Microelectronics Reliability xxx (2015) xxx–xxx

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Microelectronics Reliability

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Degradation testing and failure analysis of DC film capacitors under highhumidity conditions

Huai Wang a,⁎, Dennis A. Nielsen b, Frede Blaabjerg a

a Center of Reliable Power Electronics (CORPE), Department of Energy Technology, Aalborg University, DK-9220 Aalborg, Denmarkb Center of Reliable Power Electronics (CORPE), Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg, Denmark

⁎ Corresponding author.E-mail address: hwa@et.aau.dk (H. Wang).

http://dx.doi.org/10.1016/j.microrel.2015.06.0110026-2714/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: H.Wang, et al., Degrtronics Reliability (2015), http://dx.doi.org/1

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 May 2015Accepted 10 June 2015Available online xxxx

Keywords:Film capacitorsReliabilityPower electronics

Metallized polypropylene film capacitors are widely used for high-voltage DC-link applications in powerelectronic converters. They generally have better reliability performance compared to aluminum electrolyticcapacitors under electro-thermal stresses within specifications. However, the degradation of the film capacitorsis a concern in applications exposed to high humidity environments. This paper investigates the degradation of atype of plastic-boxedmetallizedDCfilm capacitors under different humidity conditions based on a total of 8700 hof accelerated testing and also postfailure analysis. The test results are given by themeasured data of capacitanceand the equivalent series resistance. The degradation curves in terms of capacitance reduction are obtained underthe conditions of 85% Relative Humidity (RH), 70% RH, and 55% RH. The postfailure analysis of the degraded sam-ples of interest is also presented. The study enables a better understanding of the humidity-related failure mech-anisms and reliability performance of DC film capacitors for power electronics applications.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Capacitors are important elements used for DC-link filters, AC filters,snubbers, energy storages, etc., in power electronic systems. Aluminumelectrolytic capacitors and metallized polypropylene film capacitors arewidely used for the DC-link applications due to the wide range of avail-able capacitances and voltages. These two types of capacitors have theirrespective advantages and limitations in terms of reliability, energydensity, capacitance stability, and voltage capability as discussed in [1,2]. Aluminum electrolytic capacitors are usually assumed to have ashorter lifetime than that of film capacitors due to the electrolyte evap-oration. Therefore, efforts have been devoted to replace the electrolyticcapacitors by film capacitors in the DC-links of power electronic sys-tems. However, the outcome of the reliability improvement of suchreplacement solution depends on the field operation conditions. Asdiscussed in [2,3], corrosion is a critical wear out failure mechanismfor film capacitors. Therefore, the degradation of DC film capacitorscould be significantly accelerated, when the Relative Humidity (RH) in-creases from zero to a high level (e.g., 85%). For applications in whichthe DC film capacitors are exposed to high humidity environments,the validity of the above assumption and the effectiveness of the elec-trolytic capacitor replacement solutions need to be justified cautiously.

The accelerated testing is an importantmethod to investigate the re-liability performance of DCfilm capacitors. In [4], the aging ofmetallizedpolymer capacitors has been studied under accelerated testing

adation testing and failure an0.1016/j.microrel.2015.06.01

conditions of voltage and temperature. In [5], the accelerated testing de-signed for metallized film capacitors is under high ripple current andvoltage stress conditions. While the impacts of voltage, current, andtemperature on the degradation of DC film capacitors are well investi-gated, there is still a lack of quantitative study on the impact of humid-ity. The testing procedures of film capacitors under humidity conditionsvary among capacitor manufacturers. For example, industries performthe qualification testing (i.e., testing to pass) of film capacitors undereither 40 °C and 93% RH, 64 °C and 93% RH, or 85 °C and 85% RH for aperiod of time (e.g., 1000 h) [6,7]. A study on the degradation of X2type film capacitors under humidity conditions for AC filter applicationsis presented in [8]. The prior-art testing activities and the correspondingresults suffer fromone ormore of the following limitations: 1) the influ-ence of humidity is not considered and the testing results are thereforenot applicable for applications under humid environments; 2) themainpurpose of the humidity-related testing is for component qualifica-tion [7], which is not sufficient for DC film capacitor applications; and3) detailed testing data and the corresponding postfailure analysis areusually not provided.

Common for all three of the listed limitations is that it results in alack of knowledge about themechanisms that have caused the capacitorto degrade and/or to fail. The disregard of postfailure analysis happensdespite the well-known fact that metallized film capacitors can sufferfrom various failure and degradation mechanisms, both depending onthe production quality [9] and field application [10]. Hence, by simulta-neously subjecting capacitors to high temperature, humidity, voltage,and ripple current for qualification it is not only impossible to determinethe failure/degradation mechanism, due to the correlation between

alysis of DCfilm capacitors under high humidity conditions,Microelec-1

Table 1Testing samples of metallized DC film capacitors.

Testing samples Testing conditions

Group 1 — 1100 V/40 μF (10 pcs) 85 °C and 85% RHGroup 2 — 1100 V/40 μF (10 pcs) 85 °C and 70% RHGroup 3 — 1100 V/40 μF (10 pcs) 85 °C and 55% RH

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different stressors, but also to compare and extrapolate the test resultsto real applications.

Therefore, this paper aims to investigate the humidity impact andthe respective failure mechanisms of a type of DC film capacitors usedfor power electronics applications. The study is based on the measureddata from a total of 8700 h of degradation testing. The analysis of thefilm andmetallization layer is performed for a degraded capacitor sam-ple and a new capacitor sample by visual inspection, opticalmicroscopy,and energy-dispersive x-ray (EDX) spectroscopy.

2. Degradation testing for DC film capacitors underhumidity conditions

2.1. Designed testing system

A system is built up consisting of the necessary equipment for the ac-celerated testing of film capacitors, as shown in Fig. 1. The climaticchamber is capable to control the temperature from −70 °C to 180 °Cand the humidity from 10% RH to 95% RH within a certain temperaturerange. Three ripple current testers are also available in the system tosupply voltage and ripple current stresses at different frequencies. AnLCR meter and a leakage current and insulation resistance meter areused to characterize the electrical parameters of capacitors. The systemallows the testing of a wide range of film capacitors for the DC-linkapplications in power electronics.

A type of 1100 V/ 40 μFmetallized polypropylene film capacitors arechosen for the investigation. 30 testing samples are used and dividedinto three groups as given in Table 1. The initial capacitances and Equiv-alent Series Resistances (ESR) of the samples at 100 Hz vary from38.95 μF to 40.05 μF, and from 11.75 mΩ to 30 mΩ, respectively. Thepresented testing is dedicated to study the impact of humidity andwith a constant temperature of 85 °C. Therefore, it is possible to distin-guish the failure mechanisms due to humidity from the effects of otherstressors.

2.2. Degradation testing results

According to the testing samples and testing conditions shown inTable 1, the accelerated degradation testing has lasted for 2160 h,2700 h, and 3850 h for Group 1, Group 2, and Group 3, respectively.

Fig. 1. A photo of the capacitor testing system.

Please cite this article as: H.Wang, et al., Degradation testing and failure antronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.01

The capacitance and ESR values of the samples are measured at100 Hz along with the testing time. The end-of-life criterion of a capac-itor is usually defined as the time to a specific percentage of the capac-itance drop or ESR increase with respect to its initial value before inoperation. Since the initial values of the testing samples vary fromeach other, all of the capacitances and ESRs are normalized to theirinitial values in the figures and analyses below.

Fig. 2 presents the capacitances and ESRs of the samples in the threegroups of testing. The 10 capacitors in each group are named as Cap 1,Cap 2…, and Cap 10. It can be noted that the capacitance values startto reduce with an increasing slope after around 500 h, 1000 h, and1500 h, respectively, for the testing under 85% RH, 70% RH, and 55%RH. In each group, it appears that some capacitors degrade considerablyfaster than others, for example, Cap 1 in Group 1, Cap 7 in Group 2, andCap 6 in Group 3. It implies the statistical variances among the testingsamples. The ESR values of some of the capacitors become more than100 times of their initial values at the end of testing, as shown inFig. 2(d) to (f). According to the measured data, the ESR values of allof the testing samples increase to more than 3 times after around1000 h, 1800 h, and 2400 h of testing, in Group 1, Group 2, and Group3, respectively.

Fig. 3 plots themean capacitance values of the 10 testing samples ineach group and the corresponding degradation curves based on them.The changes of the mean normalized capacitances along with the test-ing time are modeled as

Cnorm1 ¼ 0:99488−0:0012� e0:00244t ð1Þ

Cnorm2 ¼ 1:00102−0:00073� e0:00181t ð2Þ

Cnorm3 ¼ 1:00301−0:00039� e0:00166t ð3Þ

where Cnorm1, Cnorm2, and Cnorm3 are the mean normalized capacitancesof the testing samples in Group 1, Group 2, and Group 3, respectively,and t is the testing time. The three curves give a quantitative descriptionof the degradation speeds under the three different RH-levels.

3. Failure analysis of capacitor samples

3.1. Failure analysis method and strategy

Since the stressors in the degradation tests are limited to a fixed hightemperature and varying humidity level, the main focus on the failureanalysis is on corrosion of the metallization layers. The capacitor filmroll is removed from the encapsulation and investigated visually andby optical microscopy at various distances into the film and comparedto that of a new capacitor.

Additionally, the film and metallization layers are investigated byEDX [11] for the film element composition. More specifically, it is thepresence of oxygen that is under investigation, because it is a require-ment for corrosion to occur.

3.2. Visual inspection of the capacitor film

By rolling out the film from Cap 10 in the test Group 1, several fea-tures are revealed. In the first 60 cm of the capacitor film, themetalliza-tion layer is like that of a new capacitor. Then a transition occurs to anearly transparent metallization layer, which indicates corrosion of

alysis of DCfilm capacitors under high humidity conditions,Microelec-1

(a) The normalized capacitances under 85% RH.

(b) The normalized capacitances under 70% RH.

(c) The normalized capacitances under 55% RH.

(d) The normalized ESR under 85% RH.

(e) The normalized ESR under 70% RH.

(f) The normalized ESR under 55% RH.

Fig. 2. Degradation testing results in terms of capacitance ESR under different humidity conditions at 85 °C (ESR — Equivalent Series Resistance, RH — Relative Humidity).

3H. Wang et al. / Microelectronics Reliability xxx (2015) xxx–xxx

themetallization layer. An image of the capacitor film at 50 cm and 1minto the roll is shown in Fig. 4, where it is seen that at 1 m almost onlythe reinforced metallization edges are left compared to that at 50 cm.

Roughly 25 m into the capacitor roll a pattern emerges, which isshown in Fig. 5. Here it is observed that in some sections the film ismore transparent than others. The more transparent sections arethose facing the sides, top, and bottom of the capacitor, while the lesstransparent ones are those facing the corners.

3.3. Optical microscopy investigation

One film section from a new capacitor and one section from Cap 10in test Group 1 at 1 m into the roll are investigated in a Leica M3000inverted microscope. The results are shown in Fig. 6. It is seen that the

Please cite this article as: H.Wang, et al., Degradation testing and failure antronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.01

metallization layer on the segment from the new capacitor is fairly in-tact, whereas the metallization in the tested capacitor seems severelydamaged. It is observed that there are small metal islands left, wherethe rest of the metallization layer has corroded.

3.4. Energy-dispersive x-ray spectroscopy

EDXmeasurements are performed at different distances into the ca-pacitor film roll in order to detect and quantify the presence of oxygenatoms from water diffusing into the capacitor during testing. Carbon,oxygen, and zinc are detected as expected. The estimated percentagesof oxygen are listed in Table 2. An additional EDX measurement isperformed at 25 m into the new capacitor for comparison.

alysis of DCfilm capacitors under high humidity conditions,Microelec-1

Fig. 3. Mean normalized capacitances and the corresponding degradation curves underdifferent humidity levels and 85 °C (RH — Relative Humidity).

Fig. 5. Photography of the capacitor film at 25m into the capacitor roll of Cap 10 in the testGroup 1.

(a) A new capacitor sample.

(b) Cap 10 in Group 1 after the degradation testing.

4 H. Wang et al. / Microelectronics Reliability xxx (2015) xxx–xxx

In thiswork the results from the EDX analysis should be treatedwithcaution and as relative numbers. The reason for that is a low x-ray signalfrom the sample due to the very thin sample thickness (about 50 nmmetallization layer and 2.5 μm polypropylene film). That being said,the results still clearly shows a difference in oxygen content in thetwo capacitors as well as a distance dependence.

3.5. Degradation mechanisms

The results from the EDXmeasurements as well as the observationsmade in Fig. 5 indicate that the water has diffused into the capacitoraided by the high temperature during testing. Fig. 5 indicates diffusion,because the different sections of the film have different distances to theambient. Also the EDX results indicate that water has diffused into thecapacitor, because the oxygen content slowly decreases with distanceinto the capacitor roll. The oxygen content measured in the new capac-itor is expected to be from a natural oxide layer or adsorbed moisturefrom the ambient air.

Moreover, based on the observations in Fig. 4, corrosion of the met-allization layer is an electro-chemical reaction between metal and oxy-gen aided by an electrical potential. This is based on the fact that thefirst50 cm of film is like new, even though water has diffused through it. Itindicates that the outermost layers are not directly connected to theend-spray and no current will therefore run in this part of the film,when a voltage is applied. No voltage is applied during the degradationtesting, however, a small voltage is applied when measuring capaci-tance and ESR, which is likely to have enabled corrosion of the metalli-zation layers.

4. Conclusions

The degradation of DC film capacitors under high humidity condi-tions is investigated in this paper. A total of 8700 h of accelerated testing

Fig. 4. Photography of (left) the un-corrodedmetallization at 50 cm, (center) the corrodedmetallization at 1 m into the capacitor roll, and (right) 25 m of a new capacitor.

Please cite this article as: H.Wang, et al., Degradation testing and failure antronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.01

is performed for three groups of capacitors under 85 °C and 85% RH, 70%RH and 55% RH, respectively. The measured capacitance and ESR valuesalong with the testing time are presented. The degradation of the threegroups of capacitors is quantitatively modeled based on the mean ca-pacitance values in each group. The film andmetallization layers of a de-graded capacitor are analyzed by energy-dispersive x-ray spectroscopy.

Fig. 6.Microscopy images of themetallization film from a new capacitor and from a testedcapacitor (the scale bars represent a distance of 200 μm).

alysis of DCfilm capacitors under high humidity conditions,Microelec-1

Table 2Oxygen content measured by EDX of the Cap 10 in Group 1 anda new capacitor sample.

Distance Oxygen content

2 m 19.8%25 m 17.9%50 m 16.9%75 m 16.0%100 m 15.7%25 m (new) 6.7%

5H. Wang et al. / Microelectronics Reliability xxx (2015) xxx–xxx

The oxygen contents level at 25 m is 2.7 times compared to a newcapacitor in the same location into the capacitor roll due to the waterdiffusion. The corrosion of the metallization layer is likely to be anelectro-chemical reaction between metal and oxygen aided by anelectrical potential induced during the parameter measurements.

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alysis of DCfilm capacitors under high humidity conditions,Microelec-1