Identification of Missing or Insufficient Electrolyte ... - DfR Solutions

CARTS 2004: 24 th Annual Capacitor and Resistor Technology Symposium, 29 March – 1 AprilIdentification of Missing or Insufficient Electrolyte Constituents in FailedAluminum Electrolytic CapacitorsCraig Hillman and Norman HelmoldCALCE, University of MarylandCollege Park, MD 20752(301) 405-5316chillman@calce.umd.edunhelmold@calce.umd.eduIntroductionBeginning in late 2001, a number of onlinenewsgroups and industry publications started reportingthe early failures of low effective series resistance(ESR) aluminum electrolytic capacitors manufacturedin Taiwan. The most obvious characteristic of thesefailures was the bulging of the capacitor enclosureprior to loss of electrical function (see Figure 1).These capacitors had been produced in the millionsunder contract to multiple capacitor suppliers, andthey had widespread use in the mass-producedelectronic products of several companies, potentiallycausing the early failure of hundreds of thousands ofend units.While several articles describing the bulging and earlyfailure of these capacitors were published in late 2002and early 2003, the root cause of the failures was onlyspeculated to be a missing component of theirelectrolyte [1-6]. Responding to the request of acapacitor supplier, the CALCE Electronic Productsand Systems Center (EPSC) undertook an extensivestudy of 1000 microfarad, 10 volt, aluminumelectrolytic capacitors susceptible to bulging and earlyfailure.Initial Chemical PropertiesThe initial effort was to establish cause of the casebulging. Based on an understanding of electrolytechemistry and knowledge of similar failures fromolder generations of electrolytic capacitors, it wasspeculated that hydrogen evolution was initiating theincrease in internal pressure. Mass spectroscopy of gasemanating from several punctured bulged capacitorsconfirmed this hypothesis (see Figure 2).To prevent hydrogen gas formation, proprietarydepolarizers are often added to the electrolytic solutionto react with hydrogen atoms that would otherwisebond to each other, forming molecules of gas.Depolarizers, also known as degassing agents orgetters, are often nitro-substituted aromaticcompounds (nitroaromatics), sorbate compounds,amine salts, or soluble inorganic salts. The amount ofdepolarizer necessary to prevent gas formationdepends on the chemical being used. Nitroaromaticsare typically added in amounts of approximately 1wt%. Sorbic acid has been used in amounts rangingfrom about 0.1 wt% to 15 wt%, but it seems to bemost optimal in concentrations of 2 wt% to 3 wt%.Inorganic salts are used over a range from around 0.1wt% to 5 wt%.A review of ion chromatographic and massspectrographic analyses of capacitor electrolytes didnot yield an obvious depolarizer (ion chromatographycan detect ions down to 0.1 ppm while massspectroscopy is sensitive to approximately 10 ppm).The presence of impurities in the aluminum foil canalso induce gas generation. Metals such as copper,magnesium, iron, and zinc, can form galvanic coupleswith aluminum. These create electrons that mightcombine with hydrogen ions in the electrolyte solutionin the reaction2H + + 2e - → H 2to produce gaseous hydrogen. Such production wouldonly occur at the cathodic plate, as the anodic plate iscovered by the dielectric. A patent by R. Dapo of thePhilips Corporation [7] suggests that the purity of thealuminum foil should be greater than 98 wt%.To see if the excessive hydrogen was being producedby impurities in the capacitor foil, wavelengthdispersive x-ray spectrographic (WDS) analyses offoils from a capacitor from the lot of Taiwanesecapacitors known to bulge and foils from a capacitorfrom a lot of non-bulging Japanese capacitors were122

performed. A small amount of magnesium wasdetected in both the Taiwanese and Japanese foils, andcopper was detected in the Taiwanese foils alone (seeTable 1). Ignoring the topical constituents of oxygenand carbon, the purity of the cathodic aluminum foilfrom the Japanese capacitor worked out to beapproximately 99.1 wt%, which was within the limitset by Dapo. The purity of the cathodic aluminum foilfrom the Taiwanese capacitor was approximately97.5%, which was below the minimum value stated byDapo. The insufficient purity of the Taiwanesealuminum foil could cause gaseous hydrogenproduction that would not be impeded by adepolarizer, but the galvanic couples were not thoughtto be sufficient to account for the rapid production ofhydrogen gas that was necessary to cause therelatively rapid bulging of the capacitor cans.There were other anomalies in the ionchromatographic analyses, chiefly variations in theamounts of ammonium and phosphate ions present.Ammonium ions in water form ammonium hydroxide,which is strongly basic. This raised concerns about thepH of the electrolyte in the bulging capacitors, as areview of the chemical properties of aluminum oxide –the dielectric – showed that it is slightly soluble inbasic solutions (but not in acidic)[8]. Measuring thepH of electrolytes from capacitors from the Taiwaneselot known to bulge and from a Japanese lot that hadnot exhibited bulging showed that the electrolytes ofthe bulging lot were weakly basic (7 < pH < 8), whilethose of the non-bulging lot were acidic (pH 4).Initial Electrical InvestigationParametric measurements were performed oncapacitors retrieved from the field. Review of resultsshowed that some of the capacitors exhibited a risingcapacitance prior to bulging. Normally, due to loss ofelectrolyte through the base plug seal, electrolyticcapacitors have a slightly decreasing capacitance untila rapid falloff occurs at the end of life. It was possibleto explain the anomalous behavior as the result ofdielectric thinning, which could be due to dielectricdissolution in the basic electrolyte.Electrical TestingElectrolytic capacitors from a lot known to besusceptible to bulging were subjected acceleratedstress testing at 55°C with a bias of 3.3 volts (theapplication voltage). Capacitance, ESR, and leakagecurrent were periodically measured at roomtemperature. The capacitance was found to increaseslowly up to 1100 microfarads, at which point the rateof capacitance rise increased significantly and leakagecurrent exceeded manufacturer’s specifications. Thistrend continued until seal breakage and electrolyteloss, at which point capacitance and leakage currentplummeted and ESR increased (see Figures 4, 5, and6).This is in contrast to the behavior seen with capacitorsfrom a different lot and capacitors from a Japanesemanufacturer. Both capacitor sets displayed the morecommon degradation of a gradual decrease incapacitance.Chemical TestingElectrolyte samples from the suspect lot of Taiwanesecapacitors, a different lot of Taiwanese capacitors notexhibiting bulging, and a lot of capacitors from twoJapanese manufacturers were subjected to additionalpH and ion chromatographic testing (see Table 2).Both Taiwanese samples were slightly basic, whileboth Japanese samples were acidic. Increased pH wasmost likely due to the presence of ammonium ions,which are present only in the electrolyte from theTaiwanese samples. The Taiwanese sample from thebulging lot did not contain significant amounts ofphosphate ions, while that from the non-bulging lotdid. Similar results were seen with the electrolyte fromthe Japanese manufacturers, with phosphate ionspresent in one sample and absent from the other.Review of the chemical properties of aluminumphosphate showed that it is not soluble in basicsolutions [8], making it a candidate for a protectivecoating on aluminum oxide in the presence of a basicelectrolyte.Suspecting that unprotected aluminum oxidedielectrics were dissolving into the alkalineelectrolytes, energy dispersive spectroscopy (EDS)was performed on evaporated electrolyte from thecapacitor samples described previously. Theelectrolyte from the lot of Taiwanese capacitorsexhibiting bulging contained aluminum; aluminumwas not detected in the electrolyte from the othercapacitor samples (see Figure 6).DiscussionThe hypothesized root-cause of failure, insufficientbalance of pH and phosphate levels, has beenvalidated through the measurement of electrical123

ehavior of capacitors retrieved from the field.Capacitors were found to have elevated levels ofcapacitance, which is an indication of thinning of thedielectric layer. Thinning of the dielectric layer is mostlikely due to dissolution of aluminum oxide into theelectrolyte. Dissolution in electrolytic capacitorsoccurs when the concentration of phosphate isinsufficient in relation to the pH of the electrolytechemistry.It is important to note that the capacitors underinvestigation had been tested by manufacturer to thespecified lifetime at maximum rated voltage atmaximum rated temperature. All electrical parameterswere found to be within specification at thecompletion of this industry standard test. The reasonfor this experience is that the high voltage required forthis test most likely sufficiently retarded dielectricdissolution until a time beyond the rated lifetime ofthese capacitors.This experience points out a hazard of testing tospecifications, instead of testing to failure. Theindustry standard test is based upon an expectedfailure mechanism – electrolyte evaporation. If anunexpected failure mechanism is introduced due tochanges in material or architecture, the assumedacceleration factor may no longer be valid. A bestpractice approach is to test to failure, which isCALCE’s recommended testing regime for electronicparts. If the manufacturer had tested to failure, it islikely that bulging would have been seennear the end of life, and this would have indicated theexistence of an unexpected failure mechanism.1. D. Zogbi, “Problems in Taiwan's AluminumCapacitor Industry,” Market Eye: Zogbi onPassives, October 14, 20022. C. Hillman, “Substandard Electronic Parts,”CALCE What’s New, October 2002,www.calce.umd.edu/whats_new/2002/China.pdf3. D. Zogbi, “Low-ESR Aluminum ElectrolyticFailures Linked to Taiwanese Raw MaterialProblems,” Passive Component Industry, Sept/Oct2002, p. 10, 12, 314. B. Liotta, “Taiwanese Cap Makers DenyResponsibility,” Passive Component Industry,Nov/Dec 2002, p. 6, 8-105. Y.T. Chiu and S.K. Moore, “Leaking CapacitorsMuck up Motherboards,” IEEE Spectrum Online,February 2003, www.spectrum.ieee.org/WEBONLY/resource/feb03/ncap.html6. M. Langberg, “Scrimping Sometimes Doesn'tPay,” Mercury News Online, March 8, 2003,www.mercurynews.com/mld/mercurynews/business/5346302.htm7. R. Dapo, “Electrolyte containing a noveldepolarizer and an electrolytic capacitorcontaining said electrolyte,” U.S. Patent5,175,674, December 29, 1992.8. “Handbook of Chemistry and Physics,” 80 th ed.,D. Lide, Ed. Boca Raton, FL: CRC Press, 1999,pp. 4-37 and 4-38124

WDS X-RAY SPECTRA OF FOILSTaiwanese BulgingInner FoilOuter Foil(anode)(cathode)Atomic % Weight % Atomic % Weight %Aluminum 57.34 67.67 86.44 90.78Oxygen 33.25 23.27 8.41 5.23Carbon 5.52 2.90 3.65 1.71Calcium 1.00 1.75 0.46 0.72Phosporous 2.29 3.10 0.60 0.73Magnesium 0.23 0.25 0.17 0.16Copper 0.38 1.06 0.27 0.67BulgingJapanese Non-BulgingInner FoilOuter Foil(anode)(cathode)Atomic % Weight % Atomic % Weight %Aluminum 50.76 62.86 84.63 90.60Oxygen 39.32 28.88 9.94 6.31Carbon 7.63 4.21 4.74 2.26Calcium 1.99 3.65 0.08 0.09Phosporous 0.22 0.31 0.58 0.71Magnesium 0.08 0.09 0.04 0.04Copper ----- ----- ----- -----Table 1: Wavelength dispersive X-ray spectrographicanalyses of the foils of a Taiwanese bulging 1000microfarad, 10 volt, aluminum electrolytic capacitorand the foils of a Japanese non-bulging 1000microfarad, 10 volt, aluminum electrolytic capacitor.Rated Rated Origin Bulging pH NH 4 PO 4Capacitance Voltage Status1000 µF 10 V Taiwan Yes > 7 Yes No1000 µF 10 V Taiwan No > 7 Yes Yes1000 µF 10 V Japan No ~ 4 No Yes1000 µF 10 V Japan No ~ 4 No NoFigure 1: A bulged 1000 microfarad, 10 volt,aluminum electrolytic capacitor manufactured inTaiwan in 2002. The photograph ahs been retouchedto obliterate manufacturer identity information.Table 2: Ion chromatographic and pH analyses ofTaiwanese and Japanese capacitors.125

302520151050BulgedTaiwanese CapacitorN + O -H 2+1234567H +891011121314151617181920m/ZFigure 2: Mass spectrograph of the gas from aTaiwanese 1000 microfarad, 10 volt, aluminumelectrolytic capacitor that had just started to bulge.Current [µA]Leakage Current8000700060005000400030002000100000 4 8 12 16 20 24 28 32 36 40 44 48 52 56Time [days]Figure 4: Leakage current of capacitors from thebulging Taiwanese lot tested at 55°C at a bias of 3.3volts. Diamonds indicate the earliest failure, which hascomplete data. Squares indicate the second group offailures; data collection for these was terminated afterthey blew their cans off. Triangles indicate the thirdgroup of failures, which had just started to bulge.Circles indicate a capacitor just starting to produce asignificant quantity of gaseous hydrogen.Equivalent Series ResistanceOH - H 2 O + 126Capacitance [µF]Capacitance240022002000180016001400120010000 4 8 12 16 20 24 28 32 36 40 44 48 52 56Time [days]Figure 3: Capacitances of capacitors from the bulgingTaiwanese lot tested at 55°C at a bias of 3.3 volts.Diamonds indicate the earliest failure, which hascomplete data. Squares indicate the second group offailures; data collection for these was terminated afterthey blew their cans off. Triangles indicate the thirdgroup of failures, which had just started to bulge.Circles indicate a capacitor just starting to produce asignificant quantity of gaseous hydrogen.Resistance [m]20017515012510075502500 4 8 12 16 20 24 28 32 36 40 44 48 52 56Time [days]Figure 5: Equivalent series resistance of capacitorsfrom the bulging Taiwanese lot tested at 55°C at a biasof 3.3 volts. Diamonds indicate the earliest failure,which has complete data. Squares indicate the secondgroup of failures; data collection for these wasterminated after they blew their cans off. Trianglesindicate the third group of failures, which had juststarted to bulge. Circles indicate a capacitor juststarting to produce a significant quantity of gaseoushydrogen.

BulgingTaiwaneseDissolvedAluminumNon-BulgingTaiwaneseNo DissolvedAluminumFigure 6: Energy dispersive X-ray spectrographicanalyses of electrolyte samples from the Taiwanesecapacitors exhibiting bulging and the Taiwanesecapacitors not exhibiting bulging.127