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The second observation is that the first breakdown for the tantalum polymer capacitors occurs at 1.75 times rated voltage. This is roughly twice as high as the lowest voltage seen for the MnO2-based capacitors.While the precise numerical value of this advantage varies somewhat with manufacturer and rated voltage, Flood Damage Restoration For Electronics tantalum polymer capacitors consistently outperform equivalent MnO2-based capacitors on this test.

As was previously mentioned, it is thought that this improved robustness against high inrush current results from a combination of the reduced processing temperatures during polymerization and the relative softness of the polymer with respect to hard, Flood Damage Restoration For Electronics crystalline MnO2. It is also thought that this advantage is not due to the electrical properties of the polymer or its processing chemistry. 

This hypothesis is supported by theincreased DC leakage current seen in tantalum polymer capacitors as is demonstrated in Figure 17. Typical Reliability PerformanceThe most absolute measure of reliability for tantalum capacitors is time-to-failure during low-impedanceDC lifetests. While other important reliability tests exist, it still remains true that when failure rates arediscussed, it is data from hot, dry, Flood Damage Restoration For Electronics DC lifetests that dominate the discussion. 

Early lifetesting was typicallyperformed at 85oC and rated voltage. However, unless the sample sizes are exceedingly large, it takes avery long time to collect a useful amount of failure data in a practical time frame at these conditions.To address this issue, Flood Damage Restoration For Electronics accelerated lifetests were developed. Times-to-failure can be reliably accelerated byuse of voltages higher than rated voltages and/or temperatures above 85oC. 

The data of Figure 22 aretimes-to-failure for a population of MnO2-based tantalum capacitors that were tested at 1.32 times ratedvoltage at 85oC. Estimates vary, Flood Damage Restoration For Electronics but the acceleration formula of MIL-PRF-55365 indicates that the timeacceleration due to this applied voltage is approximately 400. The data are plotted on a Weibull scale thatgenerates straight lines from data that fit a Weibull distribution.

One characteristic of such data is that if the Weibull β parameter (slope of the data) is less than one, thefailure rate of the devices is decreasing in time. Manufacturers take advantage of this fact to "Weibull grade" tantalum capacitors to Flood Damage Restoration For Electronics known reliability levels by lifetesting capacitors long enough at acceleratedconditions to weed out enough of the early failures that the failure rate falls to the desired low level. 

For the data of Figure 22, the β parameter is 0.36 which is much less than 1. If one works through thecalculations presented in MIL-PRF-55365 using the data of Figure 22, he finds that after 10 hours oflifetesting at 1.32 times rated voltage, Flood Damage Restoration For Electronics the failure rate has fallen from a very high initial value to less than0.1% per 1000 hours. 

Armed with this information, typically gathered on a sample of capacitors from agiven lot, Flood Damage Restoration For Electronics the manufacturer would then subject the rest of the lot to low-impedance lifetest for 10 hours at85oC and 1.32 times rated voltage and ship the survivors with confidence that the surviving populationwould demonstrate the same 0.1%/1000 hour failure rate. 

This strategy has been used with much successto establish the failure rate of MnO2-based solid tantalum capacitors for military and aerospace applications. However, Flood Damage Restoration For Electronics when one attempts to follow the same path with tantalum polymer capacitors, the results are notthe same. Generally, even after hundreds of hours at 1.32 times rated voltage and 85C, no failures areobserved at all, or if there is a rare failure, it occurs at the very start of the test and no more failures aresubsequently observed. 

So what value of β do you use if no failures occur? How do you estimate thefailure rate if there are no failures?Basically the whole approach doesn’t work very Flood Damage Restoration For Electronics well for almost all tantalum polymer capacitors because ofthe shortage of failures. But this is a good thing. What it means is that these capacitors are inherentlymore reliable than MnO2-based tantalum capacitors. 

Indeed, field experience indicates very few failures,and almost all of those occur immediately after turn on. Moreover, Flood Damage Restoration For Electronics it is thought that the few turn-onfailures (usually measured in parts per million) are predominantly the result of damage to the dielectric that is caused by the thermo-mechanical stresses of reflow mounting rather than by defects resulting from themanufacturing process (the MnO2 impregnation process is thought to be the cause of almost all early failures of MnO2-based capacitors).

Because of the shortage of lifetest failures of tantalum polymer capacitors, there was curiosity regarding theexpected life of these devices. To explore this question, Flood Damage Restoration For Electronics very highly accelerated lifetests were performed.Figure 23 shows time-to-failure for four such lifetests on 100µF, 6V tantalum polymer capacitors. 

The testvoltage was 9.6V, or 1.6 times rated voltage. Test temperatures ranged from 85oC to 145oC. These dataand data collected at other combinations of voltage and Flood Damage Restoration For Electronics temperature lead to acceleration models thatpredicted very long life at rated voltage and 85oC. Specifically the first failure under maximum ratedconditions for the capacitors whose data appear in Figure 23 is not expected for roughly 100 years. 

Little de-rating is needed for reliable application of these capacitors, and the industry typically recommendsderating by only 20% instead of 50% for MnO2-based tantalum capacitors. There is something Flood Damage Restoration For Electronics else important about the data of Figure 23. The failure distributions are very tightlydistributed in time. 

If these data were plotted on a Weibull scale, the β parameter would be significantlygreater than one, indicating a rapidly increasing failure rate. This means that the dielectric is wearing outunder the applied stress. There is so little dielectric damage done during manufacturing that we canactually observe Flood Damage Restoration For Electronics and model the wearout of tantalum pentoxide, and we see very little difference between the best and worst performers in the sample. 

This is in direct contrast with the times-to-failure presented in Figure 22 for MnO2-based capacitors where the Flood Damage Restoration For Electronics failures are distributed over many orders of magnitude in time. Such wide distribution of times-to-failure indicates an equally-wide range of dielectric damage inflicted during the manufacturing process.

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