effort has been spent quantifying their reliability since they lack the long history of the MnO2-cathode parts, and traditional reliability assessment techniques such as Weibull grading do not appear to be directly applicable (often so few capacitors fail under normal test conditions that one cannot perform the calculations). Some benefits of these research efforts are the refinement of techniques for conducting accelerated lifetests, new insights into the

©2012 ECA (Electronics Components, Assemblies & Materials Association), Arlington, VA CARTS International 2012 Proceedings, pp. 135-146, March 26-29, Las Vegas, NV, USA

Page 1 of 11

complexity of the resulting time-to-failure data, and a nascent effort to identify and quantify the various physical mechanisms that drive this complexity. But advancements continue to occur in the incumbent (and still dominant) MnO2 technology as well. Some of the more significant improvements are being made in the higher-voltage devices where reliability has not always been as good as customers would hope. KEMET has developed superior MnO2 capacitors by employing a package of the best available manufacturing technologies in conjunction with a newly patented screening technique. These MnO2 capacitors have excellent reliability and are targeted primarily to meet the needs of hi-rel customers. So an obvious question is: How good are these new MnO2 capacitors, and how does their reliability compare with the high voltage polymer capacitors? The purpose of this paper is to address this question, look for differences between the two cathode technologies, and uncover additional clues regarding the various degradation mechanisms at work in tantalum capacitors. The strategy of the work is to perform accelerated lifetests on high voltage MnO2 capacitors that have similar case size and voltage rating as the high voltage tantalum polymer capacitors previously reported on at CARTS USA1. The resulting time-to-failure data from these lifetests are compared and contrasted with the previously reported tantalum polymer results to determine similarities and differences, and to draw inferences regarding the various underlying degradation mechanisms. The tested MnO2 capacitors performed very well and predicted life at rated conditions was much longer than required by any practical application. But the MnO2 capacitors were not necessarily superior to the tantalum polymer capacitors when judged solely on the basis of short-term dielectric strength (somewhat inferior) and projected lifespan at comparable dielectric stress levels (similar). Test Protocol The reader is asked to review the descriptions of the test and data analysis techniques that are described in the previous paper1 and its references2-4. Briefly summarized, the process involves exposing 100-piece samples of capacitors to various test voltages and temperatures while tracking the time to failure of each device. These data are plotted in log-normal format and the resulting t50 times (time when half the samples have blown a 1A fuse – often called “median life”) are estimated for each test condition. Further analysis of these t50 times versus changing voltage and temperature yields constants for simple voltage and temperature acceleration models. The voltage acceleration model is a simple power law in the form AV=(VTest/VRef)n, where n is an empirically derived exponent, usually between 12 and 20. The temperature acceleration model is a simple Arrhenius model with activation energy between about 0.8 and 1.9 eV. These models are used to extrapolate back to expected median life at rated voltage and temperature (or actual application conditions). More sophisticated physics-based models are available that better account for the interdependence of voltage and temperature acceleration, but the simple models described above are sufficient for the present need and are very easy to work with. Use of highly accelerated test conditions is necessary to produce reliability predictions in a practical timeframe (weeks rather than years). But because the test voltages are generally much higher than rated voltage, it is not unusual for some of the capacitors to fail during the 60-second voltage ramp at the start of the test. This initial voltage ramp at the start of the lifetest is similar to the procedure for common voltage breakdown tests and the resulting failure data provide useful insight into the short-term dielectric strength of the capacitors.


Page 2 of 11

The capacitors tested for this work were MnO2-cathode tantalum capacitors having EIA 7343 footprint rated at 25V, 35V, and 50V. These are the same rated voltages that were explored in the previous work on high voltage tantalum polymer capacitors. However, the formation voltages (formation voltage determines the dielectric thickness) were generally higher for the MnO2 capacitors than for the tantalum polymer capacitors. This caused the capacitances of the MnO2 parts to be somewhat lower at 35V and 50V. Specifically, the capacitances for the MnO2 parts were 22uF, 10uF, and 4.7uF for 25V, 35V, and 50V ratings, respectively, while the capacitances for the polymer parts in the previous work were 22uF, 15uF, and 10uF.

Figure 1. Lognormal Plot of Failure Percentile versus Time to Failure at 1.8Vr, 1.9Vr, 2.0Vr, and 2.1Vr for Tests Performed at 105oC on 15uF, 35V Experimental Tantalum Polymer Capacitors. Review of Previous Tantalum Polymer Results Figure 1 is a reproduction of some 105oC time-to-failure data from the previous paper concerning high voltage tantalum polymer capacitors. The significance of this plot is that it captures the complete complexity of the time-to-failure plots and, thus provides clues regarding the various degradation behaviors at work in the tested capacitors. What is seen is that the failure modes for tantalum polymer capacitors can be broken into four distinct categories: initial failures, a flat region with a relatively few additional failures, a wear out region, and in some cases an anti wear out region where catastrophic failures slowed down or stopped completely. In the previous paper, the significance of each of these categories was discussed in detail. The initial failures were caused by exceeding the dielectric strength of the failed capacitors during the initial voltage ramp to the test voltage. This does not mean that these capacitors were defective, only that the accelerated test voltage required to produce


Page 3 of 11

failures in a practical timeframe was higher than the potential barrier at the dielectric-cathode interface per MIS (metal-insulator-semiconductor) theory. It was shown that this mechanism disappears into insignificance when the test voltage is reduced to more normal levels. The flat region is hypothesized to be the result of gradual deterioration of the potential barrier at the dielectric-cathode interface. This deterioration is likely caused by trace mobile charge migration to the insulator-semiconductor interface over time as occurs in traditional semiconductor devices. The failure rate declines rapidly in this region as the trace mobile charge available at a fixed test voltage reaches its destination and the available charge supply is depleted. In the case of the tantalum polymer capacitors, very little temperature dependence was seen in the flat region which is consistent with other characterizations of the insulator-semiconductor (dielectric-cathode) interface in these devices. The wear out region is characterized by dramatically increasing failure rate. Natural thermodynamic processes, accelerated by both temperature and electric field, lead to creation of oxygen vacancies in the dielectric in the neighborhood of the metal-dielectric interface. The resulting free oxygen is absorbed into the tantalum substrate and the vacancies cause the dielectric near the interface to become progressively more conductive. This reduces the remaining “good” dielectric available to support the applied test voltage. The increased electric field in the remaining “good” dielectric further accelerates creation of more oxygen vacancies, and the field strength grows even stronger in a vicious wear out cycle characterized by sharply increasing failure rate. This mechanism is considered to be the dominant wear out mechanism at rated voltage. Its voltage and temperature dependence are easily characterized if sufficient time is available to test at conditions that clearly separate the failure regions in time. As test conditions are further accelerated, the various regions become less distinct as the underlying mechanisms start to overlap in time. The overlap occurs because the underlying mechanisms do not accelerate uniformly with voltage and temperature (e.g., the flat region is only slightly temperature dependent while the wear out region is highly temperature dependent). The anti wear out region is thought to occur only in tantalum polymer capacitors. It is thought that extended periods of exposure to very high electric field strength and elevated temperature can lead to de-doping of the cathode material where it contacts the dielectric, rendering a thin layer of it non-conductive. This process reduces the electric field in the dielectric as some of the field begins to appear in this thin but growing insulating layer adjacent to the dielectric. The capacitors stop blowing 1A fuses, but nevertheless suffer significant capacitance loss as the effective dielectric thickness increases. It is presently unknown whether this mechanism has any significance at more normal application conditions. New MnO2 Test Results Figure 2 contains time-to-failure data collected from the 10uF, 35V MnO2 samples at 125oC. As was the case for the 35V tantalum polymer capacitors, the 35V MnO2 samples provided the optimal mix of conditions to allow clear separation of the various regions in the time-to-failure distributions in a practical, but still fairly long test time. One can observe clear separation of the initial failures from the flat region, and clear separation of the flat region from the wear out region. As expected, there was no evidence of an anti wear out region for the MnO2 capacitors. The first significant observation is that there is clearly a wear out mechanism at work in these capacitors that produces increasing failure rate. A popular mantra of the MnO2 tantalum capacitor industry has been that MnO2 capacitor failure rate is always declining in time, so there must not be a wear out mechanism. Figure 2 provides hard evidence to refute this assertion. The capacitors do wear out and the wear out rate is clearly voltage dependent. This voltage dependence will be explored further.


Page 4 of 11

Figure 2. Lognormal Plot of Failure Percentile versus Time to Failure at 1.7Vr, 1.8Vr, 1.9Vr, 2.0Vr, and 2.1Vr for Tests Performed at 125oC on 10uF, 35V Experimental MnO2 Tantalum Capacitors.

Figure 3. Lognormal Plot of Failure Percentile versus Time to Failure at 1.8Vr, 2.0Vr, 2.1Vr, and 2.2Vr for Tests Performed at 105oC on 10uF, 35V Experimental MnO2 Tantalum Capacitors.


Page 5 of 11

Figure 3 contains time-to-failure data collected on the 10uF, 35V MnO2 samples at 105oC. Only a small amount of wear out behavior is observed at the highest test voltages. Casual observation of Figures 2 and 3 indicates that it takes about 10 times longer to reach wear out at 105oC as it does at 125oC at a given test voltage and the time is stretching out into the thousands of hours, even at very high test voltages. A temperature acceleration factor of 10 between 105oC and 125oC implies activation energy of about 1.5 eV in the Arrhenius temperature acceleration model. This is entirely consistent with the results observed for the high voltage tantalum polymer parts when they are tested at similar electric field stress.

It is likely that the time to wear out would extend by another factor of ten if the test temperature were reduced to 85oC. This would put the onset of wear out into the tens of thousands of hours (a few years) at 2 times rated voltage at 85oC. It might take 10 years to collect convincing evidence of wear out at 85oC and 2Vr for these samples, which is a dramatically long time for an accelerated lifetest. So it’s not really surprising that wear out is not routinely observed in MnO2 capacitors at more normal lifetest conditions.

Figure 4. Lognormal Plot of Failure Percentile versus Time to Failure at 1.8Vr, 1.9Vr, 2.0Vr, 2.1Vr, and 2.2Vr for Tests Performed at 125oC on 22uF, 25V Experimental MnO2 Tantalum Capacitors.

Figure 4 contains time-to-failure data collected on the 22uF, 25V MnO2 samples at 125oC. The initial failure region is clearly visible in the plots, but there is overlap of the flat region and the wear out region. It appears that wear out is postponed at lower test voltages, as expected, but the blurring of the mechanisms where they overlap in time makes it all but impossible to accurately quantify the voltage dependence of the wear out mechanism.

The only solution to this problem for these 22uF, 25V MnO2 parts would be to further reduce the test voltages and wait longer. But, as observed above for the 35V parts, the wait could be very long. This situation clearly demonstrates the practical limits of accelerated testing of highly reliable devices.


Page 6 of 11

Figure 5 is a plot of median time to failure versus test voltage for both the 10uF, 35V MnO2 parts and the 15uF, 35V tantalum polymer parts previously tested. The test voltages are normalized as a percentage of the dielectric formation voltage where the thickness of the dielectric is related to the formation voltage by a factor of roughly 2 nm per volt. The MnO2 parts occupy the upper left of the plot while the polymer parts occupy the lower right of the plot. A thin dashed line is drawn on the plot that represents voltage acceleration that obeys a power law with a voltage ratio exponent of 13.2.

Figure 5. Median Life versus Percent of Formation Voltage for 10uF, 35V Experimental MnO2 Tantalum Capacitors and 15uF, 35V Experimental Polymer Tantalum Capacitors.

The median life at 2 times rated voltage (~50%Vf) and 125oC for the 35V MnO2 capacitors is about 900 hours. With activation energy of about 1.5 eV and a voltage ratio exponent of 13.2, the expected median life at rated voltage and 85oC is about 0.85 billion hours or roughly 100,000 years. This is certainly long enough life for any practical application.

It is clear from the plot that the voltage dependence of the wear out mechanism is essentially the same for both kinds of parts. The similar temperature dependence of wear out mentioned earlier and the similar voltage dependence observed in Figure 5 suggest that both kinds of capacitor share a common wear out mechanism that is independent of the cathode material. This is consistent with the hypothesis that wear out occurs at the tantalum-dielectric interface, not the dielectric-cathode interface.

Another comment on Figure 5 is in order. At very high percentages of formation voltage, the slope of the curve (the voltage ratio exponent in the voltage acceleration formula) increases, indicating additional acceleration. It is hypothesized that a second wear out mechanism begins to contribute to wear out at test voltages in excess of roughly 65% of formation voltage.


Page 7 of 11

It is clear from Figure 5 that the polymer parts were tested at higher percentages of formation voltage than were the MnO2 parts. The data of Figure 6 provide the reason why. Figure 6 displays the initial failures that occurred at 125oC at the various test voltages for the 25V, 35V, and 50V MnO2 and polymer capacitors. At a given test voltage, some percentage of a sample of parts does not survive the ramp up of the test voltage. This percentage appears as a data point on the curve related to that test sample. At higher test voltages, a higher percentage of the sample fails during ramp up and vice-versa.

Figure 6. Percent of Initial Failures versus Percent of Formation Voltage Applied for 25V, 35V, and 50V Experimental MnO2 and Polymer Tantalum Capacitors at 125oC.

The test voltages were chosen to be high enough to generate at least some failures in times less than 100 hours, but not so high as to cause all the parts to fail during ramp up. It turns out that the electric field strengths that meet these criteria are lower for the MnO2 parts (they have thicker dielectric) than the polymer parts even though the ratios of test voltage to rated voltage are generally similar.

This means that the cathode material impacts the short-term dielectric strength of the capacitor. This is consistent with the hypothesis that breakdown voltage is governed by the potential barrier at the insulator-semiconductor (oxide-cathode) interface, and that this barrier behaves somewhat differently for MnO2 parts (MnO2 is an n-type semiconductor) than for PEDT polymer parts (p-type semiconductor).

Also observed in Figure 6 is that the test voltages -- as a percentage of formation voltage -- that cause early failures fall as the dielectric thickness rises. This is true for both cathode materials, and demonstrates that there are limits to the improvement in breakdown voltage that can be achieved by simply making the dielectric thicker. This implies that the road to substantially higher breakdown voltages for high-voltage tantalum capacitors is likely controlled as much by the properties of the chosen cathode material as by dielectric thickness.


Page 8 of 11

One final graph appears in Figure 7. This graph demonstrates the relative temperature dependence of the initial failures (breakdown voltages) for the 35V MnO2 and 35V polymer capacitors. It is clear that there is more temperature dependence of breakdown voltage in the MnO2 capacitors than in the polymer capacitors. This difference in temperature dependence provides additional corroboration that the dielectric strength is governed by the oxide-cathode interface and that this interface has different properties for the different cathode materials.

Figure 7. Percent of Initial Failures versus Percent of Formation Voltage Applied at 85oC, 105oC, and 125oC for 10uF, 35V Experimental MnO2 Tantalum Capacitors and 15uF, 35V Experimental Polymer Tantalum Capacitors.

It is interesting to note that while the temperature dependence is very small with no easily discerned positive or negative temperature characteristic for the tantalum polymer capacitors, it is larger with clearly negative temperature coefficient for the MnO2 capacitors. At this time it is unknown why this is true, but it is speculated that this could be related to the n-type/p-type difference in the semiconductor cathode materials and related differences in the temperature dependence of the concentration and mobility of their respective charge carriers.

Summary and Conclusions

In previous work, accelerated time-to-failure lifetesting of high voltage (25-50V) tantalum polymer capacitors was performed to determine voltage and temperature acceleration models so that lifetime could be predicted at less stressful conditions in a practical timeframe. The motivation was to assess the reliability of a new capacitor technology when traditional test methods (Weibull grading) proved inadequate. These polymer-cathode capacitors demonstrated excellent projected median life at rated conditions and very good volumetric efficiency.


Page 9 of 11

In the mean time, significant improvements were made in the manufacture and screening of the corresponding 25-50V MnO2-cathode part types, and there was interest in comparing the performance of these improved MnO2-cathode parts to the polymer-cathode parts. This motivated an accelerated lifetest investigation to determine the voltage and temperature acceleration models for the improved MnO2 capacitors. Representative samples were tested at various voltages and temperatures and the resulting time-to-failure data were analyzed using the same techniques as were applied to the polymer-cathode capacitors.

The first observation was that the structure and complexity of the time-to-failure distributions for the MnO2 capacitors were substantially similar to those of the polymer-cathode capacitors. Initial failures, a relatively flat region of declining failure rate, and a distinct wear out mode were present in the failure distributions in response to the highly accelerated test conditions. Only the anti wear out mode was absent, as was expected (no degradation of MnO2 conductivity was expected at the test conditions employed). The existence of a wear-out mode -- characterized by rapidly increasing failure rate -- was expected, but is contrary to common claims that MnO2 tantalum capacitors do not have a wear out mechanism.

The wear out mechanism for both the polymer-cathode capacitors and MnO2-cathode capacitors is thought to be the natural diffusion of oxygen from the dielectric into the tantalum substrate metal. This process leaves behind oxygen vacancies in the nearby dielectric, degrading its insulating properties. The applied voltage is now forced to drop across less “good” dielectric, progressively increasing the electric field stress. This speeds up the loss of more oxygen and the cycle continues, leading to increasing failure rate over time. This mechanism is accelerated by both temperature and voltage, and is believed to be the dominant failure mechanism at normal application conditions, albeit only after very long time periods at normal stress levels.

The voltage and temperature acceleration models for this wear out mechanism were found to be essentially the same for both kinds of tantalum capacitor with a voltage ratio exponent of 13.2 in the power law voltage acceleration formula and an activation energy of about 1.5 eV in the Arrhenius temperature acceleration formula (for 35V devices tested at 125oC and ~50% of formation voltage). For the 35V MnO2 capacitors, projected median life of the capacitors is roughly 100,000 years at rated voltage and 85oC.

In absolute terms, the projected median life of the MnO2 capacitors is longer than the projected median life of the polymer capacitors. But once the result is adjusted to account for the lower electric field stress in the MnO2 capacitors (because of their thicker dielectric), the results are essentially identical.

Attention was then directed toward the initial failure region of the time-to-failure distributions. These capacitors fail because the highly accelerated test voltage overcomes the potential barrier at the dielectric-cathode interface. This is consistent with MIS theory where the insulator-semiconductor interface corresponds to the dielectric-cathode interface. Because these failures occur during the 60s voltage ramp at the start of testing, they provide dielectric strength data at the test temperature for the failed capacitors.

It was demonstrated that the initial failure behavior of the MnO2 capacitors is significantly different from the initial failure behavior of the polymer capacitors. The failures occur at lower electric field stress for the MnO2 capacitors than the polymer capacitors. Also, the percentage of failures at a given stress level is distinctly temperature dependent for the MnO2 capacitors while it is not significantly temperature dependent for the polymer capacitors. These differences support the assertion that dielectric strength and its temperature dependence are substantially determined by the choice of cathode material in tantalum capacitors according to MIS theory.

For the MnO2 capacitors the MnO2 cathode is an n-type semiconductor while for the polymer-cathode capacitors the cathode is a p-type semiconductor. It is thought that this distinction may play some role in the temperature dependence difference between the two cathode materials.


Page 10 of 11


Page 11 of 11

The MnO2 tantalum capacitors tested for this paper have much longer median life at rated conditions than is necessary for any practical application. But they have similar percentages of initial failures at similar percentages of rated voltage when comparison is made with the polymer-cathode capacitors. This suggests that MnO2 capacitors will have excessive lifespan when the dielectric is thick enough to produce breakdown voltages that are competitive with the polymer capacitors.

This presents an opportunity for both the user and the designer of MnO2 capacitors in high-reliability applications. Clearly the MnO2 capacitors studied here could be used without any voltage de-rating (user decision) under steady-state application conditions without any practical impact on long-term reliability. Alternatively, these capacitors could be made with thinner dielectric to provide more capacitance in the same volume (designer decision) without practical reduction of reliability.

But some applications do not provide steady state conditions. These more dynamic applications expose the capacitors to high peak charging currents and occasional voltage transients of significant magnitude (or both). For these applications it is likely a good tradeoff to build in additional dielectric strength at the cost of lower volumetric efficiency. This situation provides manufacturers of MnO2 capacitors an opportunity to provide different product designs custom-tailored to the nature of the hi-rel application and an opportunity for the user to consider less conservative de-rating guidelines or higher capacitance density for well-behaved circuits.

References 1 E. Reed and G. Haddox, “Reliability of High Voltage Tantalum Polymer,” CARTS USA 2011 Proceedings of the 31st Symposium for Passive Components, p. 195 (2011). 2 J. Paulsen, E. Reed, and J. Kelly, “Reliability of Tantalum Polymer Capacitors,” CARTS’04 Proceedings of the 24th Capacitor and Resistor Technology Symposium, p. 114 (2004).

3 E. Reed, J. Kelly, and J. Paulsen, “Reliability of Low-Voltage Tantalum Polymer Capacitors,” CARTS USA 2005 Proceedings of the 25th Symposium for Passive Components, p. 189 (2005). 4 Y. Freeman, W. Harrell, I. Luzinov, B. Holman, and P. Lessner, “Electrical Characterization of Tantalum Capacitors with Poly(3,4-ethylenedioxythiophene) Counter Electrodes,” Journal of the Electrochemical Society, p. G65 (2009).

Documents

Reliability of High-Voltage MnO2 Tantalum Capacitors CARTS... · Reliability of High-Voltage MnO2 Tantalum Capacitors . Erik Reed, George Haddox . KEMET Electronics Corporation, 2835