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Charging–discharging characteristics of a wound aluminum polymer capacitor
MR-12101; No of Pages 6 Microelectronics Reliability xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Charging–discharging characteristics of a wound aluminum polymer capacitor U.H. Jeong a, J.P. Hyung b, Y.G. Yoon c, M.J. Ko b, S.G. Ha d, D.H. Lee e, H.W. Lim b, J.S. Jang a,⁎ a
Department of Industrial Engineering, Graduate School of Ajou University, Suwon, Republic of Korea Reliability Assessment Center, Korea Testing Certification, Gunpo, Republic of Korea c Medical Device Center, Korea Testing Certification, Gunpo, Republic of Korea d Technical Research Center, ENESOL Co., LTD, Yongin, Republic of Korea e Reliability Test Team, LG Display Co., LTD, Gumi, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 July 2016 Accepted 8 July 2016 Available online xxxx Keywords: Polymer Power device Degradation PEDOT:PSS Shrinkage Charging-discharging
a b s t r a c t This study characterized aluminum polymer capacitors, especially when they are charging and discharging. Tests were conducted under various conditions. The following environments were considered: three high-temperature conditions, two high temperature/high humidity conditions, and room temperature. Various operating conditions were also considered, such as charging–discharging, operating, and storage. The test results showed that the capacitance of the wound polymer aluminum capacitor degraded with charging–discharging at low temperature. At lower temperatures, this characteristic accelerated but was mitigated with a dry electrolyte. The degraded capacitances partially recovered when the capacitors were stored at a high temperature. These characteristics were not observed for a conventional liquid aluminum capacitor. This unreported special characteristic of polymer aluminum capacitors should be considered when designing systems such as power electronics. Polymer capacitors are known for their high reliability, especially at high temperatures. At low temperatures, however, the charging–discharging characteristic should be carefully considered. This paper reports on this characteristic of polymer capacitors for consideration by industries. © 2016 Published by Elsevier Ltd.
1. Introduction Since the development of polymer aluminum capacitors with organic materials [1], their application has increased due to their high reliability, low impedance, thermal stability, and long life. Conventional liquid electrolyte aluminum capacitors have many disadvantages, including high impedance, thermal instability, and liquid electrolyte leakage, because they use a low-conductivity (10− 2–10− 3 S/cm) and thermally unstable liquid electrolyte [2]. Table 1 presents types of electrolytic capacitors and their characteristics. In 1994, Kudoh et al. [3] studied the thermal stability of polymer electrolytic capacitors and showed that a hermetically sealed aluminum polymer capacitor has excellent stability and can function without any deterioration after 3600 h even at 150 °C. Even if a polymer aluminum capacitor has high reliability, international standards like IEC 60384– 26 basically use the same approach as that for conventional liquid electrolyte aluminum capacitors [4]. AEC Q200, which is a standard for the harsh automotive environment, also applies the same requirements as for liquid electrolytic capacitors [5]. Electrolytic capacitors can be used in many domains, such as avionics, automotive, and industrial. Such components are required for most power supplies and have one of the highest failure rates [6]. Compared ⁎ Corresponding author. E-mail address: [email protected] (J.S. Jang).
to conventional liquid electrolytic capacitors, the polymer aluminum capacitor is known for high reliability at high temperatures, but this should be verified for other environments or applications. This paper presents the failure mechanism and special characteristics of polymer electrolytic capacitors. 1.1. Aluminum electrolytic capacitor Fig. 1 shows the structure of an aluminum electrolytic capacitor, which has an etched aluminum anode foil, aluminum oxide layer as a dielectric, electrolyte, and cathode foil. In a polymer aluminum electrolytic capacitor, the electrolyte is a polymer, not a liquid. The capacitance can be calculated as follows: C ¼ ε0 εr
A d
ð1Þ
where C is the capacitance, A is the area of two plates, εr is the relative static permittivity between two plates, ε0 is the electric constant (8.854 × 10−12 F/m), and d is the separation between the plates. Aluminum electrolytic capacitors can yield a high capacitance despite their relatively small size because of the increased area A with the etched anode foil. Aluminum electrolytic capacitors are affected and degraded by the operating environment, especially with regard to the voltage, frequency, and temperature. If an electrolytic capacitor is
http://dx.doi.org/10.1016/j.microrel.2016.07.078 0026-2714/© 2016 Published by Elsevier Ltd.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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Table 1 Types of electrolytes and their characteristics.
Table 2 Test conditions.
Types of electrolytes Working voltage [V] Characteristics
Test environment
Test conditions
Conventional
~450
60 °C/90% RH
Polymer Tantalum
~100 ~50
30 s charging/30 s discharging 30 s charging/330 s discharging charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Storage Operating 5 s charging/5 s discharging 10 s charging/10 s discharging 30 s charging/30 s discharging 30 s charging/330 s discharging
Inexpensive unit capacitance, high temperature/frequency dependency. Low ESR, high reliability Expensive, compact, low temperature dependency, high reliability
85 °C/85% RH
125 °C
degraded, the capacitance decreases. This increases ripples in the output stage and can make a system or sub-system fail [7]. 140 °C
1.2. Degradation mechanisms of the electrolytic capacitor A conventional aluminum electrolytic capacitor is known to fail through vaporization of the electrolyte. When a capacitor is charging or discharging, it generates heat internally, which vaporizes the electrolyte. This decreases the capacitance and finally causes the capacitor to fail. This mechanism is the basis for most methods testing the lifetime of conventional electrolytic capacitors [7,8]. Other mechanisms include the charging and discharging ion exchange, which leads to electrolyte degradation, or a voltage surge that causes defects in the aluminum oxide layer. Such defects react with the electrolyte and produce gas and high internal pressure, which can cause a catastrophic failure from the popping of a rubber seal [7]. This study combined conventional high temperature test methods and a charging and discharging test to determine the failure of the
155 °C
Room temperature
electrolytic capacitor. Aluminum polymer capacitors were used as samples with a rated voltage of 63, 68 μF, and PEDOT:PSS (poly polystyrene sulfonate) as the electrolyte. Fig. 2 shows a sample.
1.3. Test conditions The test conditions were designed as combinations of high temperature and charging/discharging. This approach is based on the degradation mechanism of electrolytic capacitors, as discussed in Section 1.2. As indicated in Table 2, five samples were tested under all of the conditions. Twenty samples were tested in each environment, and five samples were tested under each condition.
Fig. 1. Structure of an aluminum capacitor.
Fig. 3. Test circuit schematic.
Fig. 2. Tested sample.
Fig. 4. Measured waveform: 500 ms/div, 10 V/div.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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Fig. 5. 60 °C/90% RH results. Fig. 8. 140 °C results.
1.4. Basic test circuit Fig. 3 shows a basic test circuit diagram, and Fig. 4 shows the measured waveform of the circuit.
2. Test results
Fig. 6. 85 °C/85% RH results.
Fig. 7. 125 °C results.
The tests were conducted for 4000–5000 h, and the capacitances were monitored in situ. Figs. 5–10 show the changes in capacitance. Two failure modes were observed. The first was catastrophic failure due to the rubber seal tearing. The tearing was accelerated with increasing temperature. At 155 °C, the rubber seals failed after about 2800 h; at 140 °C, the rubber seals failed after about 4000–4500 h. Fig. 11 uses X-rays and optics to clearly show this failure. At both wear-out failure and catastrophic failure, internal pressure can be inferred because of the geometric displacement at the upper and bottom areas relative to the non-tested normal sample. At catastrophic failure, the rubber of the sample was torn, so the rubber seal shrank. The rubber seal was observed to swell at wear-out failure.
Fig. 9. 155 °C results.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
Fig. 10. Room temperature results.
Fig. 12. 30 s charging–30 s discharging condition.
At high temperatures leading to catastrophic failure, the rubber hardened, so it tended to tear rather than swell. The second failure mode was the wear-out failure mechanism, where the capacitance decreases with repeated charging–discharging. This was only observed at room temperature. This failure mode has not been reported for polymer aluminum electrolyte capacitors. The results at room temperature under the 30 s charging–30 s discharging and 30 s charging–330 s discharging conditions were compared with the other environmental conditions as a reference, as shown in Figs. 12 and 13. The capacitance only decreased at room temperature. At 155 °C, there was catastrophic failure because of the rubber seal tearing. Catastrophic failure by gases is a well-known failure mechanism, as discussed in Section 1.2. However, the wear-out mechanism of decreasing capacitance with charging–discharging at room temperature is an unreported failure mode and should be examined closely.
3. Failure mechanism analysis To verify the failure mechanism of decreasing capacitance at room temperature, two factors were considered: whether or not decreasing the temperature accelerates this effect and whether or not a PEDOT:PSS electrolyte is affected by moisture. This is because PEDOT:PSS shrinks and swells depending on the moisture [9,10]. Dried samples were prepared, and a test was conducted at 0 °C. Previous test samples and dried samples were tested under 30 s charging– 30 s discharging and 5 s charging–5 s discharging conditions. The test was performed for 150 h. Fig. 14 shows the results. The test results showed that the dried polymer electrolyte can dramatically reduce the decrease in capacitance with charging–discharging and that the degradation is accelerated at lower temperatures. Even though drying the polymer electrolyte can reduce the effect, a completely dried polymer electrolyte cannot react well with a damaged
Fig. 11. X-ray and optical pictures of samples.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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Fig. 15. Shrinkage of polymer.
Fig. 13. 30 s charging–330 s discharging condition.
heat, the inference was made that voids due to shrinkage do not fully recover. Fig. 17 shows the rate of change in the capacitance. The recovery phenomenon was accelerated by temperature. This implies that the external heat and internal heat during charging–discharging can accelerate the capacitance recovery.
aluminum oxide layer, which can suddenly increase the leakage current and lead to failure. To analyze the physical failure mechanism, another test was designed. The electrolyte capacitor has a large capacitance because of the large area from the etched aluminum foil. When PEDOT:PSS shrinks, the contact surface with aluminum oxide layer decreases, as shown in Fig. 15. This decreases the capacitance. At high temperature, no decrease in capacitance was observed. In this case, the temperature can be expected to have a swelling effect. Thus, the test was designed to verify whether or not there is a recovery effect from swelling. The degraded capacitor was stored in a 125 °C environment. Fig. 16 shows the results. In Fig. 16, the straight line above 70 μF is the normal capacitance value at 125 °C. The line with points is recovering capacitance value. This result implies that the changing capacitance of a polymer aluminum capacitor has a counter-effect. When the capacitors are charging– discharging, the capacitance decreases. If a capacitor is stored at high temperature, recovery takes place, but it is not total. Because the internal heat and swelling pressure may be more effective than external
To verify the unique capacitance decrease, another test was conducted to directly compare polymer aluminum capacitors with liquid aluminum capacitors. The test was conducted for 48 h under the 5 s charging– 5 s discharging condition to accelerate the decreasing effect. Samples were selected with two different capacities. Table 3 presents the test results. The capacitances significantly decreased, especially for the polymer aluminum capacitor.
Fig. 14. 0 °C low-temperature acceleration test.
Fig. 16. Capacitance recovery characteristics.
4. Comparison with conventional liquid aluminum electrolyte capacitor
5. Conclusion In this study, multiple tests were performed to characterize a polymer aluminum capacitor's failure mode and mechanisms, especially under the charging–discharging condition. From a lifetime perspective, the polymer aluminum capacitor experiences catastrophic failure from a torn rubber seal due to increased internal pressure by gas produced
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
can be partially recovered from by high temperature, and drying can reduce the effect. At temperatures higher than 60 °C, the internal heat generated during charging–discharging and external heat can compensate for the decrease in capacitance. This recovery effect can be accelerated at higher temperatures. For applications using polymer aluminum capacitors that require high reliability, the decreasing capacitance and polymer shrinkage should be concerns, especially for operation at low temperature. The presented results can be used to design higher-reliability systems and isolate failure. Acknowledgement This work (Grants No. 1415143900) was supported by Reliability Infrastructure Development of Components and Materials funded Korea Institute Of Advancement of Technology. References
Fig. 17. Capacitance recovery rate with temperature.
Table 3 Comparison of test results.
Type
Capacity [μF]
Capacitance change rate
ESR change rate
Tangent loss change rate
Liquid Liquid Polymer Polymer
100 220 120 270
100.5% 95.1% 85.4% 67.6%
104% 191% 161% 196%
104% 181% 135% 132%
from the interaction between the aluminum oxide layer and polymer electrolyte during charging–discharging. There is also a wear-out mechanism from shrinkage of the polymer during charging–discharging, especially at low temperatures. This mechanism is fundamentally different from the failure of conventional liquid aluminum capacitors. While a conventional liquid aluminum capacitor is gradually degraded by vaporization of the electrolyte, the polymer aluminum capacitor is degraded by shrinkage of the polymer electrolyte. This degradation
[1] S. Yoshimura, Y. Itoh, M. Yasuda, M. Murakami, S. Takahashi, K. Hasegawa, An aluminum solid electrolytic capacitor with an organic semiconductor electrolyte, IEEE Trans. Parts Hybrids Packag. 11 (4) (1975) 315–321. [2] H. Yamamoto, M. Oshima, M. Fukuda, I. Isa, K. Yoshino, Characteristics of aluminum solid electrolytic capacitors using a conducting polymer, J. Power Sources 60 (1996) 173–177. [3] Y. Kudoh, M. Fukuyama, S. Yoshimura, Stability study of polypyrrole and application to highly thermostable aluminum solid electrolytic capacitor, Synth. Met. 66 (1994) 157–164. [4] IEC 60384–26. Fixed Aluminium Electrolytic Capacitors with Conductive Polymer Solid Electrolyte, 1.0 ed. International Electrotechnical Commission, 2010. [5] AEC-Q200 Stress Test Qualification for Passive Components, Automotive Electronics Council, 2010. [6] D. Goodman, J. Hofmeister, J. Judkins, Electronic prognostics for switched mode power supplies, Microelectron. Reliab. 47 (2007) 1902–1906. [7] C.S. Kulkarni, J. Celya, K. Goebel, G. Biswas, Physics based electrolytic capacitor degradation models, European Conf. Of the Prognostics and Health Management Soc., 3, 2013. [8] M.L. Gasperi, Life prediction model for aluminum electrolytic capacitors, IEEE Ind. Appl. Conf. 3 (1996) 1347–1351. [9] B. Friedel, J.K. Brennar, C.R. McNeill, U. Steiner, N.C. Greenham, Influence of solution heating on the properties of PEDOT:PSS colloidal solution and impact on the device performance of polymer solar cells, Org. Electron. 12 (2011) 1736–1745. [10] U. Lang, E. Muller, N. Naujoks, J. Dual, Microscopical investigation of PEDOT:PSS thin films, Adv. Funct. Mater. 19 (2009) 1215–1220.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Charging–discharging characteristics of a wound aluminum polymer capacitor U.H. Jeong a, J.P. Hyung b, Y.G. Yoon c, M.J. Ko b, S.G. Ha d, D.H. Lee e, H.W. Lim b, J.S. Jang a,⁎ a
Department of Industrial Engineering, Graduate School of Ajou University, Suwon, Republic of Korea Reliability Assessment Center, Korea Testing Certification, Gunpo, Republic of Korea c Medical Device Center, Korea Testing Certification, Gunpo, Republic of Korea d Technical Research Center, ENESOL Co., LTD, Yongin, Republic of Korea e Reliability Test Team, LG Display Co., LTD, Gumi, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 July 2016 Accepted 8 July 2016 Available online xxxx Keywords: Polymer Power device Degradation PEDOT:PSS Shrinkage Charging-discharging
a b s t r a c t This study characterized aluminum polymer capacitors, especially when they are charging and discharging. Tests were conducted under various conditions. The following environments were considered: three high-temperature conditions, two high temperature/high humidity conditions, and room temperature. Various operating conditions were also considered, such as charging–discharging, operating, and storage. The test results showed that the capacitance of the wound polymer aluminum capacitor degraded with charging–discharging at low temperature. At lower temperatures, this characteristic accelerated but was mitigated with a dry electrolyte. The degraded capacitances partially recovered when the capacitors were stored at a high temperature. These characteristics were not observed for a conventional liquid aluminum capacitor. This unreported special characteristic of polymer aluminum capacitors should be considered when designing systems such as power electronics. Polymer capacitors are known for their high reliability, especially at high temperatures. At low temperatures, however, the charging–discharging characteristic should be carefully considered. This paper reports on this characteristic of polymer capacitors for consideration by industries. © 2016 Published by Elsevier Ltd.
1. Introduction Since the development of polymer aluminum capacitors with organic materials [1], their application has increased due to their high reliability, low impedance, thermal stability, and long life. Conventional liquid electrolyte aluminum capacitors have many disadvantages, including high impedance, thermal instability, and liquid electrolyte leakage, because they use a low-conductivity (10− 2–10− 3 S/cm) and thermally unstable liquid electrolyte [2]. Table 1 presents types of electrolytic capacitors and their characteristics. In 1994, Kudoh et al. [3] studied the thermal stability of polymer electrolytic capacitors and showed that a hermetically sealed aluminum polymer capacitor has excellent stability and can function without any deterioration after 3600 h even at 150 °C. Even if a polymer aluminum capacitor has high reliability, international standards like IEC 60384– 26 basically use the same approach as that for conventional liquid electrolyte aluminum capacitors [4]. AEC Q200, which is a standard for the harsh automotive environment, also applies the same requirements as for liquid electrolytic capacitors [5]. Electrolytic capacitors can be used in many domains, such as avionics, automotive, and industrial. Such components are required for most power supplies and have one of the highest failure rates [6]. Compared ⁎ Corresponding author. E-mail address: [email protected] (J.S. Jang).
to conventional liquid electrolytic capacitors, the polymer aluminum capacitor is known for high reliability at high temperatures, but this should be verified for other environments or applications. This paper presents the failure mechanism and special characteristics of polymer electrolytic capacitors. 1.1. Aluminum electrolytic capacitor Fig. 1 shows the structure of an aluminum electrolytic capacitor, which has an etched aluminum anode foil, aluminum oxide layer as a dielectric, electrolyte, and cathode foil. In a polymer aluminum electrolytic capacitor, the electrolyte is a polymer, not a liquid. The capacitance can be calculated as follows: C ¼ ε0 εr
A d
ð1Þ
where C is the capacitance, A is the area of two plates, εr is the relative static permittivity between two plates, ε0 is the electric constant (8.854 × 10−12 F/m), and d is the separation between the plates. Aluminum electrolytic capacitors can yield a high capacitance despite their relatively small size because of the increased area A with the etched anode foil. Aluminum electrolytic capacitors are affected and degraded by the operating environment, especially with regard to the voltage, frequency, and temperature. If an electrolytic capacitor is
http://dx.doi.org/10.1016/j.microrel.2016.07.078 0026-2714/© 2016 Published by Elsevier Ltd.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
2
U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
Table 1 Types of electrolytes and their characteristics.
Table 2 Test conditions.
Types of electrolytes Working voltage [V] Characteristics
Test environment
Test conditions
Conventional
~450
60 °C/90% RH
Polymer Tantalum
~100 ~50
30 s charging/30 s discharging 30 s charging/330 s discharging charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Charging No source 30 s charging/30 s discharging 30 s charging/330 s discharging Storage Operating 5 s charging/5 s discharging 10 s charging/10 s discharging 30 s charging/30 s discharging 30 s charging/330 s discharging
Inexpensive unit capacitance, high temperature/frequency dependency. Low ESR, high reliability Expensive, compact, low temperature dependency, high reliability
85 °C/85% RH
125 °C
degraded, the capacitance decreases. This increases ripples in the output stage and can make a system or sub-system fail [7]. 140 °C
1.2. Degradation mechanisms of the electrolytic capacitor A conventional aluminum electrolytic capacitor is known to fail through vaporization of the electrolyte. When a capacitor is charging or discharging, it generates heat internally, which vaporizes the electrolyte. This decreases the capacitance and finally causes the capacitor to fail. This mechanism is the basis for most methods testing the lifetime of conventional electrolytic capacitors [7,8]. Other mechanisms include the charging and discharging ion exchange, which leads to electrolyte degradation, or a voltage surge that causes defects in the aluminum oxide layer. Such defects react with the electrolyte and produce gas and high internal pressure, which can cause a catastrophic failure from the popping of a rubber seal [7]. This study combined conventional high temperature test methods and a charging and discharging test to determine the failure of the
155 °C
Room temperature
electrolytic capacitor. Aluminum polymer capacitors were used as samples with a rated voltage of 63, 68 μF, and PEDOT:PSS (poly polystyrene sulfonate) as the electrolyte. Fig. 2 shows a sample.
1.3. Test conditions The test conditions were designed as combinations of high temperature and charging/discharging. This approach is based on the degradation mechanism of electrolytic capacitors, as discussed in Section 1.2. As indicated in Table 2, five samples were tested under all of the conditions. Twenty samples were tested in each environment, and five samples were tested under each condition.
Fig. 1. Structure of an aluminum capacitor.
Fig. 3. Test circuit schematic.
Fig. 2. Tested sample.
Fig. 4. Measured waveform: 500 ms/div, 10 V/div.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
3
Fig. 5. 60 °C/90% RH results. Fig. 8. 140 °C results.
1.4. Basic test circuit Fig. 3 shows a basic test circuit diagram, and Fig. 4 shows the measured waveform of the circuit.
2. Test results
Fig. 6. 85 °C/85% RH results.
Fig. 7. 125 °C results.
The tests were conducted for 4000–5000 h, and the capacitances were monitored in situ. Figs. 5–10 show the changes in capacitance. Two failure modes were observed. The first was catastrophic failure due to the rubber seal tearing. The tearing was accelerated with increasing temperature. At 155 °C, the rubber seals failed after about 2800 h; at 140 °C, the rubber seals failed after about 4000–4500 h. Fig. 11 uses X-rays and optics to clearly show this failure. At both wear-out failure and catastrophic failure, internal pressure can be inferred because of the geometric displacement at the upper and bottom areas relative to the non-tested normal sample. At catastrophic failure, the rubber of the sample was torn, so the rubber seal shrank. The rubber seal was observed to swell at wear-out failure.
Fig. 9. 155 °C results.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
Fig. 10. Room temperature results.
Fig. 12. 30 s charging–30 s discharging condition.
At high temperatures leading to catastrophic failure, the rubber hardened, so it tended to tear rather than swell. The second failure mode was the wear-out failure mechanism, where the capacitance decreases with repeated charging–discharging. This was only observed at room temperature. This failure mode has not been reported for polymer aluminum electrolyte capacitors. The results at room temperature under the 30 s charging–30 s discharging and 30 s charging–330 s discharging conditions were compared with the other environmental conditions as a reference, as shown in Figs. 12 and 13. The capacitance only decreased at room temperature. At 155 °C, there was catastrophic failure because of the rubber seal tearing. Catastrophic failure by gases is a well-known failure mechanism, as discussed in Section 1.2. However, the wear-out mechanism of decreasing capacitance with charging–discharging at room temperature is an unreported failure mode and should be examined closely.
3. Failure mechanism analysis To verify the failure mechanism of decreasing capacitance at room temperature, two factors were considered: whether or not decreasing the temperature accelerates this effect and whether or not a PEDOT:PSS electrolyte is affected by moisture. This is because PEDOT:PSS shrinks and swells depending on the moisture [9,10]. Dried samples were prepared, and a test was conducted at 0 °C. Previous test samples and dried samples were tested under 30 s charging– 30 s discharging and 5 s charging–5 s discharging conditions. The test was performed for 150 h. Fig. 14 shows the results. The test results showed that the dried polymer electrolyte can dramatically reduce the decrease in capacitance with charging–discharging and that the degradation is accelerated at lower temperatures. Even though drying the polymer electrolyte can reduce the effect, a completely dried polymer electrolyte cannot react well with a damaged
Fig. 11. X-ray and optical pictures of samples.
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
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5
Fig. 15. Shrinkage of polymer.
Fig. 13. 30 s charging–330 s discharging condition.
heat, the inference was made that voids due to shrinkage do not fully recover. Fig. 17 shows the rate of change in the capacitance. The recovery phenomenon was accelerated by temperature. This implies that the external heat and internal heat during charging–discharging can accelerate the capacitance recovery.
aluminum oxide layer, which can suddenly increase the leakage current and lead to failure. To analyze the physical failure mechanism, another test was designed. The electrolyte capacitor has a large capacitance because of the large area from the etched aluminum foil. When PEDOT:PSS shrinks, the contact surface with aluminum oxide layer decreases, as shown in Fig. 15. This decreases the capacitance. At high temperature, no decrease in capacitance was observed. In this case, the temperature can be expected to have a swelling effect. Thus, the test was designed to verify whether or not there is a recovery effect from swelling. The degraded capacitor was stored in a 125 °C environment. Fig. 16 shows the results. In Fig. 16, the straight line above 70 μF is the normal capacitance value at 125 °C. The line with points is recovering capacitance value. This result implies that the changing capacitance of a polymer aluminum capacitor has a counter-effect. When the capacitors are charging– discharging, the capacitance decreases. If a capacitor is stored at high temperature, recovery takes place, but it is not total. Because the internal heat and swelling pressure may be more effective than external
To verify the unique capacitance decrease, another test was conducted to directly compare polymer aluminum capacitors with liquid aluminum capacitors. The test was conducted for 48 h under the 5 s charging– 5 s discharging condition to accelerate the decreasing effect. Samples were selected with two different capacities. Table 3 presents the test results. The capacitances significantly decreased, especially for the polymer aluminum capacitor.
Fig. 14. 0 °C low-temperature acceleration test.
Fig. 16. Capacitance recovery characteristics.
4. Comparison with conventional liquid aluminum electrolyte capacitor
5. Conclusion In this study, multiple tests were performed to characterize a polymer aluminum capacitor's failure mode and mechanisms, especially under the charging–discharging condition. From a lifetime perspective, the polymer aluminum capacitor experiences catastrophic failure from a torn rubber seal due to increased internal pressure by gas produced
Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078
6
U.H. Jeong et al. / Microelectronics Reliability xxx (2016) xxx–xxx
can be partially recovered from by high temperature, and drying can reduce the effect. At temperatures higher than 60 °C, the internal heat generated during charging–discharging and external heat can compensate for the decrease in capacitance. This recovery effect can be accelerated at higher temperatures. For applications using polymer aluminum capacitors that require high reliability, the decreasing capacitance and polymer shrinkage should be concerns, especially for operation at low temperature. The presented results can be used to design higher-reliability systems and isolate failure. Acknowledgement This work (Grants No. 1415143900) was supported by Reliability Infrastructure Development of Components and Materials funded Korea Institute Of Advancement of Technology. References
Fig. 17. Capacitance recovery rate with temperature.
Table 3 Comparison of test results.
Type
Capacity [μF]
Capacitance change rate
ESR change rate
Tangent loss change rate
Liquid Liquid Polymer Polymer
100 220 120 270
100.5% 95.1% 85.4% 67.6%
104% 191% 161% 196%
104% 181% 135% 132%
from the interaction between the aluminum oxide layer and polymer electrolyte during charging–discharging. There is also a wear-out mechanism from shrinkage of the polymer during charging–discharging, especially at low temperatures. This mechanism is fundamentally different from the failure of conventional liquid aluminum capacitors. While a conventional liquid aluminum capacitor is gradually degraded by vaporization of the electrolyte, the polymer aluminum capacitor is degraded by shrinkage of the polymer electrolyte. This degradation
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Please cite this article as: U.H. Jeong, et al., Charging–discharging characteristics of a wound aluminum polymer capacitor, Microelectronics Reliability (2016), http://dx.doi.org/10.1016/j.microrel.2016.07.078